Convenient Access to Ferrocene Fused aza-Heterocycles via the Intramolecular Ritter Reaction: Synthesis of Novel Racemic Planar-Chiral 3,4-Dihydroferroceno[c]pyridines and 1H-Ferroceno[c]pyrroles

Abstract

An efficient and easy approach to the synthesis of novel racemic planar-chiral 3,4-dihydroferroceno[c]pyridines and 1H-ferroceno[c]pyrroles via the intramolecular Ritter reaction of 2-ferrocenyl-3,3-dimethylbutan-2-ol with nitriles and thiocyanates in the presence of MeSO₃H was developed. Aromatic and aliphatic nitriles, phenylacetonitriles, and β-oxonitriles produced exclusively 3,4-dihydroferroceno[c]pyridines. The condensation of 2-ferrocenyl-3,3-dimethylbutan-2-ol with various thiocyanates, including alkyl thiocyanates, benzyl thiocyanate, and ethyl 2-thiocyanatoacetate, yielded not only 3,4-dihydroferroceno[c]pyridines but also 1H-ferroceno[c]pyrroles. The selectivity of these reactions depended on the temperature and the order of addition. The size of substituents at the α-position to the sulfur atom of thiocyanates also had a significant effect on the distribution of products.

1. Introduction

One of the promising areas in the development of modern organometallic chemistry is research in the field of ferrocene. The unique properties of this compound, including specific geometry, high chemical and thermal stabilities, low toxicity, and its ability to be reversibly oxidized, result in a wide spectrum of applications for ferrocene-based compounds. Ferrocene derivatives have attracted a lot of attention in the fields of medicinal chemistry [1,2,3,4,5,6,7,8,9] and homogeneous catalysis [10,11,12,13,14]. They are widely applied in material science to create sensors [15,16,17,18,19], electro-optical materials [20,21,22], batteries [18,23], burning rate catalysts for propellants [24,25], molecular machines [26,27], and so on. Thus, the development of new approaches to the synthesis of novel functionalized ferrocene-based compounds is highly desirable.

Reactions of nucleophiles with readily available α-ferrocenyl substituted alcohols FcC(OH)RR’, in which R, R’ = H, Alk, Ar, constitute a synthetically useful and convenient approach to ferrocene-based compounds. A wide range of nucleophiles, including, for example, amines [28,29,30,31], aliphatic alcohols [30,31,32], phenols [29], thiols [28,30,31,33], enamines [34], aldehydes [35], ureas [36], 1,3-dicarbonyl [28,30,31,37,38] and electron-rich aromatic compounds [28,31,39], together with various heterocycles [28,33,37,40,41,42,43,44,45,46,47], can be employed. Reactions of FcC(OH)RR’ with nucleophiles proceeds through S_N1 mechanism via the formation of thermodynamically stable α-ferrocenyl carbocations FcC⁺RR’ to provide α-adducts. The generation of α-ferrocenyl carbocations from FcC(OH)RR’ mainly occurs under acidic conditions in the presence of Brønsted or Lewis acids. Oxidative ionization of α-ferrocenyl alcohols [37] and ionization of α-ferrocenylalkyl carbonates FcCH(R)OCOOEt formed in situ from corresponding FcCH(OH)R [29,33] are also used to obtain α-ferrocenyl carbocations.

Surprisingly, although a great deal of research was devoted to the investigation of the reactions of α-ferrocenyl alcohols with a variety of nucleophiles, there were no data in the literature related to investigations of the reactions of these alcohols with nitriles prior to our studies. Due to the great importance of ferrocene-based compounds, together with our established interest in the development of approaches to the synthesis of various aza-heterocycles via intramolecular Ritter reaction, it seemed relevant to try using α-ferrocenyl alcohols as substrates for this transformation.

In our preliminary work, we demonstrated for the first time the application of α-ferrocenyl alkyl alcohols as substrates for Ritter reaction by an example of the reaction of 1-ferrocenyl-2-methylpropan-1-ol [FcCH(OH)CH(CH₃)₂] with nitriles [48]. It was found that, in most cases, the nitriles were not nucleophilic enough to react with the secondary α-ferrocenyl carbocation FcCH⁺CH(CH₃)₂ generated from FcCH(OH)CH(CH₃)₂ under acidic conditions. Instead, the more reactive tertiary β-ferrocenyl carbocation FcCH₂C⁺(CH₃)₂, formed by a 1,2-shift of the α-carbocation, readily reacted with nitriles to give the corresponding β-nitrilium cations. The latter underwent intramolecular cyclization with the formation of novel 3,4-dihydroferroceno[c]pyridines. These structures, being analogous of 3,4-dihydroisoquinolines, are assumed to be potentially bioactive compounds and attract sufficient interest in medicinal chemistry. It should be pointed out that the synthetic routes for the preparation of 3,4-dihydroferroceno[c]pyridines to date are limited, and there are only a few reports in the literature on their synthesis [49,50,51,52].

To expand the range of α-ferrocenyl alkyl alcohols suitable to be used as substrates in the intramolecular Ritter reaction, herein we studied the reaction of 2-ferrocenyl-3,3-dimethylbutan-2-ol (1) with nitriles under acidic conditions.

2. Results

In order to demonstrate that 2-ferrocenyl-3,3-dimethylbutan-2-ol (1) could serve as a substrate for the intramolecular Ritter reaction, we first examined the model reaction of alcohol 1 with 4-methylbenzonitrile (2a) in MeSO₃H (

Table 1. Reaction of alcohol 1 with 4-methylbenzonitrile (2a). Optimization of the reaction conditions ¹.

Entry	Acid	Temp (°C)	Solvent	Time	Yield (%) 2
					3a	5 + 6 [5:6] 3	Other Products
1	MeSO3H	rt	-	5 h	59	-	4a (6)
2	MeSO3H	40	-	1.15 h	73	-	-
3	MeSO3H	60	-	15 min	73	-	-
4	MeSO3H	80	-	10 min	70	-	-
5	MeSO3H	100	-	5 min	64	-	-
6 4	MeSO3H	60	-	15 min	68	-	-
7	MeSO3H	60	DCE 5	1 h	39	15 [11:89]	-
8	MeSO3H	60	toluene 5	40 min	58	29 [10:90]	-
9	H2SO4	60	-	3 h	51	-	-
10	CF3COOH	60	-	30 min	41	19 [20:80]	-
11	CH3COOH	60	-	2.5 h	-	32 6	1 (42)

¹ Reagents and conditions: 1 (0.35 mmol, 1 equiv.), 2a (0.42 mmol, 1.2 equiv.), acid (8 equiv.). ² Isolated yields after silica gel column chromatography. ³ According to ¹H NMR analysis of the fractions of chromatography. ⁴ 1.5 equiv. of 2a. ⁵ 0.5 mL of solvent. ⁶ Only alkene 5 was isolated.

). MeSO₃H in the amount of eight equivalents was chosen since we earlier showed that it was the most effective for the synthesis of 1-R-3,3-dimethyl-3,4-dihydroferroceno[c]pyridines condensation of 1-ferrocenyl-2-methylpropan-1-ol with nitriles under acidic conditions [48].

The reaction of alcohol 1 with 1.2 equivalent of nitrile 2a in the presence of MeSO₃H at room temperature provided the desired ferroceno[c]pyridine 3a in 59% yield within 5 h (Table 1, entry 1). In addition, amide 4a (6%) was also isolated. The structure of compound 3a was unambiguously confirmed by X-ray crystallography (Figure 1). Performing the reaction at higher temperatures led to the improved product yield and significantly shorter reaction times (Table 1, entries 2–5). In these cases, 60 °C was found to be the most optimal temperature both in terms of product yield and reaction time, resulting in compound 3a in 73% yield within 15 min (Table 1, entry 3). It turned out that the use of 1.5 equivalent of nitrile 2a did not lead to the improvement in the product yield (Table 1, entry 6). The reaction at 60 °C in DCE or toluene resulted in a longer reaction time and decreased yield of compound 3a compared to the same reaction without solvent (Table 1, entries 7, 8 vs. entry 3). In addition to ferroceno[c]pyridine 3a the inseparable mixtures of alkenes 5 and 6 were also isolated. Reactions of alcohol 1 with nitrile 2a in H₂SO₄ or CF₃COOH at 60 °C also gave product 3a, but in lower yields and after longer reaction times (Table 1, entries 9, 10), indicating that these acids were inferior to MeSO₃H when inducing the studied reaction. No reaction occurred when CH₃COOH was used, and only starting alcohol 1 along with alkene 5 were isolated in 42% and 32% yields, respectively (Table 1, entry 11). Thus, based on the results obtained, the following reaction conditions were chosen as optimal: MeSO₃H, 60 °C, a ratio of 1a/2a = 1:1.2, without solvent.

With the optimized conditions identified, the reaction of alcohol 1 with a variety of nitriles and thiocyanates was further explored.

First, the use of aromatic and aliphatic nitriles 2b–t, and β-oxonitriles 2u–x, was investigated (

Table 2. Reaction of alcohol 1 with nitriles 2a–x ¹.

Entry	Nitrile 2	R	Time (min)	Product 3	Yield (%) 2	Entry	Nitrile 2	R	Time (min)	Product 3	Yield (%) 2
1	2a	4-MeC6H4	15	3a	72	13	2m	2-CF3C6H4	40	3m	65
2	2b	Ph	30	3b	75	14	2n	4-NO2C6H4	20	3n	trace 3,4
3	2c	4-MeOC6H4	40	3c	77	15	2o	Me	15	3o	76
4	2d	3-MeOC6H4	40	3d	78	16	2p	Et	20	3p	80
5	2e	2-MeOC6H4	40	3e	63	17	2q	CH2i-Pr	15	3q	75
6	2f	4-NH2C6H4	50	3f	66	18	2r	CH2Ad 5	25	3r	73
7	2g	2-NH2C6H4	180	3g	30	19	2s	Ad 5	20	3s	71
8	2h	4-BrC6H4	30	3h	82	20	2t	CH2CH2OMe	15	3t	— 4
9	2i	3-BrC6H4	40	3i	80	21	2u	CH2C(O)OEt	20	3u	59
10	2j	2-BrC6H4	40	3j	76	22	2v	CH2C(O)NH2	30	3v	34
11	2k	4-CF3C6H4	15	3k	76	23	2w	CH2C(O)Ph	20	3w	87
12	2l	3-CF3C6H4	30	3l	84	24	2x	CH2C(O)Me	20	3x	81

¹ Reagents and conditions: 1 (0.35 mmol, 1 equiv.), 2a–x (0.42 mmol, 1.2 equiv.), MeSO₃H (0.18 mL, 8 equiv.), 60 °C. ² Isolated yields after silica gel column chromatography. ³ According to GC–MS analysis of the crude residue. ⁴ A complex mixture of unidentified products. ⁵ Ad = Adamant-1-yl.

). Alcohol 1 smoothly reacted with benzonitrile (2b) and with ortho-, metha-, and para-substituted benzonitriles containing electron-donating (OMe, 2c–e and NH₂, 2f) or electron-withdrawing (Br, 2h–j or CF₃, 2k–m) substituents within 15–50 min to provide novel ferroceno[c]pyridines 3b–f, h–m in good to excellent yields (Table 2, entries 2–6, 8–13). In contrast to the other ortho-substituted benzonitriles (2e, j, m), 2-aminobenzonitrile (2g) afforded ferroceno[c]pyridine 3g in a poor yield (Table 2, entry 7). The reaction of 4-nitrobenzonitrile (2n) with alcohol 1 led to a complex mixture of unidentified products with only trace amounts of compound 3n, probably because of a low nucleophilicity of 2n (Table 2, entry 14).

Condensation of alcohol 1 with aliphatic nitriles 2o–s, including sterically hindered 1-adamantanecarbonitrile (2s), gave ferroceno[c]pyridines 3o–s within 15–25 min in high yields. According to ¹H NMR analysis of the crude residue, 3-methoxypropionitrile (2t) gave a complex mixture of products, none of which could be identified and isolated (Table 2, entry 20).

Alcohol 1 reacted smoothly with β-oxonitriles 2u–x to afford products 3u–x in 34–87% yields (Table 2, entries 21–24). The NMR spectra clearly indicated that ferroceno[c]pyridines 3u–x existed exclusively in the Z-enamine form, which was stabilized by an intramolecular hydrogen bond. That is, the ¹H NMR spectra of these compounds in CDCl₃ contained resonance signals of CH vinyl protons as singlets in the range of 4.74–5.98 ppm, together with resonance signals of NH protons as broad singlets in the range of 8.23–10.99 ppm. Cross peaks between the CH vinyl proton and H7′ proton or protons of unsubstituted Cp ring in the 2D ¹H–¹H NOESY spectra confirmed that compounds 3u–x existed as Z-isomers with respect to the >C=CH– bond (Figure 2).

Phenylacetonitriles 7a–c, like nitriles 2a–m, o–s, u–x, readily reacted with alcohol 1 to afford the corresponding ferroceno[c]pyridines 8a–c. However, products 8a–c could not be isolated in a pure form, since they were unstable. These compounds easily oxidized in air at a benzylic position, resulting in ferroceno[c]pyridines 9a–c (

Table 3. Reaction of alcohol 1 with phenylacetonitriles 7a–c ¹.

Entry	Nitrile 7	Ar	Time (min)	Time 1 (days)	Product 9	Yield (%) 2
						Method A	Method B
1	7a	3,4-(MeO)2C6H3	10	1	9a	24	21
2	7b	Ph	15	2	9b	35	24
3	7c	4-NO2C6H4	15	6	9c	16	14

¹ Reagents and conditions: 1 (0.35 mmol, 1 equiv.), 7a–c (0.42 mmol, 1.2 equiv.), MeSO₃H (0.18 mL, 8 equiv.), 60 °C; Method A: Storage of the mixtures of 8 and 9 obtained after purification of the crude residues by column chromatography at rt exposed to air. The ratio of 8/9, according to ¹H NMR analysis of the fractions of chromatography performed immediately after their isolation: 8a/9a = 78:22; 8b/9b = 66:34; 8c/9c = 65:35. Method B: Storage of the solutions of the crude residues in EtOAc at rt exposed to air. ² Isolated yields after silica gel column chromatography.

).

Purification of the crude residues by silica gel column chromatography, performed immediately after the work-up of the reaction mixtures, afforded products 8a–c as the mixtures with oxidized compounds 9a–c in ratios of 65:35 to 78:22. Storage of these mixtures at room temperature exposed to air resulted in full conversion of 8a–c to 9a–c within 1–6 days and isolation of ferroceno[c]pyridines 9a–c in 16–35% yields (Table 3, Method A). Similar results in terms of the oxidation time and yields of 9a–c were obtained after storing the solutions of crude residues in EtOAc at room temperature exposed to air (Table 3, Method B). The rate of the conversion of compounds 8a–c to 9a–c strongly depends on the nature of substituents in the aromatic ring of the benzylic fragments. Thus, the oxidation of ferroceno[c]pyridine 8a, containing MeO groups, proceeded much faster than that of 8c with NO₂ groups in the aromatic ring of the benzylic fragment (Table 3, entries 1 and 3).

Further, we investigated the reaction of alcohol 1 with thiocyanates 10a–j. It turned out that the condensation of alcohol 1 with EtSCN (10a) under optimal conditions gave a mixture of products in a 23:77 ratio, one of which was shown to be ferroceno[c]pyridine 11a and the other one ferroceno[c]pyrrole 12a (

Table 4. Reaction of alcohol 1 with EtSCN (10a) ¹.

Entry	Temp. (°C)	Time (min)	11a:12a 2	Yield (%) 3
				11a	12a
1	60	10	23:77	18	65
2	rt	15	5:95	4	87
3 4	60	10	43:57	33	53
4 4	80	3	74:26	57	5
5 4	100	1.5	88:12	53	5
6 4,6	120	1	91:9	23	7

¹ Reagents and conditions: 1 (0.35 mmol, 1 equiv.), 10a (0.42 mmol, 1.2 equiv.), MeSO₃H (0.18 mL, 8 equiv.). ² According to ¹H NMR analysis of the crude residues. ³ Isolated yields after silica gel column chromatography. ⁴ Addition of 10a to the solution of 1 in MeSO₃H preheated to the indicated temperature. ⁵ Not isolated in an analytically pure form, only as a mixture with ferroceno[c]pyridine 11a. ⁶ Considerable tar formation was observed. ⁷ Not isolated.

, entry 1). After silica gel column chromatography, compounds 11a and 12a were isolated in 18% and 65% yields, respectively. The structures of products 11a and 12a were unambiguously confirmed by X-ray crystallography (Figure 3 and Figure 4). Only a single diastereomer of ferroceno[c]pyrrole 12a was formed, in which the tert-butyl group occupied exo-position with respect to the iron atom, according to X-ray data. Next, we found that the selectivity of the reaction strongly depended on the temperature and the order of addition of the thiocyanate 10a. The reaction at room temperature led to an excellent selectivity for the formation of ferroceno[c]pyrrole 12a (11a/12a = 5:95), which was isolated in 87% yield (Table 4, entry 2). At the same time, addition of thiocyanate 10a to the solution of alcohol 1 preheated to 60 °C gave a 43:57 mixture of products 11a and 12a, which were isolated in 33% and 53% yields, respectively (Table 4, entry 3). The reaction under similar conditions at 80 °C resulted in a predominant formation of ferroceno[c]pyridine 11a (11a/12a = 74:26), isolated in 57% yield (Table 4, entry 4). Increasing the reaction temperature to 100 or 120 °C provided an even greater selectivity toward ferroceno[c]pyridine 11a, resulting in products 11a and 12a in the ratios of 88:12 and 91:9, respectively (Table 4, entries 5, 6). However, lower yields of compound 11a were obtained compared to the reaction at 80 °C, likely due to its partial decomposition.

