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Article

Rhodium-Catalyzed Linear Codimerization and Cycloaddition of Ketenes with Alkynes

by
Teruyuki Kondo
1,2,*,
Masatsugu Niimi
2,
Yuki Yoshida
2,
Kenji Wada
2,
Take-aki Mitsudo
2,
Yu Kimura
1 and
Akio Toshimitsu
3
1
Advanced Biomedical Engineering Research Unit, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
2
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
3
Office of Society-Academia Collaboration for Innovation, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
*
Author to whom correspondence should be addressed.
Molecules 2010, 15(6), 4189-4200; https://doi.org/10.3390/molecules15064189
Submission received: 11 May 2010 / Revised: 25 May 2010 / Accepted: 7 June 2010 / Published: 9 June 2010
(This article belongs to the Special Issue Cycloaddition Reactions in Organic Synthesis)
Browse Figures
Versions Notes

Abstract

:
A novel rhodium-catalyzed linear codimerization of alkyl phenyl ketenes with internal alkynes to dienones and a novel synthesis of furans by an unusual cycloaddition of diaryl ketenes with internal alkynes have been developed. These reactions proceed smoothly with the same rhodium catalyst, RhCl(PPh3)3, and are highly dependent on the structure and reactivity of the starting ketenes.

Graphical Abstract

1. Introduction

Ketenes are very important intermediates in the field of organic synthesis [1,2,3], and much attention has been focused on the ketene-metal complexes [4]. In general, ketenes coordinate to transition-metal complexes in two ways: 1) coordination through a C=C bond in ketenes [5], and 2) coordination through a C=O bond in ketenes [6,7,8,9,10]. If these coordination modes can be controlled through the selection of ketenes in transition-metal catalysis, completely different methods for the construction of novel organic molecules could be developed according to the structure and reactivity of ketenes using the same transition-metal catalyst.
We have previously developed a ruthenium-catalyzed synthesis of pyranopyrandiones by the ring-opening carbonylation of cyclopropenones [11] and a novel synthesis of 2-pyranones by the ruthenium- or rhodium catalyzed ring-opening dimerization of cyclobutenones [12], as well as a rhodium-catalyzed synthesis of 2-substituted phenols from cyclobutenones and alkenes via cleavage of a carbon-carbon bond [13]. We propose that (η4-bisketene)- and (η4-vinylketene)metal complexes are important key intermediates in these reactions; however, there are still few examples of transition-metal complex-catalyzed transformations of ketenes themselves [14,15,16,17,18,19,20]. Thus, we focused our attention on the development of novel reactions of ketenes with unsaturated compounds in the presence of ruthenium or rhodium catalysts, and recently developed rhodium-catalyzed decarbonylative coupling reactions of diphenyl ketene with 2-norbornenes and electron-deficient alkenes [21]. Then, the reactions of ketenes with alkynes were investigated in the presence of several transition-metal catalysts. After many trials, we developed the novel RhCl(PPh3)3-catalyzed linear codimerization of alkyl phenyl ketenes with internal alkynes and a novel synthesis of furans by the unusual RhCl(PPh3)3-catalyzed cycloaddition of diaryl ketenes with internal alkynes. In these reactions, the catalyst is the same but the products are completely different, depending on the structure and reactivity of the starting ketenes.

