Modes of Neighbouring Group Participation by the Methyl Selenyl Substituent in β-Methylselenylmethyl-substituted 1-Phenylethyl Carbenium Ions

Abstract

Selenium substituents which are disposed β to an electron deficient centre, such as a carbocation p-orbital, or the π* orbital of an electron deficient p-system, interact in a stabilising way by a combination of C-Se hyperconjugation (σ_Se-C–π* interaction), and a through-space homoconjugative n_Se–π* interaction. The relative importance of these two modes of interaction is dependant on the electron demand of the cation, with hyperconjugation predominating for low electron demand systems, and the n_Se–π* interaction predominating for high electron demand cations.

1. Introduction

Unimolecular solvolyses of the conformationally biased β-phenylselenyl trifluoroacetate 1 (Figure 1) occurs at a rate which is 10⁷ times faster than the corresponding unsubstituted derivative 2 (Figure 1) suggesting that the selenium substituent provides strong assistance in the departure of the trifluoroacetate leaving group [1]. The mechanism of participation by the selenium substituent might reasonably be described by conventional neighbouring group participation [2]. In this case the selenium lone pair electrons act as an internal nucleophile, displacing the leaving group to give the seleniranium ion intermediate 3 (Figure 1). This is an example of non-vertical participation [3].

Figure 1. Modes of participation by β-Selenium substituents.

Figure 1. Modes of participation by β-Selenium substituents.

However, consideration was given to the possibility, that participation by the selenium substituent might occur by σ_C-Se-p hyperconjugation (vertical participation), and involve the open carbenium ion 4 (Figure 1) as an intermediate [3,4,5,6]. This mode of participation is analogous to that provided by the trimethylsilyl substituent in the carbenium ion 5 (Figure 1) which is the basis of the silicon β-effect [7,8,9,10]. Application of the variable oxygen probe to ether and ester derivatives of the antiperiplanar β-phenylselenyl alcohol 6 (Figure 1) provided crystallographic evidence that the C-Se bond is a strong σ-donor and can therefore effectively stabilise a neighbouring carbenium ion by hyperconjugation alone [1]. More recently NMR, crystallographic and computational studies on phenylselenylmethyl-substituted pyridinium ions 7 and 8 (Figure 2) revealed that a number of orbital interactions involving the selenium substituent were responsible for stabilisation of the charge on the adjacent carbon (Figure 3) [11].

Figure 2. Selenium substituted pyridinium ions.

Figure 2. Selenium substituted pyridinium ions.

Figure 3. Orbital interactions involving the selenium substituent in 2- and 4-substituted pyridinium ions.

Figure 3. Orbital interactions involving the selenium substituent in 2- and 4-substituted pyridinium ions.

The orbital interactions include σ_Se-C-π* hyperconjugation (Figure 3A), an anomeric effect (Figure 3B) and a through-space interaction between the selenium p-type lone pair orbital and the electron deficient pyridinium ion π system (Figure 3C) which represents the early stages of the bridging interaction as represented by structure 3 above. The latter two interactions explain the preferred gauche dihedral angle about the Se-C bond in these structures, a conformation, which is also preferred in α-phenylselenyl ketones, where similar orbital interactions are plausible [12,13].

Calculations showed that C-Se hyperconjugation (σ_C-Se–π*) is the predominant mode of stabilisation in the weakly electron demanding pyridinium ions 7 and 8, where the σ_C-Se–π* hyperconjugative interaction provides 34.8 and 34.2 kJ mol⁻¹ stabilisation respectively, while the through-space n_Se–π* interaction provides 9.0 and 8.0 kJ mol⁻¹ of stabilisation. However the through-space (n_Se–π*) interaction becomes more important as the electron demand of the β-cation increases. For example, in the selenylmethyl-substituted cyclopropenium ion 9 the NBO interaction energies for the σ_C-Se–π* interaction is 104.9 kJ mol⁻¹ while the n_Se–π* through-space interaction is 73.7 kJ mol^-1. Also consistent with the increasing importance of the through space interaction as the electron demand increases is the closing of the Se-C-C(+) bond angle, which decreases from 110.9° for the pyridinium ion 8 to 101.1° in the cyclopropenium ion 9. The anomeric interaction (n_Se–σ*_CC) was found to be relatively unimportant in all ions. In this paper we investigate computationally the relative importance of the stabilising orbital interactions in the more highly electron demanding ions 10–13 (Figure 4).

Figure 4. β-Selenium-substituted ions with higher electron demand.

Figure 4. β-Selenium-substituted ions with higher electron demand.

2. Methods

Calculations were performed at the B3LYP/6-311++G** level of theory [14,15,16,17,18], a level of theory which has been previously employed to investigate stereoelectronic effects of chalcogen substituents [11,19]. Natural Bond Orbitals (NBOs) were calculated using the NBO 3.1 program [20] as implemented in the Gaussian 03 package [21].