Further, we investigated the reaction of alcohol 1 with thiocyanates 10b–f both at room temperature and at 80 °C (

Table 5. Reaction of alcohol 1 with thiocyanates 10a–f ¹.

Entry	Thiocyanate 10	R	Temp. (°C)	Time (min)	Products 11, 12	11:12 2	Yield (%) 3
							11	12
1	10a	Et	rt	25	11a, 12a	5:95	4	87
2			80	3		74:26	57	5
3	10b	Me	rt	25	11b, 12b	2:98	4	64
4			80	3		52:48	35	5
5	10c	Pr	rt	25	11c, 12c	6:94	4	75
6			80	3		74:26	53	5
7	10d	i-Pr	rt	40	11d, 12d	16:84	7	53
8			80	3		92:8	61	5
9	10e	CH2Ph	rt	25	11e, 12e	5:95	4	37
10			80	3		67:23	42	5
11	10f	n-Hex	rt	40	11f, 12f	4:96	4	65
12			80	3		79:21	55	5

¹ Reagents and conditions: 1 (0.35 mmol, 1 equiv.), 10a–f (0.42 mmol, 1.2 equiv.), MeSO₃H (0.18 mL, 8 equiv.). ² According to ¹H NMR analysis of the crude residues. ³ Isolated yields after silica gel column chromatography. ⁴ Not isolated in an analytically pure form, only as a mixture with corresponding ferroceno[c]pyrrole 12. ⁵ Not isolated in an analytically pure form, only as a mixture with corresponding ferroceno[c]pyridine 11.

). Thiocyanates 10b–f, similar to EtSCN (10a), afforded ferroceno[c]pyrroles 12b–f as the main products at room temperature, with 11/12 ratios of 16:84 to 2:98, in moderate to good yields, as single diastereomers (Table 5, entries 1, 3, 5, 7, 9 and 11). At 80 °C thiocyanates 10c–f reacted with alcohol 1 with a predominant formation of ferroceno[c]pyridines 11c–f with ratios of 11c–f to 12c–f in the range of 67:23 to 92:8 (Table 5, entries 6, 8, 10 and 12). In these cases, products 11c–f were isolated in 42–61% yields. When MeSCN (10b) was used, the reaction proceeded with no selectivity to give a 52:48 mixture 11b and 12b, and ferroceno[c]pyridine 11b was obtained only in 35% yield (Table 5, entry 4).

The selectivity of the reaction is influenced by substituents at the α-position to the sulfur atom of thiocyanates 10a–f. An increase in the substituent size results in a higher selectivity toward ferroceno[c]pyridines 11, versus ferroceno[c]pyrroles 12. For example, in the case of reactions at 80 °C, the most sterically hindered isopropyl thiocyanate (10d) afforded ferroceno[c]pyridine 11d and ferroceno[c]pyrrole 12d in a ratio of 92:8, whereas the reaction with a less sterically constrained MeSCN (12b) gave products 11b and 12b in a ratio of 52:48 (Table 5, entries 8 and 4). When the reaction of alcohol 1 with isopropyl thiocyanate (10d) was carried out at room temperature, 11d/12d ratio was 16:84, while when using MeSCN (12b) under the same conditions, a 2:98 11b/12b ratio was observed (Table 5, entries 7 and 3).

Ethyl 2-thiocyanatoacetate (10g) reacted with alcohol 1 the same way as thiocyanates 10a–f (

Table 6. Reaction of alcohol 1 with thiocyanates 10g–j ¹.

Entry	Thiocyanate 10	R	Temp. (°C)	Time	Products (Yield, %) 2
1	10g	EtO	rt	1 h	3u (trace) 3, 11g (6) 3, 12g (73); [11g/12g = 4:96] 4
2			80	15 min	3u (trace) 3, 11g (27) 3, 12g (26); [11g/12g = 59:41] 4
3	10h	NH2	rt	48 h	1 (37), 5 + 6 (14) [5/6 = 77:23] 5, 13 (13)
4			80	3 min	- 6
5	10i	Ph	rt	48 h	1 (20), 5 + 6 (18) [5/6 = 55:45] 5, 10i (38), 13 (51)
6			80	40 min	3w (8%), 5 + 6 (42) [5/6 = 91:9] 5, 10i (63)
7	10j	Me	rt	48 h	5 + 6 (22) [5/6 = 56:44] 5, 13 (48)
8			80	3 min	3x (trace) 4, 1 (38), 5 + 6 (20) [5/6 = 96:4] 5

¹ Reagents and conditions: 1 (0.35 mmol, 1 equiv.), 10g (0.53 mmol, 1.5 equiv.), 10h–j (0.42 mmol, 1.2 equiv.), MeSO₃H (0.18 mL, 8 equiv.). ² Isolated yields after silica gel column chromatography. ³ Isolated as a mixture of 11g and 3u. According to ¹H NMR analysis of the fraction of chromatography 11g/3u ratios were found to be 99:1 (entry 1) and 96:4 (entry 2). ⁴ According to ¹H NMR analysis of the crude residues. ⁵ According to ¹H NMR analysis of the fraction of chromatography. ⁶ A complex mixture of unidentified products.

, entries 1, 2). The use of thiocyanate 10g at room temperature led to the formation of a 4:96 mixture of compounds 11g and 12g, which were isolated in 6% and 73% yields, respectively (Table 6, entry 1), whilst at 80 °C products 11g and 12g were formed in a ratio of 59:41, and purification by silica gel column chromatography gave 27% of ferroceno[c]pyridine 11g and 26% of ferroceno[c]pyrrole 12g (Table 6, entry 2). In both cases the ¹H NMR spectra of ferroceno[c]pyridine 11g recorded immediately after its isolation indicated that this compound contained trace amounts of ferroceno[c]pyridine 3u with 11g/3u ratios of 99:1 and 96:4, respectively. ¹H NMR analyses showed that ferroceno[c]pyridine 11g was very slowly converted to compound 3u when stored at room temperature, both neat and in solution. Compound 3u has not been isolated in a pure form in these cases. The results of the ¹H NMR monitoring of the transformation of thioimine 11g to enaminone 3u are performed in Supplementary Materials (Figure S1, Table S1). The formation of ferroceno[c]pyridine 3u likely occurred due to the extrusion of sulfur from thioimine 11g. This reaction is a typical for thioimines, containing CH₂C(O)R fragment at the sulfur atom, and is the basis of such a synthetic method as Eschenmoser sulfide contraction [53,54]. The proposed mechanism for the transformation of thioimine 11g to enaminone 3u is performed in Supplementary Materials (Scheme S1).

In contrast to ethyl 2-thiocyanatoacetate (10g), the reactions of alcohol 1 with β-oxothiocyanates 10h–j at room temperature did not proceed (Table 6, entries 3, 5, and 7). In these cases the starting alcohol 1 together with alcohol 13 and inseparable mixtures of alkenes 5 and 6 were isolated. Among thiocyanates 10h–j only 10i was recovered in 38% yield. According to the ¹H NMR analysis of the crude residue, the condensation of 1 with 2-thiocyanatoacetamide (10h) at 80 °C resulted in the formation of a complex mixture of unidentified products, which were not isolated (Table 6, entry 4). Reaction with phenacyl thiocyanate (10i) at 80 °C within 40 min afforded 8% of ferroceno[c]pyridine 3w (Table 6, entry 6), along with a mixture of alkenes 5 and 6 in a 42% overall yield, and 63% of recovered thiocyanate 10i. It should be noted that the yield of the product 3w remained at the same level upon an increase in the reaction time up to 1.5 h. According to the ¹H NMR spectrum of the crude residue, 2-oxopropyl thiocyanate (10j) at 80 °C gave trace amounts of ferroceno[c]pyridine 3x, which was not isolated. Only an inseparable mixture of alkenes 5 and 6 in 20% yield together with 38% of starting alcohol 1 were isolated (Table 6, entry 8). Similar to the formation of 3u from 11g, products 3w and 3x, are assumed to be formed via sulfur extrusion from thioimines 11i and 11j, respectively. However, we could neither isolate nor detect ferroceno[c]pyridines 11i and 11j by ¹H NMR and GS-MS analysis of the crude residues.

The proposed mechanism for the formation of ferroceno[c]pyridines 3, 8, 11 and ferroceno[c]pyrroles 12 is shown in Scheme 1. The reaction includes initial acid-promoted ionization of starting alcohol 1 with the formation of tertiary α-ferrocenyl carbocation A, which is in an equilibrium with isomeric tertiary β-ferrocenyl carbocation B. The latter reacts with nitriles 2, 7, or 10 to give nitrilium ion C (path a), the intramolecular cyclization of which then provides ferroceno[c]pyridines 3, 8, and 11. Thiocyanates 10, being more nucleophilic than nitriles 2 and 7 because of the influence of an electron-donating sulfur atom, may react not only with carbocation B but also with the less electrophilic α-ferrocenyl carbocation A, to form nitrilium ion D (path b). Subsequent intramolecular cyclization of the intermediate D gives ferroceno[c]pyrroles 12.

The lower electrophilicity of α-ferrocenyl carbocation A compared to β-ferrocenyl carbocation B is due to the extensive delocalization of the positive charge with the participation of the ferrocenyl moiety. At room temperature an equilibrium between carbocations A and B is shifted towards the thermodynamically more stable α-ferrocenyl carbocation A, which explains the predominant formation of ferroceno[c]pyrroles 12 at these conditions. An increase in the reaction temperature shifts the A↔B equilibrium towards the carbocation B, which causes an increase in the selectivity of the formation of ferroceno[c]pyridines 11.

3. Material and Methods

3.1. General Information

Thin-layer chromatography was performed on commercially available Sorbfil silica gel plates, which were visualized under UV light (254 nm). Column chromatography was performed on silica gel 60 (0.063–0.200 mm, Macherey-Nagel). ¹H and ¹³C NMR spectra were recorded on a Bruker Avance NEO 400 spectrometer using CDCl₃, DMSO-d₆, or C₆D₆ as solvents. Chemical shifts are quoted on the δ scale, parts per million (ppm). The ¹H chemical shifts were measured relative to the internal standard HMDSO (δ_H 0.055 ppm) for CDCl₃ and DMSO-d₆, or residual C₆H₆ (δ_H 7.16 ppm) for C₆D₆. The ¹³C chemical shifts were measured relative to the solvent signal (δ_C 77.16 ppm for CDCl₃, δ_C 39.50 ppm for DMSO-d₆, δ_C 128.06 ppm for C₆D₆). The ¹⁹F chemical shifts were measured relative to the internal standard C₆F₆. The assignment of primary (CH₃), secondary (CH₂), tertiary (CH), and quaternary (C) carbon nuclei was made by using DEPT-135 spectra. The signals in the ¹H and ¹³C NMR spectra of compounds 3a, 3u–x, 11a, and 12a were assigned based on 2D ¹H–¹³C HSQC, ¹H–¹³C HMBC and ¹H–¹H NOESY experiments. Copies of NMR spectra for all new compounds are deposited in the Supplementary Materials. Low-resolution mass spectra were obtained with an Agilent 6890N/5975B GC–MS system (column: HP-5ms, 15 or 30 m × 0.25 mm, 0.25 µm; helium as a carrier gas, 1 mL/min, electron impact ionization mode (230 °C, 70 eV)) and Agilent 7890B/5977B GC–MS system (column: HP-5ms UI, 30 m × 0.25 mm, 0.25 µm; helium as a carrier gas, 1 mL/min, electron impact ionization mode (230 °C, 70 eV)). High-resolution mass spectra were recorded with a Bruker maXis HD UHR-QTOF mass spectrometer equipped with an electrospray ionization ion source. Infrared spectra were recorded on a Bruker IFS 66 FT-IR spectrometer. Elemental analysis was carried out on a Vario EL Cube analyzer. Melting points were determined using a PTP apparatus and are uncorrected.

2-Ferrocenyl-3,3-dimethylbutan-2-ol (1), ethyl 2-thiocyanatoacetate (10g) and 2-oxopropyl thiocyanate (10j) were synthesized as described below. 3-Oxo-3-phenylpropanenitrile (2w) [55], 3-oxobutyronitrile 2x [56], propyl thiocyanate (10c) [57], hexyl thiocyanate (10f) [57], 2-thiocyanatoacetamide (10h) [58] and phenacyl thiocyanate (10i) [59] were prepared as previously described. All other chemicals were purchased from commercial suppliers and used without further purification.

3.2. Experimental Procedures and Characterization Data of the New Compounds

3.2.1. Synthesis of 2-Ferrocenyl-3,3-dimethylbutan-2-ol (1)

To a stirred solution of 1-ferrocenyl-2,2-dimethylpropan-1-one [60] (4.05 g, 10 mmol) in Et₂O (100 mL) was added dropwise a solution of MeMgI prepared from Mg (2.30 g, 96 mmol) and MeI (7.47 mL, 120 mmol) in Et₂O (70 mL). The reaction mixture was refluxed for 5 h and left overnight at room temperature. The reaction mixture was then cooled in an ice bath, and saturated aqueous NH₄Cl solution (150 mL) was slowly added dropwise with vigorous stirring. The phases were separated, and the aqueous phase was extracted with Et₂O (40 mL × 3). The combined organic phases were washed with water, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The crude residue was purified by silica gel column chromatography (petroleum ether/EtOAc 30:1) to afford pure alcohol 1 (3.50 g, 82%) as a yellow solid; mp 93–93.5 °C (mp 93.5–94 °C (hexane) [61]); R_f 0.43 (petroleum ether/EtOAc 30:1). IR (thin film): 3560, 3102, 3087, 2982, 2958, 2910, 2872, 1479, 1460, 1410, 1390, 1360, 1313, 1217, 1109, 1078, 1036, 1027, 1006, 897, 843, 822, 810, 543, 492, 422 cm⁻¹. NMR data are in agreement with that previously published [62]. MS (EI) m/z (% relative intensity): 286 [M]⁺ (14), 269 [M–OH]⁺ (11), 268 [M–H₂O]⁺ (56), 229 [M–t-C₄H₉]⁺ (32), 186 [C₅H₅FeC₅H₅]⁺ (100), 121 [C₅H₅Fe]⁺ (28), 56 [Fe]⁺ (11). Analysis calculated for C₁₆H₂₂O: C, 67.15; H, 7.75. Found: C, 67.45; H, 7.91.

3.2.2. Synthesis of Ethyl 2-Thiocyanatoacetate (10g) and 2-Oxopropyl Thiocyanate (10j)

The title compounds were synthesized according to the literature procedure [59] with slight modifications.

Compound 10g. Well-ground NH₄SCN (2.09 g, 27.5 mmol) was added portionwise to a stirred solution of ethyl 2-bromoacetate (2.8 mL, 25 mmol) in i-PrOH (3 mL), and the resulting mixture was heated under reflux with stirring for 5 h. The reaction mixture was then cooled to room temperature, diluted with water (10 mL) until the complete dissolution of the precipitate, and extracted with EtOAc (10 mL × 3). The combined organic phases were washed with water, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The residue was dissolved in EtOH (20 mL), and charcoal (2 g) was added. The resulting mixture was stirred at room temperature for 30 min, and the charcoal was removed by filtration and washed with EtOH (5 mL). The combined filtrates were concentrated in vacuo to afford pure product 10g (3.25 g, 90%) as a colorless liquid; R_f 0.30 (hexane/EtOAc 5:1). IR (thin film): 2987, 2943, 2161, 1739, 1369, 1306, 1272, 1188, 1024 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 4.28 (q, 2H, J = 7.1 Hz), 3.76 (s, 2H), 1.32 (t, 3H, J = 7.2 Hz). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 166.3 (C), 110.7 (C), 63.0 (CH₂), 35.1 (CH₂), 14.1 (CH₃). MS (EI) m/z (% relative intensity): 145 [M]⁺ (11), 100 [M–OC₂H₅]⁺ (12), 72 [M–CH₂SCN]⁺ (13), 29 [M–C₂H₅]⁺ (14). Analysis calculated for C₅H₇NO₂S: C, 41.37; H, 4.86; N, 9.65; S, 22.08. Found: C, 41.62; H, 5.03; N, 9.62, S, 22.22.

Compound10j. Well-ground NH₄SCN (2.09 g, 27.5 mmol) was added portionwise to a stirred solution of chloroacetone (2 mL, 25 mmol) in i-PrOH (3 mL), and the resulting mixture was stirred at room temperature for 20 h. The reaction mixture was then worked up as described for the preparation of compound 10g to afford pure product 10j (2.03 g, 71%) as a light yellow liquid; R_f 0.20 (hexane/EtOAc 5:1). IR (thin film): 2973, 2847, 2159, 2110, 2057, 1718, 1360, 190, 1157, 575, 563 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 4.02 (s, 2H), 2.34 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 198.7 (C), 111.3 (C), 44.4 (CH₂), 28.5 (CH₃). MS (EI) m/z (% relative intensity): 115 [M]⁺ (11), 72 [M–CH₃CO]⁺ (7), 43 [M–CH₂SCN]⁺ (57). Analysis calculated for C₄H₅NOS: C, 41.72; H, 4.38; N, 12.16; S, 27.84. Found: C, 41.86; H, 4.27; N, 12.23, S, 28.06.