2. Results and Discussion

Treatment of alkyl phenyl ketenes 1a-c with internal alkynes 2 in the presence of 5 mol % RhCl(PPh3)3 in mesitylene at 120 ºC for 12 h under an argon atmosphere gave the corresponding dienones 3 in high yield with high stereoselectivity (Scheme 1). For example, RhCl(PPh3)3-catalyzed reaction of ethyl phenyl ketene (1a) with 3-hexyne (2a) gave only (2Z, 5E)-5-ethyl-3-phenylocta-2,5-dien-4-one (3a) in 92% yield, and no stereoisomers were obtained at all.
Molecules 15 04189 g003 550
Scheme 1. Rhodium-catalyzed linear codimerization of ketenes with alkynes to dienones.
Scheme 1. Rhodium-catalyzed linear codimerization of ketenes with alkynes to dienones.
Molecules 15 04189 g003
First, the catalytic activities of several transition-metal complexes were examined in the reaction of 1a with 2a, and the results are summarized in Table 1. Among the catalysts examined, RhCl(PPh3)3 (3a, 92%) showed the highest catalytic activity. RhCl(CO)(PPh3)2 (3a, 46%) and RhCl3.3H2O (3a, 26%) also showed moderate catalytic activity; however, other rhodium complexes, such as RhH(PPh3)4 and RhH(CO)(PPh3)3, as well as ruthenium complexes, such as RuCl2(PPh3)3, [RuCl2(CO)3]2, and RuH2(PPh3)4, and an iridium complex, IrCl(CO)(PPh3)2, were totally ineffective, whereas Pd(PPh3)4 showed slight catalytic activity (3a, 29%). An attempt to reduce the amount of RhCl(PPh3)3 catalyst from 5.0 mol % to 2.0 mol % resulted in vain (Entry 2).
Table
Table 1. Catalytic activity of several transition-metal complexes in the reaction of 1a with 2a to 3a.
Table 1. Catalytic activity of several transition-metal complexes in the reaction of 1a with 2a to 3a.
Molecules 15 04189 i019
EntryCatalystYield of 3a (%) aEntryCatalystYield of 3a (%) a
1RhCl(PPh3)3­927RuCl2(PPh3)31
2 bRhCl(PPh3)35 8 c[RuCl2(CO)3]20
3RhCl(CO)(PPh3)2469RuH2(PPh3)40
4RhCl3·3H2O2610Pd(PPh3)429
5RhH(PPh3)4211IrCl(CO)(PPh3)20
6RhH(CO)(PPh3)30
a GLC yield; b RhCl(PPh3)3 (2.0 mol %, 0.020 mmol) for 40 h; c [RuCl2(CO)3]2 (0.025 mmol).
Table
Table 2. RhCl(PPh3)3-catalyzed linear codimerization of alkyl phenylketenes 1a-c with internal alkynes 2a,b to dienones 3a-d.a
Table 2. RhCl(PPh3)3-catalyzed linear codimerization of alkyl phenylketenes 1a-c with internal alkynes 2a,b to dienones 3a-d.a
EntryKeteneAlkyneProductIsolated Yield (%)
1 Molecules 15 04189 i0011a Molecules 15 04189 i002 Molecules 15 04189 i003 3a 74 (92) b
2a
2 1a Molecules 15 04189 i004 Molecules 15 04189 i0053b 68
2b
3 Molecules 15 04189 i0061b 2a Molecules 15 04189 i0073c 52
4 Molecules 15 04189 i0081c 2a Molecules 15 04189 i0093d 40
a Ketene (1.0 mmol), alkyne (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) at 120 ºC for 12 h under an argon atmosphere; b GLC yield.
The results obtained in the reaction of several alkyl phenyl ketenes 1a-c with alkynes 2a and b under the optimized reaction conditions are summarized in Table 2. Ethyl phenyl ketene (1a) reacted with 6-dodecyne (2b) to give the corresponding dienone 3b in an isolated yield of 68% (Entry 2). As for ketenes, cycloalkyl phenyl ketenes, such as 1b and 1c, also reacted with 2a to give the corresponding dienones, 3c and 3d, in isolated yields of 52% and 40%, respectively (Entries 3 and 4). Unfortunately, when terminal alkynes, such as phenylacetylene and 1-hexyne, were used in RhCl(PPh3)3-catalyzed reaction with ethyl phenyl ketene (1a), the corresponding dienones were obtained in low yield (below 10%) together with various byproducts, probably due to the formation of a (vinylidene)Rh species.
In sharp contrast, treatment of diaryl ketenes 1d and e instead of alkyl phenyl ketenes 1a-c with internal alkynes 2 in the presence of the same RhCl(PPh3)3 catalyst (5 mol %) in mesitylene at 120 ºC for 12 h under an argon atmosphere gave unusual cycloadducts, the furans 4, instead of dienones 3, in good to high yields (Scheme 2). The structure of furan 4a was confirmed by 13C Inadequate NMR measurement (see Experimental, Figure 2).
Molecules 15 04189 g004 550
Scheme 2. Rhodium-catalyzed cycloaddition of ketenes with alkynes to furans.
Scheme 2. Rhodium-catalyzed cycloaddition of ketenes with alkynes to furans.