3.2.3. Synthesis of Ferroceno[c]pyridines 3a–m, o–s, u–x (Table 2)

General Procedure (GP). Nitrile 2a–m, o–s, u–x (0.42 mmol) was added to a stirred solution of alcohol 1 (100 mg, 0.35 mmol) in MeSO₃H (0.18 mL) at room temperature, and the resulting mixture was heated at 60 °C in an oil bath with vigorous stirring for the indicated time (monitored by TLC). The reaction mixture was then cooled to room temperature, neutralized with 10% aq. Na₂CO₃ solution (3 mL) and extracted with EtOAc (10 mL × 4). The combined organic phases were washed with water, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The crude residue was purified by silica gel column chromatography.

rac-3,3,4,4-Tetramethyl-1-(4-methylphenyl)-3,4-dihydroferroceno[c]pyridine (3a). The title compound was prepared according to GP using 4-methylbenzonitrile (2a) (0.05 mL, 0.42 mmol); reaction time = 15 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3a (98 mg, 73%) as an orange solid; mp 123.5–125 °C (hexane); R_f 0.08 (petroleum ether/EtOAc 25:1); R_f 0.30 (petroleum ether/TEA 100:1). IR (thin film): 3094, 2973, 2928, 2868, 1588, 1560, 1453, 1372, 1361, 1314, 1301, 1175, 1156, 1108, 1141, 1108, 1002, 933, 899, 822, 755, 745, 495, 467 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 7.96 (br d, 2H, J = 8.2 Hz, H2′, H6′), 7.31 (br d, 2H, J = 7.9 Hz, H3′, H5′), 4.45 (d, 2H, J = 1.9 Hz, H5, H6), 4.37 (t, 1H, J = 1.8 Hz, H7), 4.27 (s, 5H, H Cp), 2.41 (s, 3H, 4′-CH₃), 1.45 (s, 3H, 4-CH₃), 1.44 (s, 3H, 3-CH₃), 1.11 (s, 3H, 4-CH₃), 0.74 (s, 3H, 3-CH₃). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 162.3 (C1), 138.5 (C4′), 135.7 (C1′), 128.3 (C3′, C5′), 127.1 (C2′, C6′), 98.3 (C4a), 71.4 (C7a), 69.4 (5 CH Cp), 66.9 (C5 or C6), 65.7 (C7), 65.4 (C5 or C6), 61.7 (C3), 33.9 (C4), 28.0 (4-CH₃), 24.4 (4-CH₃), 23.7 (3-CH₃), 23.1 (3-CH₃), 20.6 (4′-CH₃). MS (EI) m/z (% relative intensity): 385 [M]⁺ (35), 370 [M–CH₃]⁺ (100), 342 [M–i-C₃H₇]⁺ (5), 329 [M–C₄H₈]⁺ (18), 314 [M–C₄H₈–CH₃]⁺ (11), 121 [C₅H₅Fe]⁺ (15), 56 [Fe]⁺ (5). Analysis calculated for C₂₄H₂₇FeN: C, 74.81; H, 7.06; N, 3.64. Found: C, 75.01; H, 7.19; N, 3.56.

rac-3,3,4,4-Tetramethyl-1-phenyl-3,4-dihydroferroceno[c]pyridine (3b). The title compound was prepared according to GP using benzonitrile (2b) (0.043 mL, 0.42 mmol); reaction time = 30 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3b (97 mg, 75%) as a red solid; mp 103–105 °C (hexane); R_f 0.10 (petroleum ether/EtOAc 25:1); R_f 0.35 (petroleum ether/TEA 100:1). IR (thin film): 3093, 2973, 2930, 2868, 1592, 1565, 1452, 1372, 1361, 1317, 1302, 1174, 1156, 1108, 1002, 935, 897, 823, 780, 754, 725, 697, 473, 453 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 30 °C): δ 8.06–8.03 (m, 2H), 7.55–7.49 (m, 3H), 4.49 (d, 2H, J = 1.8 Hz), 4.40 (t, 1H, J = 1.8 Hz), 4.30 (s, 5H), 1.46 (s, 3H), 1.45 (s, 3H), 1.11 (s, 3H), 0.78 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 162.7 (C), 138.4 (C), 129.2 (CH), 127.9 (2 CH), 127.3 (2 CH), 98.3 (C), 71.3 (C), 69.6 (5 CH), 67.1 (CH), 65.8 (CH), 65.6 (CH), 61.9 (C), 33.9 (C), 28.1 (CH₃), 24.7 (CH₃), 23.8 (CH₃), 23.2 (CH₃). MS (EI) m/z (% relative intensity): 371 [M]⁺ (47), 356 [M–CH₃]⁺ (100), 328 [M–i-C₃H₇]⁺ (6), 315 [M–C₄H₈]⁺ (22), 300 [M–C₄H₈–CH₃]⁺ (13), 121 [C₅H₅Fe]⁺ (9), 56 [Fe]⁺ (3). Analysis calculated for C₂₃H₂₅FeN: C, 74.40; H, 6.79; N, 3.77. Found: C, 74.70; H, 7.06; N, 3.77.

rac-1-(4-Methoxyphenyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3c). The title compound was prepared according to GP using 4-methoxybenzonitrile (2c) (0.056 mg, 0.42 mmol); reaction time = 40 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3c (108 mg, 77%) as an orange solid; mp 140.5–142 °C (hexane); R_f 0.02 (petroleum ether/EtOAc 25:1); R_f 0.15 (petroleum ether/TEA 100:1). IR (thin film): 3091, 2973, 2933, 2868, 2836, 1608, 1589, 1562, 1513, 1453, 1441, 1372, 1361, 1309, 1251, 1168, 1155, 1108, 1033, 1002, 933, 899, 836, 825, 751, 660, 544, 518, 478, 461 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 60 °C): δ 8.07–8.03 (m, 2H), 7.07–7.03 (m, 2H), 4.45 (d, 2H, J = 1.9 Hz), 4.38 (t, 1H, J = 1.8 Hz), 4.27 (s, 5H), 3.86 (s, 3H), 1.45 (s, 3H), 1.43 (s, 3H), 1.12 (s, 3H), 0.71 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 161.9 (C), 160.3 (C), 130.8 (C), 128.8 (2 CH), 113.3 (2 CH), 98.3 (C), 71.4 (C), 69.5 (5 CH), 67.0 (CH), 66.0 (CH), 65.7 (CH), 61.7 (C), 55.1 (CH₃), 33.9 (C), 28.5 (CH₃), 24.2 (CH₃), 24.1 (CH₃), 23.0 (CH₃). MS (EI) m/z (% relative intensity): 401 [M]⁺ (38), 386 [M–CH₃]⁺ (100), 371 [M–CH₂O]⁺ (7), 358 [M–i-C₃H₇ and/or M–CH₃–CO]⁺ (6), 345 [M–C₄H₈]⁺ (23), 330 [M–C₄H₈–CH₃]⁺ (15), 165 (11), 134 (11), 121 [C₅H₅Fe]⁺ (28), 56 [Fe]⁺ (8). Analysis calculated for C₂₄H₂₇FeNO: C, 71.83; H, 6.78; N, 3.49. Found: C, 71.64; H, 7.13; N, 3.44.

rac-1-(3-Methoxyphenyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3d). The title compound was prepared according to GP using 3-methoxybenzonitrile (2d) (0.051 mL, 0.42 mmol); reaction time = 40 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3d (110 mg, 78%) as an orange solid; mp 100–102 °C (hexane); R_f 0.03 (petroleum ether/EtOAc 25:1); R_f 0.20 (petroleum ether/TEA 100:1). IR (thin film): 3094, 2973, 2937, 2869, 2834, 1599, 1568, 1487, 1462, 1435, 1372, 1361, 1305, 1285, 1274, 1238, 1168, 1152, 1108, 1049, 1039, 1002, 948, 882, 824, 802, 791, 755, 742, 697, 663, 534, 516, 473, 449 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 70 °C): δ 7.61 (dt, 1H, J = 7.6, 1.3 Hz), 7.56 (dd, 1H, J = 2.7, 1.5 Hz), 7.41 (t, 1H, J = 7.9 Hz), 7.06 (ddd, 1H, J = 8.2, 2.7, 1.0 Hz), 4.47 (d, 2H, J = 1.8 Hz), 4.39 (t, 1H, J = 1.8 Hz), 4.27 (s, 5H), 3.88 (s, 3H), 1.47 (s, 3H), 1.46 (s, 3H), 1.11 (s, 3H), 0.78 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 162.5 (C), 158.9 (C), 139.8 (C), 129.0 (CH), 119.9 (CH), 114.9 (CH), 112.7 (CH), 98.3 (C), 71.3 (C), 69.6 (5 CH), 67.1 (CH), 65.8 (CH), 65.6 (CH), 62.0 (C), 55.1 (CH₃), 33.9 (C), 28.1 (CH₃), 24.6 (CH₃), 23.8 (CH₃), 23.2 (CH₃). MS (EI) m/z (% relative intensity): 401 [M]⁺ (51), 386 [M–CH₃]⁺ (100), 371 [M–CH₂O]⁺ (6), 358 [M–i-C₃H₇ and/or M–CH₃–CO]⁺ (6), 345 [M–C₄H₈]⁺ (22), 330 [M–C₄H₈–CH₃]⁺ (12), 121 [C₅H₅Fe]⁺ (10), 56 [Fe]⁺ (3). Analysis calculated for C₂₄H₂₇FeNO: C, 71.83; H, 6.78; N, 3.49. Found: C, 72.05; H, H 7.20; N, 3.49.

rac-1-(2-Methoxyphenyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3e). The title compound was prepared according to GP using 2-methoxybenzonitrile (2e) (0.051 mL, 0.42 mmol); reaction time = 40 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3e (88 mg, 63%) as a brown solid; mp 100–101 °C (hexane); R_f 0.05 (petroleum ether/EtOAc 25:1); R_f 0.25 (petroleum ether/TEA 100:1). IR (thin film): 3096, 2972, 2933, 2869, 2834, 1597, 1494, 1463, 1435, 1372, 1361, 1312, 1267, 1243, 1153, 1117, 1108, 1049, 1027, 1003, 939, 922, 900, 821, 751, 659, 512, 476, 455 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 7.42–7.38 (m, 1H), 7.28 (dd, 1H, J = 7.4, 1.7 Hz), 7.11 (br d, 1H, J = 8.3 Hz), 7.05 (td, 1H, J = 7.4, 1.0 Hz), 4.32 (dd, 1H, J = 2.2, 1.2 Hz), 4.29 (t, 1H, J = 2.4 Hz), 4.12 (s, 5H), 4.02 (dd, 1H, J = 2.4, 1.2 Hz), 3.79 (s, 3H), 1.50 (s, 3H), 1.47 (s, 3H), 1.01 (s, 3H), 1.00 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 162.9 (C), 156.3 (C), 129.6 (C), 129.0 (CH), 128.9 (CH), 119.9 (CH), 111.6 (CH), 97.7 (C), 73.3 (C), 69.2 (5 CH), 66.6 (CH), 64.3 (CH), 64.0 (CH), 61.4 (C), 55.5 (CH₃), 34.1 (C), 26.2 (CH₃), 26.2 (CH₃), 24.4 (CH₃), 22.7 (CH₃). MS (EI) m/z (% relative intensity): 401 [M]⁺ (100), 386 [M–CH₃]⁺ (61), 371 [M–CH₂O]⁺ (11), 358 [M–i-C₃H₇ and/or M–CH₃–CO]⁺ (15), 345 [M–C₄H₈]⁺ (50), 330 [M–C₄H₈–CH₃]⁺ (11), 263 (10), 121 [C₅H₅Fe]⁺ (9), 56 [Fe]⁺ (3). Analysis calculated for C₂₄H₂₇FeNO∙0.15C₆H₁₄: C, 72.20; H, 7.08; N, 3.38. Found: C, 72.43; H, 7.10; N, 3.05.

rac-4-(3,3,4,4-Tetramethyl-3,4-dihydroferroceno[c]pyridin-1-yl)aniline (3f). The title compound was prepared according to GP using 4-aminobenzonitrile (2f) (49 mg, 0.42 mmol); reaction time = 50 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 5:1–petroleum ether/TEA 100:1) gave pure compound 3f (89 mg, 66%) as an orange solid; mp 167.5–169 °C (hexane); R_f 0.02 (petroleum ether/EtOAc 5:1); R_f 0.30 (petroleum ether/TEA 100:1). IR (thin film): 3464, 3433, 3378, 3321, 3204, 3094, 2973, 2931, 2868, 1621, 1609, 1584, 1556, 1518, 1454, 1372, 1361, 1315, 1296, 1171, 1155, 1107, 1002, 834, 754, 660, 504, 470 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 7.88–7.85 (m, 2H), 6.68–6.65 (m, 2H), 5.31 (br s, 2H), 4.44–4.42 (m, 2H), 4.37 (t, 1H, J = 1.8 Hz), 4.26 (s, 5H), 1.44 (s, 3H), 1.39 (s, 3H), 1.12 (s, 3H), 0.65 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 161.8 (C), 150.1 (C), 128.6 (2 CH), 126.0 (C), 112.8 (2 CH), 98.4 (C), 71.9 (C), 69.4 (5 CH), 66.6 (CH), 66.3 (CH), 65.4 (CH), 61.4 (C), 33.9 (C), 28.7 (CH₃), 24.4 (CH₃), 24.0 (CH₃), 23.0 (CH₃). MS (EI) m/z (% relative intensity): 386 [M]⁺ (44), 371 [M–CH₃]⁺ (100), 330 [M–C₄H₈]⁺ (19), 315 [M–C₄H₈–CH₃]⁺ (12), 121 [C₅H₅Fe]⁺ (10), 56 [Fe]⁺ (4). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₃H₂₇FeN₂, 387.1518; found, 387.1523.

rac-2-(3,3,4,4-Tetramethyl-3,4-dihydroferroceno[c]pyridin-1-yl)aniline (3g). The title compound was prepared according to GP using 2-aminobenzonitrile (2g) (49 mg, 0.42 mmol); reaction time = 180 min. Purification by silica gel column chromatography (petroleum ether/acetone 15:1) gave pure compound 3g (40 mg, 30%) as an orange oil; R_f 0.20 (petroleum ether/acetone 15:1). IR (thin film): 3452, 3202, 3093, 2973, 2929, 2868, 1613, 1580, 1540, 1450, 1372, 1361, 1308, 1264, 1159, 1108, 1002, 936, 818, 750, 659, 493, 472, 453 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 8.25 (dd, 1H, J = 7.9, 1.6 Hz), 7.14–7.10 (m, 1H), 6.87 (br s, 2H), 6.75 (dd, 1H, J = 8.2, 1.3 Hz), 6.70–6.65 (m, 1H), 4.47 (t, 1H, J = 2.4 Hz), 4.44 (dd, 1H, J = 2.4, 1.2 Hz), 4.33 (dd, 1H, J = 2.5, 1.2 Hz), 4.29 (s, 5H), 1.46 (s, 3H), 1.43 (s, 3H), 1.13 (s, 3H), 0.75 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 165.3 (C), 148.5 (C), 130.1 (CH), 129.6 (CH), 118.0 (C), 116.0 (CH), 114.0 (CH), 98.1 (C), 71.9 (C), 69.6 (5 CH), 67.3 (CH), 67.1 (CH), 65.5 (CH), 61.6 (C), 33.2 (C), 28.2 (CH₃), 24.2 (CH₃), 23.8 (CH₃), 23.75 (CH₃). MS (EI) m/z (% relative intensity): 386 [M]⁺ (100), 371 [M–CH₃]⁺ (96), 330 [M–C₄H₈]⁺ (27), 303 (10), 265 [M–C₅H₅Fe]⁺ (11), 121 [C₅H₅Fe]⁺ (16), 56 [Fe]⁺ (7). Analysis calculated for C₂₃H₂₆FeN₂: C, 71.51; H, 6.78; N, 7.25. Found: C, 71.56; H, 7.17; N, 7.04.