Molecules 15 04189 g004
The catalytic activities of several transition-metal complexes were also examined in the reaction of diphenyl ketene (1d) with 3-hexyne (2a), and the results are summarized in Table 3. Among the catalysts examined, only RhCl(PPh3)3 showed catalytic activity (4a, 74%). Other rhodium, ruthenium, iridium and palladium complexes were totally ineffective in the present reaction.
Table
Table 3. Catalytic activity of several trasition-metal complexes in the reaction of 1d with 2a to 4a.
Table 3. Catalytic activity of several trasition-metal complexes in the reaction of 1d with 2a to 4a.
Molecules 15 04189 i020
EntryCatalystYield of 4a (%) aEntryCatalystYield of 4a (%) a
1RhCl(PPh3)3­747[Cp*RuCl2]20
2RhCl(CO)(PPh3)228RuCl2(PPh3)30
3RhCl3·3H2O09[RuCl2(CO)3]20
4[RhCl(cod)]2010IrCl(CO)(PPh3)20
5[RhCl(C2H4)2]2011Pd(PPh3)40
6RhH(PPh3)40
a GLC yield.
6-Dodecyne (2b), as well as 4-octyne (2c) and 5-decyne (2d), reacted with 1d to give the corresponding furans 4b-d in moderate yields (Entries 2-4 in Table 4). As for ketenes, di(4-chloro-phenyl) ketene (1e) also reacted with 2a to give the corresponding furan 4e in an isolated yield of 51% (Entry 5).
Table
Table 4. RhCl(PPh3)3-catalyzed unusual cycloaddition of diaryl ketenes 1d, e with internal alkynes 2a-d to furans 4a-e.a
Table 4. RhCl(PPh3)3-catalyzed unusual cycloaddition of diaryl ketenes 1d, e with internal alkynes 2a-d to furans 4a-e.a
EntryKeteneAlkyneProductIsolated Yield (%)
1 Molecules 15 04189 i0101d Molecules 15 04189 i002 Molecules 15 04189 i0114a 64 (74) b
2a
2 1d Molecules 15 04189 i004 Molecules 15 04189 i0124b 43
2b
3 1d Molecules 15 04189 i013 Molecules 15 04189 i0144c 52
2c
4 1d Molecules 15 04189 i015 Molecules 15 04189 i0164d 43
2d
5 Molecules 15 04189 i0171e 2a Molecules 15 04189 i0184e 51 (70)b
a Ketene (1.0 mmol), alkyne (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) at 120 ºC for 12 h under an argon atmosphere; b GLC yield.
While the reaction mechanism is not yet clear, the possible mechanisms are illustrated in Scheme 3 and Scheme 4. Scheme 3 shows a possible mechanism of the reaction of alkyl phenyl ketenes 1a-c with internal alkynes 2 to give dienones 3. We now believe that the initial step is the coordination of alkyl phenyl ketenes 1 to an active rhodium center through a C=C bond in ketenes. Oxidative cyclization of alkyl phenyl ketenes 1a-c with alkynes 2 would give rhodacyclopentenone intermediates I [5]. Stereoselective β-hydrogen elimination, followed by reductive elimination, would give the corresponding dienones 3 stereoselectively. In addition, we now consider that a catalytically active species is a Rh(I) bearing a chloro ligand, and RhCl3.3H2O would be reduced to a Rh(I)-Cl species by crystal water to show some catalytic activity (Entry 4 in Table 1).
Molecules 15 04189 g005 550
Scheme 3. A possible mechanism of linear codimerization of alkyl phenyl ketenes 1a-c with internal alkynes 2 to give dienones 3.
Scheme 3. A possible mechanism of linear codimerization of alkyl phenyl ketenes 1a-c with internal alkynes 2 to give dienones 3.
Molecules 15 04189 g005
Molecules 15 04189 g006 550
Scheme 4. A possible mechanism of the synthesis of furans 4 by unusual cycloaddition of diaryl ketenes 1d and e with internal alkynes 2.
Scheme 4. A possible mechanism of the synthesis of furans 4 by unusual cycloaddition of diaryl ketenes 1d and e with internal alkynes 2.
Molecules 15 04189 g006
On the other hand, a possible mechanism for the reaction of diaryl ketenes 1d and e with internal alkynes 2 to furans 4 is shown in Scheme 4. In the synthesis of furans 4, the reaction starts from the coordination of diaryl ketenes to an active rhodium center through a C=O bond in ketenes (not through a C=C bond in ketenes). Oxidative cyclization of diaryl ketenes 1d, and e with alkynes 2 gives an oxametallacycle intermediate II [22,23,24,25]. β-Hydrogen elimination and insertion of an allenyl group in an intermediate III into a Rh-H bond, followed by reductive elimination/isomerization, would give the desired furans 4.

3. Experimental

3.1. General

GLC analyses were carried out on a Shimadzu GC-18A gas chromatograph equipped with a glass column (2.8 mm i.d. × 3 m) packed with Silicone OV-17 (2% on Chromosorb W(AW-DMCS), 60–80 mesh). Recycling preparative HPLC was performed with an LC-918 (Japan Analytical Industry Co. Ltd.) equipped with JAIGEL-1H and 2H columns (GPC) using CHCl3 as an eluent. 1H-NMR spectra were recorded at 300 or 400 MHz, and 13C-NMR spectra were recorded at 75 or 100 MHz. Samples were analyzed in CDCl3, and the chemical shift values are expressed relative to Me4Si as an internal standard. IR spectra were obtained on a Nicolet Impact 410 spectrometer. Elemental analyses were performed at the Microanalytical Center of Kyoto University.

3.2. Materials

Ketenes were synthesized as described in the literature [26,27]. Alkynes were obtained commercially and purified before use by standard procedures. RhCl3.·3H2O, [RhCl(cod)]2, [Cp*RhCl2]2, [RuCl2(CO)3]2, IrCl(CO)(PPh3)2, and Pd(PPh3)4 were obtained commercially and used without further purification. RhCl(PPh3)3 [28], RhCl(CO)(PPh3)2 [29], RhH(PPh3)4 [30], RhH(CO)(PPh3)3 [31], [RhCl(C2H4)2]2 [32], RuCl2(PPh3)3 [33], and RuH2(PPh3)4[34] were prepared as described in the literature.