rac-1-(4-Bromphenyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3h). The title compound was prepared according to GP using 4-bromobenzonitrile (2h) (76 mg, 0.42 mmol); reaction time = 30 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3h (129 mg, 82%) as a brown solid; mp 85–86.5 °C (hexane); R_f 0.12 (petroleum ether/EtOAc 25:1); R_f 0.40 (petroleum ether/TEA 100:1). IR (thin film): 3094, 2974, 2931, 2869, 1588, 1556, 1488, 1452, 1391, 1372, 1361, 1310, 1155, 1108, 1070, 1011, 934, 898, 824, 754, 697, 518, 506, 475, 440 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 8.00–7.99 (m, 2H), 7.68–7.65 (m, 2H), 4.43 (d, 2H, J = 1.8 Hz), 4.34 (t, 1H, J = 1.9 Hz), 4.24 (s, 5H), 1.40 (s, 6H), 1.07 (s, 3H), 0.70 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 161.6 (C), 137.4 (C), 130.8 (2 CH), 129.2 (2 CH), 122.6 (C), 98.2 (C), 70.9 (C), 69.4 (5 CH), 67.1 (CH), 65.6 (CH), 65.5 (CH), 62.0 (C), 33.8 (C), 28.0 (CH₃), 24.3 (CH₃), 23.6 (CH₃), 22.9 (CH₃). MS (EI) m/z (% relative intensity): 449 [M]⁺ (64), 434 [M–CH₃]⁺ (100), 406 [M–i-C₃H₇]⁺ (4), 393 [M–C₄H₈]⁺ (23), 378 [M–C₄H₈–CH₃]⁺ (3), 369 [M–HBr]⁺ (3), 314 (43), 299 (57), 191 (13), 121 [C₅H₅Fe]⁺ (14), 56 [Fe]⁺ (5). Analysis calculated for C₂₃H₂₄BrFeN∙0.1C₆H₁₄: C, 61.78; H, 5.58; N, 3.05. Found: C, 61.61; H, 5.92; N, 3.05.

rac-1-(3-Bromphenyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3i). The title compound was prepared according to GP using 3-bromobenzonitrile (2i) (76 mg, 0.42 mmol); reaction time = 40 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3i (125 mg, 80%) as a brown solid; mp 131–132 °C (hexane); R_f 0.03 (petroleum ether/EtOAc 25:1); R_f 0.35 (petroleum ether/TEA 100:1). IR (thin film): 3094, 2974, 2930, 2869, 1591, 1555, 1451, 1372, 1361, 1302, 1186, 1155, 1108, 1070, 999, 937, 902, 870, 824, 791, 756, 738, 709, 692, 518, 474, 453 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 8.23 (t, 1H, J = 1.8 Hz), 7.92 (dt, 1H, J = 7.8, 1.2 Hz), 7.53 (ddd, 1H, J = 7.9, 2.1, 1.1 Hz), 7.29 (t, 1H, J = 7.8 Hz), 4.35–4.33 (m, 2H), 4.27 (dd, 1H, J = 2.3, 1.4 Hz), 4.23 (s, 5H), 1.49 (s, 3H), 1.46 (s, 3H), 1.10 (s, 3H), 0.81 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, 30 °C): 163.5 (C), 141.4 (C), 132.3 (CH), 131.1 (CH), 129.9 (CH), 126.3 (CH), 122.4 (C), 99.2 (C), 71.9 (C), 70.0 (5 CH), 67.4 (CH), 66.2 (CH), 66.0 (CH), 62.9 (C), 34.7 (C), 28.7 (CH₃), 25.2 (CH₃), 24.0 (CH₃), 23.6 (CH₃). MS (EI) m/z (% relative intensity): 449 [M]⁺ (90), 434 [M–CH₃]⁺ (100), 406 [M–i-C₃H₇]⁺ (10), 393 [M–C₄H₈]⁺ (48), 378 [M–C₄H₈–CH₃]⁺ (7), 314 (32), 299 (16), 249 (12), 234 (12), 218 (14), 191 (22), 121 [C₅H₅Fe]⁺ (25), 56 [Fe]⁺ (10). Analysis calculated for C₂₃H₂₄BrFeN: C, 61.36; H, 5.37; N, 3.11. Found: C, 61.60; H, 5.39; N, 3.04.

rac-1-(2-Bromphenyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3j). The title compound was prepared according to GP using 2-bromobenzonitrile (2j) (76 mg, 0.42 mmol); reaction time = 40 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3j (120 mg, 76%) as an orange solid; mp 110–111 °C (hexane); R_f 0.02 (petroleum ether/EtOAc 25:1); R_f 0.20 (petroleum ether/TEA 100:1). IR (thin film): 3095, 2973, 2930, 2868, 1604, 1476, 1452, 1432, 1342, 1372, 1361, 1312, 1193, 1154, 1108, 1086, 1049, 1029, 1021, 1003, 923, 861, 822, 767, 751, 742, 693, 546, 486, 468, 447 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 7.65 (dd, 1H, J = 8.0, 1.1 Hz), 7.51–7.43 (m, 2H), 7.36–7.31 (m, 1H), 4.36 (dd, 2H, J = 2.4, 1.2Hz), 4.32 (t, 1H, J = 2.4 Hz), 4.17 (s, 5H), 3.90 (dd, 1H, J = 2.4, 1.1 Hz), 1.47 (s, 3H), 1.46 (s, 3H), 1.03 (s, 3H), 0.99 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): 163.4 (C), 140.1 (C), 132.3 (CH), 129.51 (C), 129.46 (C), 127.1 (CH), 120.2 (C), 97.8 (C), 71.8 (C), 69.4 (5 CH), 67.0 (CH), 64.4 (CH), 64.1 (CH), 61.8 (C), 34.1 (C), 26.6 (CH₃), 25.8 (CH₃), 24.4 (CH₃), 22.2 (CH₃). MS (EI) m/z (% relative intensity): 449 [M]⁺ (100), 406 [M–i-C₃H₇]⁺ (5), 393 [M–C₄H₈]⁺ (28), 384 (13), 370 [M–Br]⁺ (20), 355 [M–CH₃–Br]⁺ (22), 354 [M–CH₃–HBr]⁺ (22), 326 (19), 312 (18), 262 (11), 248 (59), 234 (62), 218 (22), 192 (29), 178 (31), 165 (15), 152 (10), 121 [C₅H₅Fe]⁺ (18), 56 [Fe]⁺ (8). Analysis calculated for C₂₃H₂₄BrFeN∙0.2C₆H₁₄: C, 62.18; H, 5.78; N, 3.00. Found: C, 62.44; H, 5.63; N, 3.02.

rac-3,3,4,4-Tetramethyl-1-(4-(trifluoromethyl)phenyl)-3,4-dihydroferroceno[c]pyridine (3k). The title compound was prepared according to GP using 4-(trifluoromethyl)benzonitrile (2k) (71 mg, 0.42 mmol); reaction time = 15 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave pure compound 3k (117 mg, 76%) as an orange solid; mp 121.5–123 °C (hexane); R_f 0.16 (petroleum ether/EtOAc 50:1); R_f 0.30 (petroleum ether/TEA 100:1). IR (thin film): 3095, 2976, 2934, 2871, 1620, 1594, 1564, 1453, 1373, 1362, 1327, 1313, 1165, 1127, 1107, 1081, 1067, 1018, 1003, 849, 824, 758, 474, 444 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 8.24 (br d, 2H, J = 7.9 Hz), 7.87 (br d, 2H, J = 7.9 Hz), 4.50 (d, 2H, J = 1.8 Hz), 4.41 (t, 1H, J = 1.8 Hz), 4.31 (s, 5H), 1.464 (s, 3H), 1.456 (s, 3H), 1.11 (s, 3H), 0.78 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): 161.8 (C), 141.9 (C), 129.3 (q, ²J_C,F = 31.8 Hz, C), 127.9 (s, 2 CH), 124.8 (q, ³J_C,F = 3.8 Hz, 2 CH), 124.1 (q, ¹J_C,F = 271.0 Hz, CF₃), 98.2 (C), 70.8 (C), 69.5 (5 CH), 67.2 (CH), 65.6 (CH), 65.4 (CH), 62.2 (C), 33.9 (C), 27.9 (CH₃), 24.5 (CH₃), 23.5 (CH₃), 23.0 (CH₃). ¹⁹F NMR (377 MHz, DMSO-d₆, 50 °C): δ 101.5 (s, CF₃). MS (EI) m/z (% relative intensity): 439 [M]⁺ (88), 424 [M–CH₃]⁺ (60), 396 [M–i-C₃H₇]⁺ (8), 383 [M–C₄H₈]⁺ (55), 368 (16), 284 (100), 243 (11), 227 (13), 121 [C₅H₅Fe]⁺ (13), 56 [Fe]⁺ (3). Analysis calculated for C₂₄H₂₄F₃FeN: C, 65.62; H, 5.51; N, 3.19. Found: C, 65.92; H, 5.78; N, 3.11.

rac-3,3,4,4-Tetramethyl-1-(3-(trifluoromethyl)phenyl)-3,4-dihydroferroceno[c]pyridine (3l). The title compound was prepared according to GP using 3-(trifluoromethyl)benzonitrile (2l) (0.056 mL, 0.42 mmol); reaction time = 30 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave pure compound 3l (129 mg, 84%) as a brown oil; R_f 0.15 (petroleum ether/EtOAc 50:1); R_f 0.30 (petroleum ether/TEA 100:1). IR (thin film): 3093, 2976, 2931, 2870, 1599, 1574, 1454, 1433, 1373, 1362, 1336, 1296, 1266, 1185, 1166, 1127, 1109, 1095, 1072, 1002, 825, 808, 702, 690, 473, 448 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 8.40–8.39 (m, 1H), 8.33–8.30 (m, 1H), 7.84–7.81 (m, 1H), 7.75–7.71 (m, 1H), 4.49–4.47 (m, 2H), 4.34 (dd, 1H, J = 2.4, 1.3 Hz), 4.27 (s, 5H), 1.419 (s, 3H), 1.415 (s, 3H), 1.10 (s, 3H), 0.70 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 161.5 (C), 138.9 (C), 131.0 (CH), 129.1 (CH), 128.8 (q, ²J_C,F = 31.6 Hz, C), 125.6 (q, ³J_C,F = 3.7 Hz, CH), 124.1 (q, ¹J_C,F = 272.3 Hz, CF₃), 123.5 (q, ³J_C,F = 4.2, CH), 98.3 (C), 70.8 (C), 69.4 (5 CH), 67.2 (CH), 65.8 (CH), 65.4 (CH), 62.2 (C), 33.9 (C), 28.2 (CH₃), 24.1 (CH₃), 23.6 (CH₃), 22.7 (CH₃). ¹⁹F NMR (377 MHz, DMSO-d₆, 50 °C): δ 101.6 (s, CF₃). MS (EI) m/z (% relative intensity): 439 [M]⁺ (100), 424 [M–CH₃]⁺ (44), 396 [M–i-C₃H₇]⁺ (17), 383 [M–C₄H₈]⁺ (74), 368 [M–C₄H₈–CH₃]⁺ (11), 284 (59), 121 [C₅H₅Fe]⁺ (10), 56 [Fe]⁺ (2). Analysis calculated for C₂₄H₂₄F₃FeN: C, 65.62; H, 5.51; N, 3.19. Found: C, 65.99; H, 5.63; N, 3.21.

rac-3,3,4,4-Tetramethyl-1-(2-(trifluoromethyl)phenyl)-3,4-dihydroferroceno[c]pyridine (3m). The title compound was prepared according to GP using 2-(trifluoromethyl)benzonitrile (2m) (0.056 mL, 0.42 mmol); reaction time = 40 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave pure compound 3m (100 mg, 65%) as brown oil; R_f 0.04 (petroleum ether/EtOAc 50:1); R_f 0.20 (petroleum ether/TEA 100:1). IR (thin film): 3095, 2977, 2932, 2870, 1603, 1580, 1456, 1447, 1373, 1362, 1315, 1264, 1163, 1136, 1109, 1059, 1034, 1003, 823, 768, 683, 545, 449 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 30 °C): δ 7.80–7.76 (m, 2H), 7.65–7.61 (m, 2H), 4.36 (dd, 1H, J = 2.4, 1.2 Hz), 4.33 (t, 1H, J = 2.5 Hz), 4.22 (s, 5H), 3.81 (dd, 1H, J = 2.5, 1.2 Hz), 1.46 (s, 3H), 1.45 (s, 3H), 1.04 (s, 3H), 0.92 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 162.1 (C), 138.1 (C, broad), 131.7 (CH), 129.8 (CH), 128.0 (CH), 126.3 (q, ²J_C,F = 30.6 Hz, C), 126.0 (q, ³J_C,F = 4.9 Hz, CH), 123.8 (q, ¹J_C,F = 274.3 Hz, CF₃), 97.8 (C), 72.2 (C), 69.5 (5 CH), 67.2 (CH), 64.1 (CH), 63.7 (CH), 61.7 (C), 34.2 (C), 26.8 (CH₃), 25.3 (CH₃), 24.5 (CH₃), 21.9 (CH₃). ¹⁹F NMR (376 MHz, DMSO-d₆, 50 °C): δ 106.7 (s, CF₃). MS (EI) m/z (% relative intensity): 439 [M]⁺ (100), 424 [M–CH₃]⁺ (10), 396 [M–i-C₃H₇]⁺ (19), 383 [M–C₄H₈]⁺ (19), 368 [M–C₄H₈–CH₃]⁺ (2), 284 (22), 121 [C₅H₅Fe]⁺ (5), 56 [Fe]⁺ (1). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₄H₂₅F₃FeN, 440.1283; found, 440.1291.

rac-1,3,3,4,4-Pentamethyl-3,4-dihydroferroceno[c]pyridine (3o). The title compound was prepared according to GP using acetonitrile (2o) (0.02 mL, 0.42 mmol); reaction time = 15 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3o (82 mg, 76%) as a red oil; R_f 0.07 (petroleum ether/EtOAc 25:1); R_f 0.25 (petroleum ether/TEA 100:1). IR (thin film): 3094, 2972, 2868, 1619, 1465, 1438, 1387, 1373, 1361, 1301, 1164, 1135, 1108, 1027, 1001, 854, 821, 658, 511, 487, 475, 458 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 30 °C): δ 4.46 (t, 1H, J = 1.8 Hz), 4.32 (d, 2H, J = 1.8 Hz), 4.23 (s, 5H), 2.18 (s, 3H), 1.40 (s, 3H), 1.31 (s, 3H), 1.00 (s, 3H), 0.72 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 162.5 (C), 97.8 (C), 73.5 (C), 69.2 (5 CH), 66.3 (CH), 65.1 (CH), 63.5 (CH), 61.2 (C), 33.9 (C), 28.2 (CH₃), 25.1 (CH₃), 23.8 (CH₃), 23.6 (CH₃), 22.7 (CH₃). MS (EI) m/z (% relative intensity): 309 [M]⁺ (100), 294 [M–CH₃]⁺ (24), 266 [M–i-C₃H₇]⁺ (52), 251 [M–i-C₃H₇–CH₃]⁺ (11), 226 (12), 186 [C₅H₅FeC₅H₅]⁺ (13), 162 (14), 146 (11), 129 (10), 121 [C₅H₅Fe]⁺ (44), 115 (20), 91 (10), 56 [Fe]⁺ (19), 42 (12). Analysis calculated for C₁₈H₂₃FeN: C, 69.91; H, 7.88; N, 4.53. Found: C, 69.55; H, 7.89; N, 4.49.

rac-1-Ethyl-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3p). The title compound was prepared according to GP using propiononitrile (2p) (0.03 mL, 0.42 mmol); reaction time = 20 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave pure compound 3p (90 mg, 80%) as a red oil; R_f 0.05 (petroleum ether/EtOAc 50:1); R_f 0.10 (petroleum ether/TEA 100:1). IR (thin film): 3095, 2972, 2932, 2870, 1615, 1465, 1442, 1387, 1360, 1287, 1248, 1194, 1158, 1128, 1108, 1028, 1002, 922, 870, 821, 511, 482 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 4.33 (dd, 1H, J = 2.4, 1.1 Hz), 4.25 (t, 1H, J = 2.4 Hz), 4.20 (dd, 1H, J = 2.4, 1.1 Hz), 4.15 (s, 5H), 2.50 (qt, 2H, J = 7.6, 3.6 Hz), 1.40 (s, 6H), 1.25 (t, 3H, J = 7.6 Hz), 0.97 (s, 3H), 0.89 (s, 3H). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 168.0 (C), 99.0 (C), 73.1 (C), 69.7 (5 CH), 66.9 (CH), 65.0 (CH), 63.7 (CH), 61.4 (C), 34.7 (C), 30.6 (CH₂), 27.7 (CH₃), 26.3 (CH₃), 24.5 (CH₃), 23.6 (CH₃), 13.1 (CH₃). MS (EI) m/z (% relative intensity): 323 [M]⁺ (100), 308 [M–CH₃]⁺ (18), 280 [M–i-C₃H₇]⁺ (34), 267 [M–C₄H₈]⁺ (18), 265 [M–i-C₃H₇–CH₃]⁺ (13), 121 [C₅H₅Fe]⁺ (20), 56 [Fe]⁺ (9). HRMS-ESI (m/z): [M + H]⁺ calculated for C₁₉H₂₆FeN, 324.1409; found, 324.1411.

rac-1-Isobutyl-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3q). The title compound was prepared according to GP using isovaleronitrile (2q) (0.044 mL, 0.42 mmol); reaction time = 15 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave pure compound 3q (92 mg, 75%) as a red oil; R_f 0.07 (petroleum ether/EtOAc 25:1); R_f 0.30 (petroleum ether/TEA 100:1). IR (thin film): 3097, 2969, 2930, 2868, 1613, 1464, 1386, 1372, 1361, 1304, 1259, 1245, 1224, 1194, 1159, 1144, 1135, 1108, 1028, 1002, 862, 821, 754, 510, 447 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 50 °C): δ 4.06 (dd, 1H, J = 2.5, 1.1 Hz), 4.00 (s, 6H), 3.96 (t, 1H, J = 2.4 Hz), 2.49–2.32 (m, 3H), 1.52 (s, 3H), 1.37 (s, 3H), 1.09 (d, 3H, J = 6.2 Hz), 1.06 (d, 3H, J = 6.3 Hz,), 1.01 (s, 3H), 0.97 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 50 °C): δ 164.3 (C), 99.2 (C), 74.8 (C), 69.8 (5 CH), 66.8 (CH), 65.0 (CH), 63.6 (CH), 61.9 (C), 46.0 (CH₂), 34.9 (C), 28.0 (CH₃), 26.8 (CH), 26.3 (CH₃), 24.8 (CH₃), 24.0 (CH₃), 23.2 (CH₃), 23.0 (CH₃). MS (EI) m/z (% relative intensity): 351 [M]⁺ (100), 336 [M–CH₃]⁺ (45), 308 [M–i-C₃H₇]⁺ (40), 251 (19), 121 [C₅H₅Fe]⁺ (47), 56 [Fe]⁺ (12). Analysis calculated for C₂₁H₂₉FeN: C, 71.80; H, 8.32; N, 3.99. Found: C, 71.82; H, 8.73; N, 3.80.

rac-1-(Adamantan-1-yl)methyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3r). The title compound was prepared according to GP using 1-adamantaneacetonitrile (2r) (61 mg, 0.42 mmol); reaction time = 25 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave pure compound 3r (113 mg, 73%) as a red solid; mp 137–139.5 °C (MeOH); R_f 0.10 (petroleum ether/EtOAc 50:1); R_f 0.40 (petroleum ether/TEA 100:1). IR (thin film): 3096, 2970, 2902, 2846, 1460, 1387, 1372, 1360, 1314, 1293, 1262, 1208, 1150, 1121, 1108, 1029, 1002, 821, 753, 660, 508, 448 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.11 (dd, 1H, J = 2.5, 1.2 Hz), 4.01 (s, 5H), 3.99 (dd, 1H, J = 2.4, 1.1 Hz), 3.96 (t, 1H, J = 2.4 Hz), 2.41 (d, 1H, J = 13.0 Hz), 2.35 (d, 1H, J = 13.0 Hz), 2.00–1.90 (m, 6H), 1.76–1.70 (m, 9H), 1.55 (s, 3H), 1.36 (s, 3H), 1.11 (s, 3H), 0.97 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 163.5 (C), 99.3 (C), 76.0 (C), 69.9 (5 CH), 66.9 (CH), 64.70 (CH), 64.68 (CH), 61.9 (C), 50.5 (CH₂), 43.8 (3 CH₂), 37.6 (3 CH₂), 34.7 (C), 34.4 (C), 29.5 (3 CH), 27.6 (CH₃), 26.8 (CH₃), 25.5 (CH₃), 23.8 (CH₃). MS (EI) m/z (% relative intensity): 443 [M]⁺ (100), 428 [M–CH₃]⁺ (11), 400 [M–i-C₃H₇]⁺ (33), 385 [M–i-C₃H₇–CH₃]⁺ (12), 359 (44), 308 [M–Ad]⁺ (12), 251 (19), 135 [Ad]⁺ (14), 121 [C₅H₅Fe]⁺ (33), 56 [Fe]⁺ (6). Analysis calculated for C₂₈H₃₇FeN∙0.55MeOH: C, 74.37; H, 8.57; N, 3.04. Found: C, 74.73; H, 8.95; N, 3.00.

rac-1-(Adamantan-1-yl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (3s). The title compound was prepared according to GP using adamantane-1-carbonitrile (2s) (68 mg, 0.42 mmol); reaction time = 20 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 75:1–petroleum ether/TEA 100:1) gave pure compound 3s (107 mg, 71%) as an orange oil; R_f 0.10 (petroleum ether/EtOAc 75:1); R_f 0.40 (petroleum ether/TEA 100:1). IR (thin film): 3096, 2970, 2927, 2903, 2848, 1597, 1450, 1370, 1359, 1292, 1250, 1162, 1108, 1002, 820, 758, 475, 456 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 50 °C): δ 4.34 (dd, 1H, J = 2.5, 1.1 Hz), 4.04 (t, 1H, J = 2.5 Hz), 4.02 (s, 5H), 4.00 (dd, 1H, J = 2.5, 1.1 Hz), 2.22–2.11 (m, 6H), 2.09–2.06 (m, 3H), 1.79 (br t, 6H, J = 3.2 Hz), 1.51 (s, 3H), 1.34 (s, 3H), 1.05 (s, 3H), 0.94 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 50 °C): δ 169.3 (C), 100.2 (C), 70.8 (C), 70.1 (5 CH), 67.2 (CH), 66.4 (CH), 63.9 (CH), 60.9 (C), 42.2 (C), 41.8 (3 CH₂), 37.7 (3 CH₂), 34.6 (C), 29.6 (3 CH), 26.7 (CH₃), 26.4 (CH₃), 25.1 (CH₃), 23.6 (CH₃). MS (EI) m/z (% relative intensity): 429 [M]⁺ (100), 414 [M–CH₃]⁺ (14), 386 [M–C₃H₇]⁺ (17), 373 [M–C₄H₈]⁺ (37), 294 [M–Ad]⁺ (11), 121 [C₅H₅Fe]⁺ (11), 56 [Fe]⁺ (2). Analysis calculated for C₂₇H₃₅FeN: C, 75.52; H, 8.22; N, 3.26. Found: C, 74.98; H, 8.49; N, 3.04.

rac-Ethyl (Z)-2-(3,3,4,4-Tetramethyl-3,4-dihydroferroceno[c]pyridin-1(2H)-ylidene)acetate (3u). The title compound was prepared according to GP using ethyl cyanoacetate 2u (0.045 mL, 0.42 mmol); reaction time = 20 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1) gave pure product 3u (79 mg, 59%) as an orange oil; R_f 0.30 (petroleum ether/EtOAc 25:1). IR (thin film): 3288, 3096, 2975, 2930, 2870, 1646, 1602, 1499, 1465, 1442, 1366, 1295, 1187, 1154, 1108, 1072, 1044, 1002, 822, 784, 756, 629, 470 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 8.23 (br d, 1H, NH), 4.84 (s, 1H, H2), 4.53 (dd, 1H, J = 2.5, 1.2 Hz, H7′), 4.27 (t, 1H, J = 2.5 Hz, H6′), 4.21 (dd, 1H, J = 2.5, 1.2 Hz, H5′), 4.18–4.09 (m, 2H, OCH₂CH₃, partially overlapped), 4.14 (s, 5H, H Cp), 1.55 (s, 3H, 4′-CH₃), 1.37 (s, 3H, 3′-CH₃), 1.30 (t, 3H, J = 7.1 Hz, OCH₂CH₃), 1.06 (s, 3H, 4′-CH₃), 0.95 (s, 3H, 3′-CH₃). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 170.9 (C1), 160.7 (C1′), 98.3 (C4a’), 78.6 (C2), 74.1 (C7a’), 70.5 (5 CH, Cp), 67.3 (C6′), 66.0 (C5′), 64.2 (C7′), 58.4 (OCH₂CH₃), 57.5 (C3′), 36.4 (C4′), 28.9 (4′-CH₃), 25.8 (3′-CH₃), 25.1 (4′-CH₃), 23.8 (3′-CH₃), 14.9 (OCH₂CH₃). GC–MS analyses of product 3u indicated the presence of a peak in the chromatogram that, based on its MS spectra, belonged to compound 3o. The latter was likely produced by the thermolysis of the parent compound in the GC instrument injector. HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₁H₂₈FeNO₂, 382.1464; found, 382.1458.

rac-(Z)-2-(3,3,4,4-Tetramethyl-3,4-dihydroferroceno[c]pyridin-1(2H)-ylidene)acetamide (3v). The title compound was prepared according to GP using cyanoacetamide 2v (35 mg, 0.42 mmol); reaction time = 30 min. Purification by silica gel column chromatography (petroleum ether/acetone 3:1) followed by recrystallization from hexane gave pure product 3v (42 mg, 34%) as an orange solid, m.p. 147–149 °C (hexane); R_f 0.30 (petroleum ether/acetone 3:1). IR (thin film): 3440, 3326, 3213, 3098, 2972, 2927, 2870, 2856, 1684, 1621, 1572, 1457, 1364, 1337, 1151, 1108, 1002, 823, 756, 666, 507, 471 cm⁻¹. ¹H NMR (400 MHz, CDCl₃,30 °C): δ 8.80 (br s, 1H, NH), 4.74 (s, 1H, H2), 4.67 (br s, 2H, NH₂), 4.47 (dd, 1H, J = 2.5, 1.2 Hz, H7′), 4.24 (t, 1H, J = 2.5 Hz, H6′), 4.20 (dd, 1H, J = 2.5, 1.2 Hz, H5′), 4.14 (s, 5H, H Cp), 1.55 (s, 3H, 4′-CH₃), 1.36 (s, 3H, 3′-CH₃), 1.05 (s, 3H, 4′-CH₃), 0.94 (s, 3H, 3′-CH₃). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 172.9 (C1), 158.9 (C1′), 98.5 (C4a’), 80.0 (C2), 74.4 (C7a’), 70.4 (5 CH, Cp), 66.9 (C6′), 65.9 (C5′), 63.6 (C7′), 57.1 (C3′), 36.3 (C4′), 28.8 (4′-CH₃), 25.8 (3′-CH₃), 25.1 (4′-CH₃), 23.8 (3′-CH₃). GC–MS analyses of product 3v indicated the presence of a peak in the chromatogram that, based on its MS spectra, belonged to compound 3o. The latter was likely produced by the thermolysis of the parent compound in the GC instrument injector. HRMS-ESI (m/z): [M + H]⁺ calculated for C₁₉H₂₅FeN₂O, 353.1311; found, 353.1307.

rac-(Z)-1-Phenyl-2-(3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridin-1(2H)-ylidene)ethan-1-one (3w). The title compound was prepared according to GP using 3-oxo-3-phenylpropanenitrile 2w (61 mg, 0.42 mmol); reaction time = 20 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 10:1) gave pure compound 3w (126 mg, 87%) as a red oil; R_f 0.30 (petroleum ether/EtOAc 10:1). IR (thin film): 2975, 2929, 1594, 1580, 1543, 1451, 1438, 1366, 1324, 1304, 1265, 1229, 1153, 1108, 1055, 1026, 1002, 824, 755, 732, 711, 620, 512, 487, 461 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 10.99 (br d, 1H, NH), 7.95–7.91 (m, 2H, H2′’, H6′’), 7.44–7.41 (m, 3H, H3′’, H4′’, H5′’), 5.98 (s, 1H, H2), 4.69 (dd, 1H, J = 2.6, 1.2 Hz, H7′), 4.38 (t, 1H, J = 2.5 Hz, H6′), 4.31 (dd, 1H, J = 2.5, 1.2 Hz, H5′), 4.18 (s, 5H, H Cp), 1.57 (s, 3H, 4′-CH₃), 1.46 (s, 3H, 3′-CH₃), 1.10 (s, 3H, 4′-CH₃), 1.03 (s, 3H, 3′-CH₃). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 187.2 (C1), 163.2 (C1′), 141.2 (C1′’), 130.4 (C4′’), 128.3 (C3′’, C5′’), 127.0 (C2′’, C6′’), 98.3 (C4a’), 88.0 (C2), 73.5 (C7a’), 70.7 (5 CH, Cp), 67.9 (C6′), 66.6 (C5′), 64.8 (C7′), 57.9 (C3′), 36.3 (C4′), 28.8 (4′-CH₃), 25.7 (3′-CH₃), 25.0 (4′-CH₃), 23.5 (3′-CH₃). MS (EI) m/z (% relative intensity): 413 [M]⁺ (100), 370 [M–i-C₃H₇]⁺ (19), 348 (42), 308 [M–C(O)C₆H₅]⁺ (12), 121 [C₅H₅Fe]⁺ (9), 56 [Fe]⁺ (2). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₅H₂₈FeNO, 414.1420; found, 414.1515.

rac-(Z)-1-(3,3,4,4-Tetramethyl-3,4-dihydroferroceno[c]pyridin-1(2H)-ylidene)propan-2-one (3x). The title compound was prepared according to GP using 3-oxobutyronitrile 2x (0.036 mL, 0.42 mmol); reaction time = 20 min. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1) gave pure compound 3x (99 mg, 81%) as a red oil, which solidified on long-term standing; mp 158.5–162 °C; R_f 0.20 (petroleum ether/EtOAc 25:1). IR (thin film): 3094, 2974, 2927, 2870, 1603, 1566, 1509, 1449, 1365, 1354, 1317, 1262, 1201, 1153, 1108, 1005, 961, 823, 757, 715, 508, 469 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 10.43 (br s, 1H, NH), 5.28 (s, 1H, H2), 4.55 (dd, 1H, J = 2.5, 1.2 Hz, H7′), 4.32 (t, 1H, J = 2.5 Hz, H6′), 4.25 (dd, 1H, J = 2.4, 1.1 Hz, H5′), 4.15 (s, 5H, H Cp), 2.07 (s, 3H, 1-CH₃), 1.53 (s, 3H, 4′-CH₃), 1.39 (s, 3H, 3′-CH₃), 1.06 (s, 3H, 4′-CH₃), 0.97 (s, 3H, 3′-CH₃). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 194.3 (C1), 161.2 (C1′), 98.4 (C4a’), 91.1(C2), 73.2 (C7a’), 70.6 (5 CH, Cp), 67.6 (C6′), 66.4 (C5′), 64.5 (C7′), 57.5 (C3′), 36.2 (C4′), 29.2 (1-CH₃), 28.7 (4′-CH₃), 25.6 (3′-CH₃), 25.1 (4′-CH₃), 23.6 (3′-CH₃). MS (EI) m/z (% relative intensity): 351 [M]⁺ (100), 308 [M–C(O)CH₃) or/and i-C₃H₇]⁺ (27), 286 (61), 242 (10), 228 (10), 121 [C₅H₅Fe]⁺ (9), 56 [Fe]⁺ (2). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₀H₂₆FeNO, 352.1358; found, 352.1354.

3.2.4. Synthesis of Ferroceno[c]pyridines 8a–c and 9a–c (Table 3)

General Procedure 1 (GP1). Phenylacetonitrile 7a–c (0.42 mmol) was added to the stirred solution of alcohol 1 (100 mg, 0.35 mmol) in MeSO₃H (0.18 mL) at room temperature, and the resulting mixture was heated at 60 °C in an oil bath with vigorous stirring for the indicated time (monitored by TLC). The reaction mixture was then cooled to room temperature, neutralized with 10% aq. Na₂CO₃ solution (3 mL), and extracted with EtOAc (10 mL × 4). The combined organic phases were washed with water, dried over anhydrous Na₂SO₄, and filtered. Method A: Dried organic extract was concentrated in vacuo, and the crude residue was purified by silica gel column chromatography to obtain a mixture of compounds 8a–c and 9a–c; the ratios of 8a–c and 9a–c were determined by NMR ¹H immediately after the isolation of the mixed fraction. Storage of this mixture at room temperature exposed to air until full conversion of 8a–c to 9a–c (monitored by TLC, reaction time 1), followed by purification by silica gel column chromatography, afforded pure compound 9a–c. Method B: Dried organic extract was stored at room temperature exposed to air until full conversion of 8a–c to 9a–c (monitored by TLC, reaction time 1) and then concentrated in vacuo. The crude residue was purified by silica gel column chromatography to afford pure compound 9a–c.