3.3. General procedure for the rhodium-catalyzed reaction of ketenes with alkynes to give dienones and furans

A mixture of ketene 1 (1.0 mmol), alkyne 2 (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) was placed in a two-necked 20-mL Pyrex flask equipped with a magnetic stirring bar and a reflux condenser under a flow of argon. The reaction was carried out at 120 ºC for 12 h with stirring. After the reaction mixture was cooled, the products were analyzed by GLC and isolated by Kugelrohr distillation followed by recycling preparative HPLC.
(2Z,5E)-5-Ethyl-3-phenylocta-2,5-dien-4-one (3a). Yellow liquid; b.p. 130 ºC (3.0 mmHg, Kugelrohr); IR (cm-1) 1652 (CO); 1H-NMR (CDCl3, 300 MHz): δ 0.98 (t, 3H, J = 7.52 Hz, C8-H), 1.03 (t, 3H, J = 7.52 Hz, 5-CH2CH3), 1.72 (d, 3H, J = 7.16 Hz, C1-H), 2.24 (dq, 2H, J = 7.52 Hz, C7-H), 2.42 (q, 2H, J =7.52 Hz, 5-CH2CH3), 6.15 (q, 1H, J = 7.16 Hz, C2-H), 6.64 (t, 1H, J = 7.52 Hz, C6-H), 7.19–7.37 (m, 5H, 3-phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.09 (5-CH2CH3), 13.79 (C8), 15.33 (C1), 18.12 (5-CH2CH3), 22.22 (C7), 125.06 (C2), 125.55 (3-phenyl), 127.25 (3-phenyl), 128.48 (3-phenyl), 129.19 (C3 or C5), 142.85 (3-phenyl-C1), 145.41 (C3 or C5), 150.28 (C6), 200.61 (C4); MS (EI) m/z 228 (M+). Anal. Calcd. for C16H20O: C 84.36, H 9.00. Found: C 84.14, H 8.83. A nuclear Overhauser enhancement (NOE) study was undertaken to determine the stereochemistry of dienone 3a. Irradiation of olefinic CH at δ 6.15 ppm gave a 6.7% NOE of the phenyl group at δ 7.26–7.28 ppm, while irradiation of CH2 in ethyl group at δ 2.24 ppm showed 5.2% NOE with CH3 in the other ethyl group at δ 1.03 ppm. The stereochemistry of 3a was therefore assigned to 2Z and 5E (Figure 1). The same method was used to determine the stereochemistry of 3b-d.
Molecules 15 04189 g001 550
Figure 1. The NOE study of 3a.
Figure 1. The NOE study of 3a.
Molecules 15 04189 g001
(2Z,5E)-5-Pentyl-3-phenylundeca-2,5-dien-4-one (3b). Colorless liquid; b.p. 140 ºC (1.0 mmHg, Kugelrohr); IR (cm-1): 1652 (CO) ; 1H-NMR (CDCl3, 400 MHz): δ = 0.84 (t, 3H, C11-H), 0.89 (t, 3H, 5-CH2(CH2)3CH3), 1.17–1.35(m, 12H, 5-CH2(CH2)3CH3/C8-H/C9-H/C10-H), 1.72 (d, 3H, J = 7.16 Hz, C1-H), 2.21 (dt, 2H, C7-H), 2.37 (t, 2H, 5-CH2(CH2)3CH3), 6.14 (q, 1H, J = 7.16 Hz, C2-H), 6.66 (t, 1H, C6-H), 7.18-7.31 (m, 5H, 3-phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.93, 14.04, 15.38 (C1), 22.40, 22.51, 24.97, 28.09, 28.61, 29.03, 31.89, 44.87, 125.09 (C2), 125.63 (3-phenyl), 127.14 (3-phenyl), 128.61(3-phenyl), 134.58 (C3 or C5), 141.94 (3-phenyl-1), 144.80 (C3 or C5), 149.70 (C6), 200.62 (C4); MS (EI) m/z 312 (M+).
(3E)-1-Cyclopentylidene-3-ethyl-1-phenylhex-3-en-2-one (3c). Yellow liquid; b.p. 140 oC (2.0 mmHg, Kugelrohr); IR (cm-1) 1650 (CO); 1H-NMR (CDCl3, 400 MHz): δ 0.96 (t, 3H, J =7.52 Hz, C6-H), 0.98 (t, 3H, J = 7.52 Hz, 3-CH2CH3), 1.61-1.71 (m, 4H, 1-cyclopentylidene-H), 2.21 (q, 2H, J = 7.52 Hz, 1-cyclopentylidene-H), 2.30-2.46 (m, 6H, 1-cyclopentylidene-H/C5-H/3-CH2CH3), 6.63 (t, 1H, J = 7.52 Hz, C4-H), 7.17-7.33 (m, 5H, 1-phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.31 (3-CH2CH3), 13.77 (C6), 18.51 (3-CH2CH3), 22.19 (C5), 26.34 (1-cyclopentylidene), 26.38 (1-cyclopentylidene), 32.18 (1-cyclopentylidene), 32.39 (1-cyclopentylidene), 126.50 (1-phenyl), 127.82 (1-phenyl), 128.06 (1-phenyl), 132.91 (1-cyclopentylidene), 138.53 (C1 or C3), 142.08 (1-phenyl-1), 146.26 (C1 or C3), 148.48 (C4), 200.16(C2); MS (EI) m/z 268 (M+).
(3E)-1-Cyclohexylidene-3-ethyl-1-phenylhex-3-en-2-one (3d). Yellow liquid; b.p. 160 oC (8.0 mmHg, Kugelrohr); IR (cm-1) 1641 (CO); 1H-NMR (CDCl3, 400 MHz): δ 0.91 (t, 3H, C6-H), 1.04 (t, 3H, 3-CH2CH3), 1.60 (br, 6H, 1-cyclohexylidene-H), 2.13 (br, 4H, 1-cyclohexylidene-H), 2.23 (br, 2H, C5-H), 2.31 (q, 2H, J = 7.32 Hz, 3-CH2CH3), 6.78 (t, 1H, J = 7.32 Hz, C4-H), 7.21–7.31 (m, 5H, 1-phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.62 (3-CH2CH3), 13.79 (C6), 18.46 (3-CH2CH3), 22.35 (C5), 26.54 (1-cyclohexylidene), 27.96 (1-cyclohexylidene), 28.36 (1-cyclohexylidene), 30.90 (1-cyclohexylidene), 32.88 (1-cyclohexylidene), 126.61 (1-phenyl), 128.77 (1-phenyl), 129.03 (1-phenyl), 133.32 (1-cyclohexylidene), 137.20 (C1 or C3), 140.50 (C1 or C3), 142.81 (1-phenyl-1), 148.96 (C4), 200.32(C2); MS (EI) m/z 282 (M+).