rac-1-(3,4-Dimethoxybenzyl)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (8a) and rac-(3,4-Dimethoxyphenyl)(3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine)methanone (9a). The title compounds were prepared according to GP1 using 3,4-dimethoxyphenylacetonitrile (7a) (74 mg, 0.42 mmol); reaction time = 10 min; reaction time 1 = 1 day. Method A: Purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 3:1–petroleum ether/EtOAc/Et₃N 100:5:3) gave an inseparable mixture of 7a, 8a and 9a (108 mg, 7a/8a/9a = 31:54:15; 8a/9a = 78:22). Storage of the mixture of 7a, 8a and 9a at room temperature exposed to air until full conversion of 8a to 9a, followed by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 25:1), afforded pure compound 9a (39 mg, 24%). Method B: Purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 25:1) gave pure 9a (33 mg, 21%). Data for 8a [spectroscopic data for 8a were obtained only from the mixture of 7a, 8a and 9a]: R_f 0.15 (petroleum ether/EtOAc 5:1). ¹H NMR (DMSO-d₆, 400 MHz, 30 °C): δ 7.04 (d, 1H, J = 1.8 Hz), 7.01–6.94 (m, 2H, overlapped), 4.32–4.24 (m, 3H), 4.00 (s, 5H), 3.78 (s, 3H, overlapped), 3.76 (s, 3H), 3.71 (d, 1H, J = 13.5 Hz), 3.66 (d, 1H, J = 13.5 Hz), 1.42 (s, 3H), 1.40 (s, 3H), 0.96 (s, 3H), 0.93 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): 148.5 (C), 147.4 (C), 131.2 (C), 120.7 (CH), 112.2 (CH), 111.9 (CH), 98.0 (C), 72.0 (C, broad), 69.3 (5 CH), 66.8 (CH, broad), 64.2 (CH, broad), 63.8 (CH, broad), 61.2 (C), 55.5 (CH₃O), 55.3 (CH₃O), 42.5 (CH₂), 34.0 (C), 26.5 (CH₃), 26.3 (CH₃), 24.6 (CH₃), 22.9 (CH₃); one quaternary carbon peak is missing, probably due to the broadening of the signal. MS (EI) m/z (% relative intensity): 445 [M]⁺ (100), 402 [M–i-C₃H₇]⁺ (13), 364 (10), 294 [M–(CH₃O)₂C₆H₃CH₂]⁺ (17), 266 (15), 121 [C₅H₅Fe]⁺ (9), 56 (2). Data for 9a: red oil; R_f 0.27 (petroleum ether/EtOAc 5:1). IR (thin film): 3085, 2975, 2936, 1658, 1593, 1514, 1463, 1417, 1321, 1274, 1260, 1242, 1209, 1175, 1138, 1117, 1025, 1003, 823, 757, 666, 448 cm⁻¹. ¹H NMR(DMSO-d₆, 400 MHz, 30 °C): δ 7.82 (dd, 1H, J = 8.4, 2.0 Hz), 7.70 (d, 1H, J = 1.9 Hz), 7.22 (d, 1H, J = 8.5 Hz), 4.51–4.50 (m, 2H), 4.45 (t, 1H, J = 2.4 Hz), 4.17 (s, 5H), 3.92 (s, 3H), 3.88 (s, 3H), 1.54 (s, 3H), 1.47 (s, 3H), 1.10 (s, 3H), 0.89 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 30 °C): δ 189.9 (C), 163.4 (C), 153.7 (C), 148.5 (C), 127.5 (C), 125.5 (CH), 111.8 (CH), 111.0 (CH), 98.0 (C), 69.5 (5 CH), 67.8 (CH), 65.9 (CH), 64.6 (CH), 62.3 (C), 55.8 (CH₃O), 55.4 (CH₃O), 34.3 (C), 28.3 (CH₃), 25.4 (CH₃), 23.2 (CH₃). MS (EI) m/z (% relative intensity): 459 [M]⁺ (100), 444 [M–CH₃]⁺ (13), 403 [M–C₄H₈]⁺ (40), 321 [M–(CH₃O)₂C₆H₄]⁺ (13), 306 [M–(CH₃O)₂C₆H₄–CH₃]⁺ (19), 294 [M–(CH₃O)₂C₆H₃C(O)]⁺ (21), 237 (17), 165 [(CH₃O)₂C₆H₃C(O)]⁺ (22), 121 [C₅H₅Fe]⁺ (16), 56 [Fe]⁺ (3). Analysis calculated for C₂₆H₂₉FeNO₃: C, 67.98; H, 6.36; N, 3.05. Found: C, 68.20; H, 6.80; N, 3.15.

rac-1-Benzyl-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (8b) and rac-Phenyl(3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine)methanone (9b). The title compounds were prepared according to GP1 using phenylacetonitrile (7b) (0.48 mL, 0.42 mmol); reaction time = 15 min; reaction time 1 = 2 days. Method A: Purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, 9b (20 mg, 14%) and an inseparable mixture of 8b and 9b (56 mg, 8b/9b = 89:11); calculated 8b/9b = 66:34. Storage of the mixture of 8b and 9b at room temperature exposed to air until full conversion of 8b to 9b, followed by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 25:1), afforded pure compound 9b (29 mg, 21%). Overall yield of 9b: 49 mg, 35%. Method B: Purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 25:1) gave pure 9b (33 mg, 24%). Data for 8b [spectroscopic data for 8b were obtained only from the mixture of 8b and 9b]: R_f 0.10 (petroleum ether/EtOAc 25:1). ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 7.47–7.43 (m, 2H), 7.40–7.36 (m, 2H), 7.27–7.24 (m, 1H), 4.27–4.25 (m, 3H), 3.97 (s, 5H), 3.79 (d, 1H, J = 13.8 Hz), 3.75 (d, 1H, J = 13.8 Hz), 1.42 (s, 3H), 1.41 (s, 3H), 0.93 (s, 3H), 0.91 (s, 3H). The ¹³C spectrum of mixture 8b and 9b was complex, including broadening and overlapping of some signals, and it was impossible to clearly assign the peaks corresponding to 8b. MS (EI) m/z (% relative intensity): 385 [M]⁺ (100), 342 [M–i-C₃H₇]⁺ (14), 329 [M–C₄H₈]⁺ (10), 304 (11), 121 [C₅H₅Fe]⁺ (16), 56 [Fe]⁺ (5). Data for 9b: red solid; mp 120.5–122 °C (hexane); R_f 0.20 (petroleum ether/EtOAc 25:1). IR (thin film): 3086, 2975, 2930, 2868, 1669, 1595, 1458, 1448, 1374, 1362, 1324, 1274, 1225, 1154, 1122, 1108, 1002, 876, 830, 737, 692, 493, 451 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 8.14–8.12 (m, 2H), 7.75–7.71 (m, 1H), 7.65–7.61 (m, 2H), 4.55 (dd, 1H, J = 2.4, 1.2 Hz), 4.49 (dd, 1H, J = 2.4, 1.2 Hz), 4.46 (t, 1H, J = 2.4 Hz), 4.18 (s, 5H), 1.55 (s, 3H), 1.48 (s, 3H), 1.09 (s, 3H), 0.91 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 191.2 (C), 163.1 (C), 135.0 (C), 133.3 (CH), 129.8 (2 CH), 128.3 (2 CH), 97.9 (C), 69.3 (5 CH), 67.7 (CH), 65.7 (CH), 64.5 (CH), 62.4 (C), 34.2 (C), 28.0 (CH₃), 25.3 (CH₃), 23.1 (CH₃), 22.9 (CH₃). MS (EI) m/z (% relative intensity): 399 [M]⁺ (100), 384 [M–CH₃]⁺ (13), 343 [M–C₄H₈]⁺ (33), 294 [M–CH₂C₆H₅]⁺ (11), 246 (18), 237 (13), 121 [C₅H₅Fe]⁺ (19), 56 [Fe]⁺ (6). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₄H₂₆FeNO, 400.1358; found, 400.1363.

rac-3,3,4,4-Tetramethyl-1-(4-nitrobenzyl)-3,4-dihydroferroceno[c]pyridine (8c) and rac-(4-Nitrophenyl)(3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine)methanone (9c). The title compounds were prepared according to GP1 using 2-(4-nitrophenyl)acetonitrile (7c) (68 mg, 0.42 mmol); reaction time = 15 min; reaction time 1 = 6 days. Method A: Purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave an inseparable mixture of 8c and 9c (41 mg, 8c/9c = 65:35; contaminated with unidentified compounds). Storage of the mixture of 8c and 9c at room temperature exposed to air until full conversion of 8c to 9c, followed by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1), afforded pure compound 9c (25 mg, 16%). Method B: Purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1) gave pure compound 9c (21 mg, 14%). Data for 8c [spectroscopic data for 8c were obtained only from the mixture of 8c and 9c, contaminated with identified compounds]: R_f 0.13 (petroleum ether/EtOAc 25:1). ¹H NMR (400 MHz, C₆D₆, 30 °C): 7.90–7.88 (m, 2H), 7.14–7.11 (m, 2H), 3.93 (dd, 1H, J = 2.5, 1.1 Hz), 3.89 (t, 1H, J = 2.4 Hz), 3.79 (dd, 1H, J = 2.5, 1.1 Hz), 3.77 (s, 5H), 3.63 (d, 1H, J = 14 Hz), 3.57 (d, 1H, J = 14.1 Hz). A great signal overlapping was observed in the region of 0.73–1.16 ppm, and it was impossible to clearly assign the peaks of methyl groups corresponding to 8c. ¹³C spectrum of mixture 8c and 9c was complex, including overlapping of the some signals, and it was impossible to clearly assign the peaks corresponding to 8c. MS (EI) m/z (% relative intensity): 430 [M]⁺ (100), 400 [M–C₂H₆]⁺ (13), 387 [M–i-C₃H₇]⁺ (11), 374 [M–C₄H₈]⁺ (10), 294 (14), 263 (10), 237 (10), 121 [C₅H₅Fe]⁺ (10), 56 [Fe]⁺ (3). Data for 9c: brown oil; R_f 0.25 (petroleum ether/EtOAc 25:1). IR (thin film): 3105, 3047, 2978, 2926, 2867, 2850, 1677, 1601, 1588, 1527, 1457, 1374, 1362, 1347, 1318, 1273, 1217, 1155, 1122, 1108, 1002, 989, 882, 857, 824, 737, 711, 697, 507, 483, 447 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 8.04–8.01 (m, 2H), 7.77–7.74 (m, 2H), 4.92 (dd, 1H, J = 2.4, 1.2 Hz), 4.08 (t, 1H, J = 2.4 Hz), 4.05 (dd, 1H, J = 2.5, 1.2 Hz), 4.00 (s, 5H), 1.44 (s, 3H), 1.35 (s, 3H), 0.95 (s, 3H), 0.89 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 189.9 (C), 163.7 (C), 150.4 (C), 141.1 (C), 131.6 (2 CH), 123.2 (2 CH), 98.7 (C), 70.2 (5 CH), 68.6 (CH), 66.6 (CH), 66.2 (CH), 63.9 (C), 35.0 (C), 29.0 (CH₃), 25.7 (CH₃), 23.4 (CH₃), 23.4 (CH₃). MS (EI) m/z (% relative intensity): 444 [M]⁺ (100), 429 [M–CH₃]⁺ (14), 414 [M–C₂H₆]⁺ (10), 388 [M–C₄H₈]⁺ (41), 306 (10), 294 (15), 277 (13), 237 (17), 121 [C₅H₅Fe]⁺ (19), 56 [Fe]⁺ (4). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₄H₂₅FeN₂O₃, 445.1209; found, 445.1208.

3.2.5. Synthesis of Ferroceno[c]pyridines 11a–g and Ferroceno[c]pyrroles 12a–g (Table 5 and Table 6)

General Procedure 2 (GP2). Thiocyanate 10a–g (0.42 mmol) was added to the stirred solution of alcohol 1 (100 mg, 0.35 mmol) in MeSO₃H (0.18 mL) at room temperature, and the resulting mixture was stirred at this temperature for the indicated time (monitored by TLC). The reaction mixture was then worked up as described in GP.

General Procedure 3 (GP3). Thiocyanate 10a–g (0.42 mmol) was added to the solution of 1 in MeSO₃H, and preheated to 80 °C with stirring. The resulting mixture was then stirred at this temperature for 3 min, cooled to room temperature, and worked up as described in GP.

rac-1-(Ethylthio)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (11a) and (1R*,S_p*)-1-(tert-Butyl)-3-(ethylthio)-1-methyl-1H-ferroceno[c]pyrrole (12a). The title compounds were prepared according to GP2 (reaction time = 25 min) and GP3 using ethyl thiocyanate (10a) (0.037 mL, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, a mixture of 11a and 12a (6 mg, 5%, 11a/12a = 19:81) and 12a (108 mg, 87%). GP3: purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 100:1–petroleum ether/TEA 100:1) gave, in order of elution, 11a (70 mg, 57%) and a mixture of 11a and 12a (36 mg, 29%, 11a/12a = 22:78). Data for 11a: brown solid; mp 81–83 °C (hexane); R_f 0.61 (petroleum ether/EtOAc 25:1). IR (thin film): 3096, 2971, 2927, 2868, 1580, 1448, 1372, 1361, 1287, 1239, 1151, 1141, 1108, 1097, 1002, 932, 899, 822, 512, 478 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.41 (dd, 1H, J = 2.4, 1.1 Hz, H7), 4.10 (s, 5H, H Cp), 3.99 (dd, 1H, J = 2.4, 1.1 Hz, H5), 3.92 (t, 1H, J = 2.4 Hz, H6), 3.22–3.12 (m, 1H, SCH₂CH₃), 3.11–3.02 (m, 1H, SCH₂CH₃), 1.50 (s, 3H, 3-CH₃), 1.40 (s, 3H, 4-CH₃), 1.28 (t, 3H, J = 7.3 Hz, SCH₂CH₃), 0.98 (s, 3H, 4-CH₃), 0.88 (s, 3H, 3-CH₃). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 161.3 (C1), 99.1 (C4a), 75.0 (C7a), 70.5 (5 CH Cp), 66.5 (C6), 65.7 (C5), 64.3 (C3), 63.9 (C7), 35.7 (C4), 29.2 (4-CH₃), 25.2 (4-CH₃), 24.8 (3-CH₃), 24.2 (3-CH₃), 22.9 (SCH₂CH₃), 15.1 (SCH₂CH₃). MS (EI) m/z (% relative intensity): 355 [M]⁺ (100), 340 [M–CH₃]⁺ (1), 326 [M–C₂H₅]⁺ (34), 293 (17), 269 (75), 172 (11), 121 [C₅H₅Fe]⁺ (12), 117 (22), 56 [Fe]⁺ (4). Analysis calculated for C₁₉H₂₅FeNS: C, 64.23; H, 7.09; N, 3.94; S, 9.02. Found: C, 64.04; H, 7.36; N, 3.83; S, 8.74. Data for 12a: brown solid; mp 79.5–81.5 °C (hexane); R_f 0.47 (petroleum ether/EtOAc 25:1). IR (thin film): 3096, 2969, 2936, 2908, 2871, 1522, 1451, 1391, 1366, 1299, 1149, 1141, 1106, 1003, 887, 821, 648, 508, 488 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 50 °C): δ 4.08 (dd, 1H, J = 2.3, 0.6 Hz, H4), 4.05 (dd, 1H, J = 2.1, 0.6 Hz, H6), 4.04 (s, 5H, H Cp), 3.87 (t, 1H, J = 2.3 Hz, H5), 3.25–3.16 (m, 1H, SCH₂CH₃), 3.07–2.98 (m, 1H, SCH₂CH₃), 1.76 (s, 3H, 1-CH₃), 1.30 (t, 3H, J = 7.3 Hz, SCH₂CH₃), 0.90 (s, 9H, C(CH₃)₃). ¹³C NMR (100 MHz, C₆D₆, 50 °C): δ 165.8 (C3), 110.1 (C6a), 90.8 (C3a), 78.5 (C1), 71.0 (C5), 69.8 (5 CH Cp), 62.8 (C6), 57.0 (C4), 37.8 (C(CH₃)₃), 26.1 (C(CH₃)₃), 24.5 (SCH₂CH₃), 23.1 (1-CH₃), 15.2 (SCH₂CH₃). MS (EI) m/z (% relative intensity): 355 [M]⁺ (23), 298 [M–t-C₄H₉]⁺ (100), 270 (18), 121 [C₅H₅Fe]⁺ (6), 56 [Fe]⁺ (2). Analysis calculated for C₁₉H₂₅FeNS: C, 64.23; H, 7.09; N, 3.94; S, 9.02. Found: C, 63.94; H, 7.32; N, 3.86; S, 8.80.

rac-3,3,4,4-Tetramethyl-1-(methylthio)-3,4-dihydroferroceno[c]pyridine (11b) and (1R*,S_p*)-1-(tert-Butyl)-1-methyl-3-(methylthio)-1H-ferroceno[c]pyrrole (12b). The title compounds were prepared according to GP2 (reaction time = 25 min) and GP3 using methyl thiocyanate (10b) (0.028 mL, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, a mixture of 11b and 12b (33 mg, 28%, 11b/12b = 9:91) and 12b (76 mg, 64%). GP3: purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 100:1–petroleum ether/TEA 100:1) gave, in order of elution, 11b (41 mg, 35%) and a mixture of 11b and 12b (54 mg, 45%, 11b/12b = 14:86). Data for 11b: brown solid; mp 70–74 °C (hexane); R_f 0.61 (petroleum ether/EtOAc 25:1). IR (thin film): 2973, 2924, 1581, 1449, 1372, 1361, 1286, 1241, 1151, 1108, 1097, 1002, 899, 822, 626, 512, 478 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.41 (dd, 1H, J = 2.5, 1.1 Hz), 4.09 (s, 5H), 3.99 (dd, 1H, J = 2.4, 1.2 Hz), 3.92 (t, 1H, J = 2.4 Hz), 2.40 (s, 3H), 1.52 (s, 3H), 1.40 (s, 3H), 0.98 (s, 3H, CH₃), 0.88 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 161.9 (C), 99.1 (C), 74.9 (C), 70.5 (5 CH), 66.6 (CH), 65.7 (CH), 64.4 (C), 63.8 (CH), 35.8 (C), 29.2 (CH₃), 27.3 (C), 25.2 (CH₃), 24.7 (CH₃), 24.3 (CH₃), 11.4 (CH₃). MS (EI) m/z (% relative intensity): 341 [M]⁺ (100), 326 [M–CH₃]⁺ (16), 293 (22), 269 (32), 252 (15), 237 (11), 173 (11), 121 [C₅H₅Fe]⁺ (10), 117 (21), 89 (10), 56 [Fe]⁺ (4). Analysis calculated for C₁₈H₂₃FeNS: C, 63.35; H, 6.79; N, 4.10; S, 9.39. Found: C, 63.34; H, 6.66; N, 4.04; S, 9.60. Data for 12b: brown solid; mp 97–106.5 °C (hexane); R_f 0.38 (petroleum ether/EtOAc 25:1). IR (thin film): 3079, 2981, 2970, 2953, 2939, 2907, 2871, 1525, 1453, 1364, 1297, 1286, 1144, 1103, 1004, 926, 812, 653, 508, 482 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 50 °C): δ 4.07 (dd, 1H, J = 2.3, 0.7 Hz), 4.05 (dd, 1H, J = 2.2, 0.7 Hz), 4.03 (s, 5H), 3.87 (t, 1H, J = 2.3 Hz), 2.42 (s, 3H), 1.76 (s, 3H), 0.89 (s, 9H). ¹³C NMR (100 MHz, C₆D₆, 50 °C): δ 166.5 (C), 110.4 (C), 90.5 (C), 78.5 (C), 71.0 (CH), 69.8 (5 CH), 62.9 (CH), 56.9 (CH), 37.7 (C), 26.0 (3 CH₃), 23.2 (CH₃), 12.7 (CH₃). MS (EI) m/z (% relative intensity): 341 [M]⁺ (23), 284 [M–t-C₄H₉]⁺ (100), 269 (10), 218 (14), 121 [C₅H₅Fe]⁺ (12), 56 [Fe]⁺ (3). Analysis calculated for C₁₈H₂₃FeNS: C, 63.35; H, 6.79; N, 4.10; S, 9.39. Found: C, 63.13; H, 6.99; N, 4.06; S, 9.05.

rac-3,3,4,4-Tetramethyl-1-(propylthio)-3,4-dihydroferroceno[c]pyridine (11c) and (1R*,S_p*)-1-(tert-Butyl)-1-methyl-3-(propylthio)-1H-ferroceno[c]pyrrole (12c). The title compounds were prepared according to GP2 (reaction time = 25 min) and GP3 using propyl thiocyanate (10c) (0.043 mL, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, a mixture of 11c and 12c (29 mg, 23%, 11c/12c = 24:76) and 12c (97 mg, 75%). GP3: purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 100:1–petroleum ether/TEA 100:1) gave, in order of elution, 11c (68 mg, 53%) and a mixture of 11c and 12c (37 mg, 29%, 11c/12c = 14:86). Data for 11c: brown oil; R_f 0.67 (petroleum ether/EtOAc 25:1). IR (thin film): 3097, 2969, 2930, 2870, 1580, 1449, 1372, 1361, 1286, 1238, 1151, 1141, 1108, 1097, 1002, 932, 899, 822, 512, 478 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.43 (dd, 1H, J = 2.4, 1.1 Hz), 4.11 (s, 5H), 3.99 (dd, 1H, J = 2.4, 1.1 Hz), 3.93 (t, 1H, J = 2.4 Hz), 3.24–3.09 (m, 2H), 1.70 (h, 2H, J = 7.3 Hz), 1.49 (s, 3H), 1.40 (s, 3H), 0.98 (s, 3H), 0.96 (t, 3H, J = 7.3 Hz), 0.88 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 161.4 (C), 99.1 (C), 75.0 (C), 70.5 (5 CH), 66.6 (CH), 65.7 (CH), 64.4 (C), 63.9 (CH), 35.7 (C), 30.4 (CH₂), 29.2 (CH₃), 25.2 (CH₃), 24.7 (CH₃), 24.2 (CH₃), 23.5 (CH₂), 13.7 (CH₃). MS (EI) m/z (% relative intensity): 369 [M]⁺ (100), 326 [M–C₃H₇]⁺ (38), 293 (20), 269 (70), 252 (7), 172 (11), 121 [C₅H₅Fe]⁺ (12), 117 (27), 56 [Fe]⁺ (3). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₀H₂₈FeNS, 370.1286; found, 370.1288. Data for 12c: brown oil; R_f 0.51 (petroleum ether/EtOAc 25:1). IR (thin film): 3096, 2966, 2935, 2871, 1521, 1453, 1366, 1288, 1149, 1108, 1003, 933, 887, 820, 648, 508, 488 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.09 (dd, 1H, J = 2.3, 0.7 Hz), 4.04 (s, 5H), 4.03 (dd, 1H, J = 2.3, 0.7 Hz), 3.85 (t, 1H, J = 2.3 Hz), 3.32–3.26 (m, 1H), 3.07–3.00 (m, 1H), 1.76 (s, 3H), 1.79–1.65 (m, 2H), 0.94–0.90 (m, 12H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 165.9 (C), 110.0 (C), 90.7 (C), 78.4 (C), 71.1 (CH), 69.8 (5 CH), 62.8 (CH), 57.0 (CH), 37.7 (C), 32.0 (CH₂), 26.0 (3 CH₃), 23.5 (CH₂), 23.2 (CH₃), 13.5 (CH₃). MS (EI) m/z (% relative intensity): 369 [M]⁺ (19), 312 [M–t-C₄H₉]⁺ (100), 270 (24), 204 (5), 121 [C₅H₅Fe]⁺ (7), 56 [Fe]⁺ (2). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₀H₂₈FeNS, 370.1286; found, 370.1287.

rac-1-(Isopropylthio)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (11d) and (1R*,S_p*)-1-(tert-Butyl)-3-(isopropylthio)-1-methyl-1H-ferroceno[c]pyrrole (12d). The title compounds were prepared according to GP2 (reaction time = 40 min) and GP3 using isopropyl thiocyanate (10d) (0.044 mL, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, 11d (9 mg, 7%), a mixture of 11d and 12d (30 mg, 23%, 11d/12d = 18:82) and 12d (68 mg, 53%). GP3: purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 100:1–petroleum ether/TEA 100:1) gave, in order of elution, 11d (79 mg, 61%) and a mixture of 11d and 12d (25 mg, 19%, 11d/12d = 70:30). Data for 11d: brown oil; R_f 0.47 (petroleum ether/EtOAc 100:1). IR (thin film): 3097, 2971, 2927, 2866, 1578, 1463, 1449, 1415, 1388, 1372, 1286, 1234, 1151, 1141, 1116, 1108, 1095, 1057, 1035, 1023, 1002, 932, 899, 822, 651, 512, 478 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.40 (dd, 1H, J = 2.4, 1.1 Hz), 4.19 (hept, 1H, J = 6.8 Hz), 4.10 (s, 5H), 3.99 (dd, 1H, J = 2.5, 1.2 Hz), 3.91 (t, 1H, J = 2.4 Hz), 1.51 (s, 3H), 1.41 (s, 3H), 1.40 (d, 3H, J = 6.6 Hz), 1.35 (d, 3H, J = 6.8 Hz), 0.98 (s, 3H), 0.90 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 161.8 (C), 99.0 (C), 75.0 (C), 70.5 (5 CH), 66.5 (CH), 65.7 (CH), 64.3 (C), 63.9 (CH), 35.6 (C), 33.5 (CH), 29.3 (CH₃), 25.1 (CH₃), 24.8 (CH₃), 24.2 (CH₃), 23.4 (CH₃), 23.2 (CH₃). MS (EI) m/z (% relative intensity): 369 [M]⁺ (52), 326 [M–i-C₃H₇]⁺ (38), 269 (100), 172 (10), 121 [C₅H₅Fe]⁺ (17), 117 (25), 56 [Fe]⁺ (5). Analysis calculated for C₂₀H₂₇FeNS: C, 65.04; H, 7.37; N, 3.79; S, 8.68. Found: C, 65.44; H, 7.45; N, 3.57; S, 8.66. HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₀H₂₈FeNS, 370.1286; found, 370.1284. Data for 12d: brown oil; R_f 0.50 (petroleum ether/EtOAc 25:1). IR (thin film): 3097, 2966, 2934, 2911, 2868, 1519, 1478, 1451, 1424, 1392, 1382, 1366, 1294, 1241, 1226, 1148, 1108, 1102, 1004, 934, 886, 820, 660, 513, 487 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.14 (hept, 1H, J = 6.8 Hz), 4.07 (dd, 1H, J = 2.3, 0.7 Hz), 4.04 (s, 5H), 4.03 (dd, 1H, J = 2.2, 0.8 Hz), 3.85 (t, 1H, J = 2.2 Hz), 1.76 (s, 3H), 1.39 (d, 3H, J = 6.8 Hz), 1.35 (d, 3H, J = 6.9 Hz), 0.91 (s, 9H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 165.9 (C), 109.7 (C), 90.9 (C), 78.6 (C), 71.1 (CH), 69.8 (5 CH), 62.8 (CH), 57.0 (CH), 37.8 (C), 35.3 (CH), 26.1 (3 CH₃), 23.6 (CH₃), 23.2 (CH₃), 23.1 (CH₃). MS (EI) m/z (% relative intensity): 369 [M]⁺ (18), 312 [M–t-C₄H₉]⁺ (61), 270 (100), 204 (15), 121 [C₅H₅Fe]⁺ (14), 56 [Fe]⁺ (6). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₀H₂₈FeNS, 370.1286; found, 370.1288.

rac-1-(Benzylthio)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (11e) and (1R*,S_p*)-3-(Benzylthio)-1-(tert-butyl)-1-methyl-1H-ferroceno[c]pyrrole (12e). The title compounds were prepared according to GP2 (reaction time = 25 min) and GP3 using benzyl thiocyanate (10e) (62.6 mg, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave, in order of elution, a mixture of 11e and 12e (80 mg, 55%, 11e/12e = 22:78) and 12e (54 mg, 37%). GP3: purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 100:1–petroleum ether/TEA 100:1) gave, in order of elution, a mixture of starting thiocyanate 10e and 11e (70 mg, 10e/11e = 16:84) and a mixture of 11e and 12e (57 mg, 39%, 11e/12e = 27:73). The mixture of 10e and 11e was additionally chromatographed on silica gel (benzene–benzene/TEA 100:1) to yield pure compound 11e (61 mg, 42%). Data for 11e: brown solid; mp 62.5–67.5 °C; R_f 0.79 (petroleum ether/EtOAc 25:1). IR (thin film): 3086, 3062, 3028, 2972, 2926, 2867, 2855, 1581, 1495, 1450, 1372, 1361, 1287, 1236, 1151, 1108, 1096, 1035, 1002, 932, 898, 823, 759, 700, 512, 477 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 7.42–7.40 (m, 2H), 7.16–7.11 (m, 3H), 7.05–7.01 (m, 1H), 4.44 (dd, 1H, J = 13.6 Hz), 4.40 (d, 1H, J = 13.6 Hz), 4.36 (dd, 1H, J = 2.4, 1.1 Hz), 4.05 (s, 5H), 3.97 (dd, 1H, J = 2.5, 1.1 Hz), 3.90 (t, 1H, J = 2.4 Hz), 1.52 (s, 3H), 1.38 (s, 3H), 0.97 (s, 3H), 0.89 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 161.1 (C), 139.9 (C), 129.6 (2 C), 128.5 (2 C), 127.0 (C), 99.1 (C), 74.6 (C), 70.5 (5 CH), 66.7 (CH), 65.8 (CH), 64.6 (C), 63.8 (CH), 35.7 (C), 32.8 (CH₂), 29.2 (CH₃), 25.2 (CH₃), 24.6 (CH₃), 24.3 (CH₃). MS (EI) m/z (% relative intensity): 417 [M]⁺ (69), 402 [M–CH₃]⁺ (2), 326 [M–CH₂C₆H₅]⁺ (40), 269 (100), 132 (9), 121 [C₅H₅Fe]⁺ (9), 56 [Fe]⁺ (2). Analysis calculated for C₂₄H₂₇FeNS: C, 69.06; H, 6.52; N, 3.36; S, 7.68. Found: C, 69.13; H, 6.75; N, 3.05; S, 7.83. Data for 12e: brown oil, R_f 0.65 (petroleum ether/EtOAc 25:1). IR (thin film): 3088, 3063, 3029, 2969, 2952, 2909, 2870, 1521, 1495, 1478, 1453, 1392, 1366, 1299, 1225, 1194, 1149, 1103, 1003, 929, 886, 821, 767, 699, 649, 508, 488, 472, 439 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆, 50 °C): δ 7.50–7.47 (m, 2H), 7.36–7.31 (m, 2H), 7.29–7.24 (m, 1H), 4.46 (d, 1H, J = 13.6 Hz), 4.41 (dd, 1H, J = 2.2, 0.7 Hz), 4.41 (d, 1H, J = 13.6 Hz), 4.31 (dd, 1H, J = 2.3, 0.7 Hz), 4.26 (t, 1H, J = 2.2 Hz), 4.14 (s, 5H), 1.71 (s, 3H), 0.74 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆, 50 °C): δ 164.3 (C), 138.4 (C), 128.6 (2 CH), 127.9 (2 CH), 126.7 (CH), 108.9 (C), 89.0 (C), 77.5 (C), 70.7 (CH), 69.0 (5 CH), 62.4 (CH), 56.2 (CH), 32.8 (CH₂), 25.2 (3 CH₃), 22.4 (CH₃). MS (EI) m/z (% relative intensity): 417 [M]⁺ (23), 360 [M–t-C₄H₉]⁺ (100), 269 (37), 121 [C₅H₅Fe]⁺ (7), 56 [Fe]⁺ (2). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₄H₂₈FeNS, 418.1286; found, 418.1294.

rac-1-(Hexylthio)-3,3,4,4-tetramethyl-3,4-dihydroferroceno[c]pyridine (11f) and (1R*,S_p*)-1-(tert-Butyl)-3-(hexylthio)-1-methyl-1H-ferroceno[c]pyrrole (12f). The title compounds were prepared according to GP2 (reaction time = 40 min) and GP3 using hexyl thiocyanate (10f) (0.065 mL, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/EtOAc 100:1–petroleum ether/TEA 100:1) gave, in order of elution, a mixture of 11f and 12f (38 mg, 26%, 11f/12f = 22:78) and 12f (94 mg, 65%). GP3: purification by silica gel column chromatography (petroleum ether–petroleum ether/EtOAc 200:1–petroleum ether/EtOAc 50:1–petroleum ether/TEA 100:1) gave, in order of elution, 11f (79 mg, 55%) and a mixture of 11f and 12f (45 mg, 31%, 11f/12f = 42:58). Data for 11f: brown oil; R_f 0.83 (petroleum ether/EtOAc 25:1). IR (thin film): 3097, 2970, 2927, 2857, 1580, 1449, 1372, 1361, 1286, 1240, 1151, 1141, 1108, 1097, 1035, 1002, 932, 899, 822, 512, 478, 453 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.44 (dd, 1H, J = 2.4, 1.2 Hz), 4.12 (s, 5H), 3.99 (dd, 1H, J = 2.4, 1.1 Hz), 3.93 (t, 1H, J = 2.4 Hz), 3.30–3.15 (m, 2H), 1.76–1.69 (m, 2H), 1.52 (s, 3H), 1.45–1.36 (m, 2H, partially overlapped), 1.41 (s, 3H), 1.28–1.22 (m, 4H), 0.99 (s, 3H), 0.90 (s, 3H), 0.88–0.85 (m, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 161.5 (C), 99.1 (C), 75.1 (C), 70.5 (5 CH), 66.6 (CH), 65.7 (CH), 64.4 (C), 63.9 (CH), 35.7 (C), 31.8 (CH₂), 30.1 (CH₂), 29.2 (CH₃), 29.0 (CH₂), 28.4 (CH₂), 25.2 (CH₃), 24.8 (CH₃), 24.3 (CH₃), 22.9 (CH₂), 14.2 (CH₃). MS (EI) m/z (% relative intensity): 411 [M]⁺ (100), 326 [M–C₆H₁₃]⁺ (58), 293 (28), 269 (84), 172 (13), 132 (11), 121 [C₅H₅Fe]⁺ (15), 117 (29), 56 [Fe]⁺ (3). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₃H₃₄FeNS, 412.1756; found, 412.1760. Data for 12f: brown oil; R_f 0.56 (petroleum ether/EtOAc 25:1). IR (thin film): 3096, 2955, 2930, 2871, 2858, 1521, 1478, 1453, 1425, 1391, 1366, 1298, 1286, 1225, 1193, 1149, 1108, 1035, 1003, 933, 887, 820, 648, 508, 487, 472, 439 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.11 (dd, 1H, J = 2.4, 0.7 Hz), 4.06 (s, 5H), 4.04 (dd, 1H, J = 2.3, 0.5 Hz), 3.86 (t, 1H, J = 2.2 Hz), 3.37–3.31 (m, 1H), 3.16–3.09 (m, 1H), 1.81–1.69 (m, 2H, partially overlapped), 1.77 (s, 3H), 1.41–1.33 (m, 2H), 1.27–1.18 (m, 4H), 0.92 (s, 9H), 0.87–0.83 (m, 3H). ¹³C NMR (100 MHz, C₆D₆, 50 °C): 166.1 (C), 110.1 (C), 90.8 (C), 78.5 (C), 71.1 (CH), 69.9 (5 CH), 62.9 (CH), 57.0 (CH), 37.8 (C), 31.8 (CH₂), 30.3 (CH₂), 30.2 (CH₂), 28.9 (CH₂), 26.1 (3 CH₃), 23.2 (CH₃), 22.9 (CH₂), 14.1 (CH₃). MS (EI) m/z (% relative intensity): 411 [M]⁺ (17), 354 [M–t-C₄H₉]⁺ (100), 270 (21), 121 [C₅H₅Fe]⁺ (3), 56 [Fe]⁺ (1). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₃H₃₄FeNS, 412.1756; found, 412.1762.