2-(Diphenylmethyl)-3-ethyl-5-methylfuran (4a). Yellow liquid; b.p. 135–145 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 1.05 (t, 3H, J = 7.57 Hz, 3-CH2CH3), 2.20 (s, 3H, 5-CH3), 2.29 (q, 2H, J = 7.49 Hz, 3-CH2CH3), 5.40 (s, 1H, 2-CHPh2), 5.85 (s, 1H, C4-H), 7.18–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.85 (5-CH3), 15.31 (3-CH2CH3), 18.24 (3-CH2CH3), 48.43 (2-CHPh2), 107.14 (C4), 123.09 (C3), 126.16 (phenyl), 128.24 (phenyl), 128.73 (phenyl), 142.30 (phenyl-1), 147.46 (C2), 150.22 (C5); MS (EI) m/z 276 (M+). Anal. Calcd for C20H20O: C 86.92, H 7.29. Found: C 87.02, H 7.29. The relationship of the substituted group is confirmed by 13C Inadequate NMR measurement (Figure 2).
2-(Diphenylmethyl)-5-n-butyl-3-n-pentylfuran (4b). Yellow liquid; b.p. 170 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 0.82 (t, 3H, J = 6.84 Hz), 0.89 (t, 3H, J = 7.32 Hz), 1.22 (m, 6H, 3-(CH2)2(CH2)2CH3/5-(CH2)2CH2CH3), 1.43 (m, 2H, 3-CH2CH2(CH2)2CH3), 1.56 (m, 2H, 5-CH2CH2CH2CH3), 2.27 (dt, 2H, J = 2.12 Hz, 7.69 Hz, 3-CH2(CH2)3CH3), 2.53 (t, 2H, J = 7.33 Hz, 5-CH2(CH2)2CH3), 5.37 (s, 1H, 2-CHPh2), 5.82 (s, 1H, C4-H), 7.18–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.99, 14.16, 22.36, 22.60, 24.97 (3-CH2(CH2)3CH3), 27.86 (5-CH2(CH2)2CH3), 30.20 (5-CH2CH2CH2CH3), 30.34 (3-CH2CH2(CH2)2CH3), 31.04, 48.38 (2-CHPh2), 106.54 (C4), 121.41 (C3), 126.09 (phenyl), 128.01 (phenyl), 128.73 (phenyl), 142.48 (phenyl-1), 147.54 (C2), 154.63 (C5); MS (EI) m/z 360 (M+).
Molecules 15 04189 g002 550
Figure 2. 13C Inadequate NMR spectrum of 4a.
Figure 2. 13C Inadequate NMR spectrum of 4a.
Molecules 15 04189 g002
2-(Diphenylmethyl)-5-ethyl-3-n-propylfuran (4c). Yellow liquid; b.p. 140–150 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 0.85 (t, 3H, J = 7.33 Hz, 3-CH2CH2CH3), 1.17 (t, 3H, J = 7.33 Hz, 5-CH2CH3), 1.47 (sixtet, 2H, J = 7.42 Hz, 3-CH2CH2CH3), 2.26 (t, 2H, J = 7.57 Hz, 3-CH2CH2CH3), 2.56 (q, 2H, J = 7.53 Hz, 5-CH2CH3), 5.38 (s, 1H, 2-CHPh2), 5.83 (s, 1H, C4-H), 7.10–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 12.14 (5-CH2CH3), 14.06 (3-CH2CH2CH3), 21.46 (5-CH2CH3), 23.79 (3-CH2CH2CH3), 27.08 (3-CH2CH2CH3), 48.36 (2-CHPh2), 105.86 (C4), 121.32 (C3), 126.18 (phenyl), 128.11 (phenyl), 128.82 (phenyl), 142.56 (phenyl-1), 147.81 (C2), 156.00 (C5); MS (EI) m/z 304 (M+).
2-(Diphenylmethyl)-3-n-butyl-5-n-propylfuran (4d). Yellow liquid; b.p. 150–160 ºC (0.1 mmHg, Kugelrohr); 1H-NMR (CDCl3, 300 MHz): δ 0.84 (t, 3H, J = 7.25 Hz, 3-CH2(CH2)2CH3), 0.91 (t, 3H, J = 7.52 Hz, 5-(CH2)2CH3), 1.24 (m, 4H, 3-CH2(CH2)2CH3), 1.59 (sixtet, 2H, J = 7.38 Hz, 5-CH2CH2CH3), 2.27 (t, 2H, J = 7.52 Hz, 3-CH2(CH2)2CH3), 2.50 (t, 2H, J = 7.43 Hz, 5-CH2CH2CH3), 5.37 (s, 1H, 2-CHPh2), 5.83 (s, 1H, C4-H), 7.17-7.27 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.71, 13.89, 21.39 (5-CH2CH2CH3), 22.41, 24.58 (3-CH2(CH2)2CH3), 30.10 (5-CH2CH2CH3), 32.72, 48.36 (2-CHPh2), 106.79 (C4), 121.52 (C3), 126.23 (phenyl), 128.16 (phenyl), 128.88 (phenyl), 142.67 (phenyl-1), 147.77 (C2), 154.67 (C5); MS (EI) m/z 332 (M+). Anal. Calcd for C24H26O: C 86.70, H 8.43. Found: C 86.43, H 8.67.
2-[Bis(4-chloropheny)methyl]-3-ethyl-5-methylfuran (4e). Yellow liquid; b.p. 150 ºC (3.0 mmHg, Kugelrohr); 1H-NMR (CDCl3, 300 MHz): δ 1.05 (t, 3H, J = 7.57 Hz, 3-CH2CH3), 2.20 (s, 3H, 5-CH3), 2.28 (q, 2H, J = 7.49 Hz, 3-CH2CH3), 5.31 (s, 1H, 2-CH(p-ClPh)2), 5.86 (s, 1H, C4-H), 7.06–7.31 (m, 8H, phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.63 (5-CH3), 15.10 (3-CH2CH3), 18.06 (3-CH2CH3), 47.08 (2-CH(p-ClPh)2), 107.35 (C4), 128.47 (C3), 130.13 (phenyl), 132.03 (phenyl), 132.38 (phenyl), 140.56 (phenyl-1), 146.54 (C2), 150.87 (C5); MS (EI) m/z 344 (M+).

4. Conclusions

In conclusion, we have developed a novel rhodium-catalyzed cross-coupling reaction of ketenes with alkynes. The different coordination modes of ketenes to rhodium, which highly depend on the structure and reactivity of the starting ketenes, realized the selective formation of totally different products, dienones and furans in the presence of the same rhodium catalyst, RhCl(PPh3)3. Both reactions proceed via characteristic rhodacyclic intermediates.

Acknowledgements

This work was supported in part by a Grant-in-Aid for Scientific Research (B) (No. 20350048) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. T.K. acknowledges financial support from Yazaki Memorial Foundation for Science and Technology. This research was conducted in part at the Advanced Research Institute (the Research Project of Engineering for Sustainable Environment), Katsura-Int’tech Center, Graduate School of Engineering, Kyoto University.

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  • Sample Availability: Samples of the compounds 3a-d and 4a-e are available from the authors.

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MDPI and ACS Style

Kondo, T.; Niimi, M.; Yoshida, Y.; Wada, K.; Mitsudo, T.-a.; Kimura, Y.; Toshimitsu, A. Rhodium-Catalyzed Linear Codimerization and Cycloaddition of Ketenes with Alkynes. Molecules 2010, 15, 4189-4200. https://doi.org/10.3390/molecules15064189

AMA Style

Kondo T, Niimi M, Yoshida Y, Wada K, Mitsudo T-a, Kimura Y, Toshimitsu A. Rhodium-Catalyzed Linear Codimerization and Cycloaddition of Ketenes with Alkynes. Molecules. 2010; 15(6):4189-4200. https://doi.org/10.3390/molecules15064189

Chicago/Turabian Style

Kondo, Teruyuki, Masatsugu Niimi, Yuki Yoshida, Kenji Wada, Take-aki Mitsudo, Yu Kimura, and Akio Toshimitsu. 2010. "Rhodium-Catalyzed Linear Codimerization and Cycloaddition of Ketenes with Alkynes" Molecules 15, no. 6: 4189-4200. https://doi.org/10.3390/molecules15064189

APA Style

Kondo, T., Niimi, M., Yoshida, Y., Wada, K., Mitsudo, T. -a., Kimura, Y., & Toshimitsu, A. (2010). Rhodium-Catalyzed Linear Codimerization and Cycloaddition of Ketenes with Alkynes. Molecules, 15(6), 4189-4200. https://doi.org/10.3390/molecules15064189

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