rac-Ethyl 2-((3,3,4,4-Tetramethyl-3,4-dihydroferroceno[c]pyridine)thio)acetate (11g) and (1R*,S_p*)-Ethyl 2-((1-(tert-Butyl)-1-methyl-1H-ferroceno[c]pyrrole)thio)acetate (12g). The title compounds were prepared according to GP2 (reaction time = 1 h) and GP3 (reaction time = 15 min) using ethyl 2-thiocyanatoacetate (10g) (0.064 mL, 0.53 mmol). GP2: purification by silica gel column chromatography (petroleum ether/EtOAc 25:1) gave, in order of elution, 11g (8 mg, 6%; according to ¹H NMR analysis contained ~1% of 3u; 11g/3u = 99:1) and 12g (106 mg, 73%). GP3: purification by silica gel column chromatography (petroleum ether/EtOAc 25:1) gave, in order of elution, 11g (41 mg, according to ¹H NMR analysis contained 4% of 3u; 11g/3u = 96:4; calculated yield of 11g: 27%) and 12g (38 mg, 26%). Data for 11g: brown oil; R_f 0.25 (petroleum ether/EtOAc 25:1). IR (thin film): 3096, 2975, 2927, 2869, 2856, 1739, 1647, 1587, 1463, 1448, 1389, 1372, 1363, 1289, 1261, 1242, 1151, 1108, 1097, 1034, 1002, 933, 899, 823, 744, 712, 627, 583, 512, 479, 453 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 30 °C): δ 4.36 (dd, 1H, J = 2.5, 1.1 Hz), 4.13 (s, 5H), 4.02–3.97 (m, 4H), 3.91 (t, 1H, J = 2.4 Hz), 3.77 (d, 1H, J = 15.8 Hz), 1.44 (s, 3H), 1.37 (s, 3H), 0.99 (t, 3H, J = 7.1 Hz), 0.95 (s, 3H), 0.85 (s, 3H). ¹³C NMR (100 MHz, C₆D₆, 30 °C): δ 169.2 (C), 160.2 (C), 99.2 (C), 74.2 (C), 70.6 (5 CH), 66.8 (CH), 65.8 (CH), 64.6 (C), 63.6 (CH), 61.0 (CH₂), 35.8 (C), 31.2 (CH₂), 29.1 (CH₃), 25.2 (CH₃), 24.5 (CH₃), 24.1 (CH₃), 14.3 (CH₃). GC–MS analyses of product 11g indicated the presence in the chromatogram of a peak that, based on corresponding MS spectra, belonged to compound 3o. The latter was likely produced by the thermolysis of the parent compound in the GC instrument injector. HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₁H₂₈FeNO₂S, 414.1185; found, 414.1186. Data for 12g: orange oil, which solidified on long-term standing; mp 51–52.5 °C; R_f 0.32 (petroleum ether/EtOAc 25:1). IR (thin film): 3096, 2973, 2954, 2908, 2871, 1740, 1527, 1478, 1452, 1392, 1367, 1294, 1262, 1226, 1153, 1104, 1032, 1004, 934, 887, 822, 650, 488 cm⁻¹. ¹H NMR (400 MHz, C₆D₆, 50 °C): δ 4.07 (s, 5 CH), 4.05 (dd, 1H, J = 2.4, 0.6 Hz), 4.04 (dd, 1H, J = 2.2, 0.7 Hz), 4.00–3.95 (m, 2H), 3.93 (d, 1H, J = 15.7 Hz, partially overlapped), 3.85 (d, 1H, J = 15.8 Hz), 3.86 (t, 1H, J = 2.3 Hz), 1.71 (s, 3H), 0.98 (t, 3H, J = 7.1 Hz), 0.86 (s, 9H). ¹³C NMR (100 MHz, C₆D₆, 50 °C): 168.6 (C), 164.8 (C), 110.4 (C), 89.6 (C), 78.7 (C), 71.2 (CH), 70.0 (5 CH), 63.1 (CH), 61.2 (CH₂), 57.0 (CH), 37.7 (C), 32.5 (CH₂), 25.9 (3 CH₃), 23.0 (CH₃), 14.2 (CH₃). GC–MS analysis of compound 12g indicated the presence of a peak of substance with a molecular mass of 309. MS (EI) m/z (% relative intensity): 309 [M]⁺ (31), 253 [M–C₄H₈]⁺ (60), 252 [M–t-C₄H₉]⁺ (100), 211 (8), 121 [C₅H₅Fe]⁺ (11), 56 [Fe]⁺ (3). Apparently, this is a 1-(tert-butyl)-1,3-dimethyl-1H-ferroceno[c]pyrrole yielded by the thermolysis of 12g in the GC injector. The proposed mechanism for the thermolysis of ferroceno[c]pyrrole 12g is performed in Supplementary Materials (Scheme S2). HRMS-ESI (m/z): [M + H]⁺ calculated for C₂₁H₂₈FeNO₂S, 414.1185; found, 414.1191.

3.2.6. Reaction of Alcohol 1 with 2-Thiocyanatoacetamide (10h) (Table 6, Entries 3, 4)

According to GP2 (reaction time = 48 h) and GP3 using 10h (48 mg, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/EtOAc 25:1) gave, in order of elution, an inseparable mixture of alkenes 5 and 6 (13 mg, 14%, 5/6 = 77:23), recovered alcohol 1 (37 mg, 37%) and alcohol 13 (13 mg, 13%). GP3: complex mixture of unidentified products was formed. 3-Ferrocenyl-2,3-dimethylbutan-2-ol (13): yellow solid; mp 61.5–63 °C (hexane); R_f 0.25 (petroleum ether/EtOAc 25:1). IR (thin film): 3348, 2958, 2924, 2871, 2853, 1737, 1710, 1652, 1547, 1462, 1457, 1380, 1138, 1105, 1081, 1026, 1003, 879, 818, 529, 486 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 4.15 (s, 5H), 4.13–4.12 (m, 2H), 4.09–4.08 (m, 2H), 1.37 (s, 6H), 1.21 (br s, 1H), 1.05 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 97.4 (C), 74.4 (C), 68.7 (5 CH), 67.9 (2 CH), 67.3 (2 CH), 41.5 (C), 25.6 (2 CH₃), 24.0 (2 CH₃). MS (EI) m/z (% relative intensity): 286 [M]⁺ (27), 229 (51), 186 [C₅H₅FeC₅H₅]⁺ (100), 149 (13), 121 [C₅H₅Fe]⁺ (8), 56 [Fe]⁺ (2). HRMS-ESI (m/z): [M]⁺ calculated for C₁₆H₂₂FeO, 286.1015; found, 286.1011.

Data for alkenes 5 and 6 see below.

3.2.7. Reaction of Alcohol 1 with Phenacyl Thiocyanate (10i) (Table 6, Entries 5, 6)

According to GP2 (reaction time = 48 h) and GP3 (reaction time = 40 min) using 10i (74 mg, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/CH₂Cl₂ 1:1–petroleum ether/acetone 5:1) gave, in order of elution, an inseparable mixture of alkenes 5 and 6 (17 mg, 18%, 5/6 = 55:45), recovered alcohol 1 (20 mg, 20%), thiocyanate 10i (28 mg, 38%) and alcohol 13 (51 mg, 51%). GP3: purification by silica gel column chromatography (benzene–benzene/TEA 100:1) gave, in order of elution, an inseparable mixture of alkenes 5 and 6 (39 mg, 42%, 5/6 = 91:9), recovered thiocyanate 10i (29 mg, 39%) and a mixture of compound 3w and thiocyanate 10i (31 mg, 3w/10i = 33:67). The mixture of 3w and 10i was additionally chromatographed (benzene–benzene/TEA 100:1) to yield thiocyanate 10i (18 mg, 24%) and pure compound 3w (11 mg, 8%).

3.2.8. Reaction of Alcohol 1 with 2-Oxopropyl Thiocyanate (10j) (Table 6, Entries 7, 8)

According to GP2 (reaction time = 48 h) and GP3 using 10j (0.04 mL, 0.42 mmol). GP2: purification by silica gel column chromatography (petroleum ether/EtOAc 10:1, 5:1) gave, in order of elution, an inseparable mixture of alkenes 5 and 6 (21 mg, 22%, 5/6 = 56:44) and alcohol 13 (48 mg, 48%). GP3: purification by silica gel column chromatography (petroleum ether/EtOAc 10:1, 5:1) gave, in order of elution, an inseparable mixture of alkenes 5 and 6 (19 mg, 20%, 5/6 = 94:6), recovered alcohol 1 (38 mg, 38%) and 15 mg of a mixture of unidentified products containing trace amounts of compound 3x according to NMR ¹H analysis.

3.2.9. Optimization of the Reaction of Alcohol 1 with 4-Methylbenzonitrile (2a). Representative Examples (Table 1, Entries 1, 10 and 11)

Table 1, entry 1. Nitrile 2a (0.05 mL, 0.42 mmol) was added to a stirred solution of alcohol 1 (100 mg, 0.35 mmol) in MeSO₃H (0.18 mL) at room temperature, and the resulting mixture was stirred at this temperature for 5 h. The reaction mixture was then worked up as described in GP. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, amide 4a (9 mg, 6%) and ferroceno[c]pyridine 3a (79 mg, 59%).

Table 1, entry 10. Nitrile 2a (0.05 mL, 0.42 mmol) was added to a stirred solution of alcohol 1 (100 mg, 0.35 mmol) in CF₃COOH (0.21 mL) at room temperature, and the resulting mixture was heated at 60 °C in an oil bath with vigorous stirring for 30 min. The reaction mixture was then worked up as described in GP. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, inseparable mixture of alkenes 5 and 6 (18 mg, 19%, 5/6 = 20:80) and ferroceno[c]pyridine 3a (55 mg, 41%).

Table 1, entry 11. Nitrile 2a (0.05 mL, 0.42 mmol) was added to a stirred solution of alcohol 1 (100 mg, 0.35 mmol) in CH₃COOH (0.16 mL) at room temperature, and the resulting mixture was heated at 60 °C in an oil bath with vigorous stirring for 2.5 h. The reaction mixture was then worked up as described in GP. Purification by silica gel column chromatography (petroleum ether/EtOAc 25:1–petroleum ether/TEA 100:1) gave, in order of elution, alkene 5 (30 mg, 32%) and recovered alcohol 1 (42 mg, 42%).

N-(2,3-Dimethyl-3-ferrocenylbutan-2-yl)-4-methylbenzamide (4a): yellow solid; mp 115.5–117 °C (hexane); R_f 0.39 (petroleum ether/EtOAc 25:1). IR (thin film): 3419, 3095, 2955, 2925, 2870, 2854, 1667, 1612, 1525, 1496, 1457, 1393, 1374, 1299, 1284, 1261, 1190, 1154, 1143, 1116, 1107, 1039, 1034, 1001, 890, 878, 822, 750, 514, 495, 447 cm⁻¹. ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 7.47–7.44 (m, 2H), 7.15–7.13 (m, 2H), 6.06 (br s, NH), 4.20 (t, 2H, J = 1.8 Hz), 4.18 (s, 5H), 4.15 (t, 2H, J = 1.9 Hz), 2.34 (s, 3H), 1.44 (s, 6H), 1.42 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 166.7 (C), 141.2 (C), 133.8 (C), 129.2 (CH), 126.7 (2 CH), 97.1 (C), 68.9 (5 CH), 67.8 (CH), 67.7 (CH), 59.1 (C), 41.6 (C), 24.1 (2 CH₃), 22.7 (2 CH₃), 21.5 (CH₃). MS (EI) m/z (% relative intensity): 403 [M]⁺ (6), 227 (65), 176 (21), 121 [C₅H₅Fe]⁺ (16), 119 [CH₃C₆H₄CO]⁺ (100), 91 [CH₃C₆H₄]⁺ (15), 56 [Fe]⁺ (3). Analysis calculated for C₂₄H₂₉FeNO: C, 71.47; H, 7.25; N, 3.47. Found: C, 71.13; H, 7.66; N, 3.25.

(3,3-Dimethylbut-1-en-2-yl)ferrocene (5): Orange oil. R_f 0.90 (petroleum ether/EtOAc 25:1). NMR data are in agreement with that previously published [60]. MS (EI) m/z (% relative intensity): 268 [M]⁺ (100), 238 [M–C₂H₆]⁺ (10), 211 [M–t-C₄H₉]⁺ (14), 121 [C₅H₅Fe]⁺ (19), 56 [Fe]⁺ (8). Analysis calculated for C₁₆H₂₀: C, 71.66; H, 7.52. Found: C, 71.93; H, 7.58.

(2,3-Dimethylbut-3-en-2-yl)ferrocene (6) [spectroscopic data for 6 were obtained only from the mixture of 5 and 6]: R_f 0.90 (petroleum ether/EtOAc 25:1). ¹H NMR (400 MHz, CDCl₃, 30 °C): δ 4.58–4.57 (m, 2H), 4.16 (s, 5H), 4.11–4.10 (m, 2H), 4.03 (t, 2H, J = 1.9 Hz), 1.61 (t, 3H, J = 1.1 Hz), 1.47 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, 30 °C): δ 154.0 (C), 108.6 (CH₂), 99.9 (C), 68.5 (5 CH), 66.9 (2 CH), 66.7 (2 CH), 38.8 (C), 27.8 (2 CH₃), 20.2 (CH₃). MS (EI) m/z (% relative intensity): 268 [M]⁺ (100), 253 [M–CH₃]⁺ (34), 238 [M–C₂H₆]⁺ (26), 227 [M–C₃H₅]⁺ (26), 186 [C₅H₅FeC₅H₅]⁺ (10), 121 [C₅H₅Fe]⁺ (22), 56 [Fe]⁺ (7).

3.3. X-ray Diffraction Analysis

Single crystals of compounds 3a, 11a, and 12a suitable for X-ray diffraction analysis were obtained by slow crystallization from hexane. X-ray data for 3a, 11a and 12a were collected on an Xcalibur Ruby diffractometer equipped with the CCD detector using the standard procedure (MoK-irradiation, graphite monochromator, ω-scans with 1^o step at T = 295(2) K). The empirical absorption correction was introduced by multi-scan method using SCALE3 ABSPACK algorithm [63]. Using OLEX2 [64], the structures were solved with the SHELXS program [65] and refined by the full-matrix least-squares method in the anisotropic approximation for all non-hydrogen atoms with the SHELXL program [66]. Hydrogen atoms were included in the refinement using a rider model with dependent isotropic thermal parameters. Crystallographic data for compounds 3a, 11a, and 12a have been deposited with the Cambridge Crystallographic Data Centre (CCDC Nos. 2210002, 2210003, 2210004). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44(0)-1223-336033 or e-mail: [email protected]].

Compound3a (CCDC No. 2210002). C₂₄H₂₇FeN, orange crystal, M = 385.31; crystal data: monoclinic, space group P2₁/n, a = 10.2339(17) Å, b = 17.372(3) Å, c = 10.952(2) Å, β = 103.462(19)°, V = 1893.59 ų, Z = 4, d_calc = 1.352 g/cm³, μ(Mo Kα) = 0.803 mm^–1; 9378 reflections measured, 4464 unique (R_int = 0.0312) which were used in all calculations. The final wR₂ was 0.1299 (all data) and R₁ was 0.0473 (I > 2σ(I)).

Compound11a (CCDC No. 2210003). C₁₉H₂₅FeNS, red crystal, M = 355.31; crystal data: monoclinic, space group P2₁/c, a = 8.8836(13) Å, b = 15.552(2) Å, c = 13.226(2) Å, β = 108.990(17)°, V = 1727.83 ų, Z = 4, d_calc = 1.366 g/cm³, μ(Mo Kα) = 0.990 mm^–1; 12,014 reflections measured, 3989 unique (R_int = 0.0315) which were used in all calculations. The final wR₂ was 0.0927 (all data) and R₁ was 0.0366 (I > 2σ(I)).

Compound12a (CCDC No. 2210004). C₁₉H₂₅FeNS, red crystal, M = 355.31; crystal data: monoclinic, space group P2₁/c, a = 7.5335(12) Å, b = 17.276(3) Å, c = 13.931(3) Å, β = 96.182(15)°, V = 1802.56 ų, Z = 4, d_calc = 1.309 g/cm³, μ(Mo Kα) = 0.949 mm^–1; 9501 reflections measured, 4197 unique (R_int = 0.0257) which were used in all calculations. The final wR₂ was 0.0987 (all data) and R₁ was 0.0380 (I > 2σ(I)).

4. Conclusions

In summary, we demonstrated that 2-ferrocenyl-3,3-dimethylbutan-2-ol was a suitable substrate in the intramolecular Ritter reaction. Condensation of starting alcohol with a variety of nitriles in the presence of MeSO₃H yielded novel 3,4-dihydroferroceno[c]pyridines. The reaction of 2-ferrocenyl-3,3-dimethylbutan-2-ol with thiocyanates gave not only 3,4-dihydroferroceno[c]pyridines but also 1H-ferroceno[c]pyrroles. The selectivity of this reaction depended on the temperature, the order of addition, and the size of substituents at the α-position to the sulfur atom of thiocyanates. The simplicity of the procedure and the availability of starting materials make the reaction of 2-ferrocenyl-3,3-dimethylbutan-2-ol with nitriles and thiocyanates very convenient and attractive to the synthesis of variously functionalized ferrocene-fused aza-heterocycles.