3. Results and Discussion

The parent benzylically-stabilised β-selenium substituted 1-phenylethyl cation 12 has two low energy conformations, both of which allow vertical and non-vertical modes of participation to occur. In both conformations the C-Se bond is aligned with the direction of the carbenium ion p-orbital, allowing for σ_C-Se–π* hyperconjugation to occur effectively, in addition the CH₃-Se-CH₂-C(+) dihedral angle is close to orthogonal, which allows the through-space n_Se–π* interaction between the selenium p-type lone pair orbital and the carbenium ion p-orbital to occur. This gives rise to the exo conformation 12a and the endo conformation 12b these conformations are very similar energetically, with the exo conformer being slightly favoured (1.1 kJ mol⁻¹) (Figure 5). For practical purposes the comparisons made below apply to the exo conformer.

Figure 5. Low energy conformations of the seleniranium ion 12.

Figure 5. Low energy conformations of the seleniranium ion 12.

The computed structures of the β-selenyl-carbenium ions 10–13 are presented in Figure 6, while selected geometrical parameters and NBO orbital interaction energies are presented in Table 1 [20]. A convenient measure of electron demand of a cation is the pK_R+ value, those, which are available from the literature have been included in this table.

Figure 6. Calculated structures for the β-selenium substituted 1-phenylethyl cations 10–13.

Figure 6. Calculated structures for the β-selenium substituted 1-phenylethyl cations 10–13.

Decreasing stabilisation of the carbenium ions by delocalization into the aromatic ring is demonstrated by the C(Ar)-C⁺ distance which increases from 1.389 Å in 10 to 1.452 Å in 13 where there is little resonance interaction. The general trend apparent from Table 1 is that as the magnitude of both σ_C-Se–π hyperconjugation and the through-space n_Se–p interaction increases with increasing electron demand of the 1-phenylethyl cation. Increasing strength of σ_C-Se–π hyperconjugation is evident from the decreasing population of the σ_C-Se orbital with increasing electron demand, and the increasing magnitude of the orbital overlap term [F(i,j)], while the increasing strength of the through-space n_Se-p interaction is evident from the decreasing population of the n_Se p-type lone pair orbital with increasing electron demand, and an increasing orbital overlap term [F(I,j)]. However the relative importance of the through-space stabilising interaction increases with increasing electron demand. For example in the relatively stable 4-amino-1-phenylethyl cation 10 σ_C-Se–π hyperconjugation is the most important stabilising interaction (55.7 kJ mol⁻¹ vs. 17.2 kJ mol⁻¹ involving the selenium substituent, however while this stabilising interaction increases with increasing electron demand, the n_Se–p through space interaction increases more profoundly, and in the parent cation 12 the through space through-space n_Se–p interaction is the most important stabilising interaction (418.4 vs. 211.6 kJ mol⁻¹). The increasing importance of the through-space interaction is consistent with the steady closing of the Se-CH₂-C(+) bond angle from 10–12, while in the most electron deficient cation, the p-nitrophenylethyl cation the ion is bridged, and the individual contributions from hyperconjugation and the through-space interaction can no-longer be deconvoluted.

Table 1. Structural and orbital properties, and NBO interaction energies of β-selenium substituted 1-phenylethyl cations 10–13.

	10	11	12	13
Se-CH2 (Å)	2.012	2.011	2.010	2.010
Se-CH2-C+ (°)	98.84	93.41	84.78	78.96
Se…C(+) Å	2.664	2.554	2.372	2.245
C(Ar)-C+	1.389	1.403	1.423	1.452
pKR+ a		−12.4 [ 22]	<−20 [ 23]
Vertical interaction E(2) (kJ mol−1)	55.7	73.3	211.6	-
σC-Seenergy (a.u.)	−0.641	−0.653	−0.660
σC-Sepopulation	1.891	1.866	1.820
Overlap, F(i,j) (a.u.)	0.076	0.087	0.139
Nonvertical interaction E(2) (kJ mol−1)	17.2	49.0	418.4	-
nSeenergy (a.u.)	−0.363	−0.379	−0.397
nSepopulation	1.799	1.727	1.606
F(i,j) (a.u.)	0.037	0.046	0.083

^a pK_R+ values for the corresponding non-substituted carbenium ions.

Table 1. Structural and orbital properties, and NBO interaction energies of β-selenium substituted 1-phenylethyl cations 10–13.

	10	11	12	13
Se-CH2 (Å)	2.012	2.011	2.010	2.010
Se-CH2-C+ (°)	98.84	93.41	84.78	78.96
Se…C(+) Å	2.664	2.554	2.372	2.245
C(Ar)-C+	1.389	1.403	1.423	1.452
pKR+ a		−12.4 [ 22]	<−20 [ 23]
Vertical interaction E(2) (kJ mol−1)	55.7	73.3	211.6	-
σC-Seenergy (a.u.)	−0.641	−0.653	−0.660
σC-Sepopulation	1.891	1.866	1.820
Overlap, F(i,j) (a.u.)	0.076	0.087	0.139
Nonvertical interaction E(2) (kJ mol−1)	17.2	49.0	418.4	-
nSeenergy (a.u.)	−0.363	−0.379	−0.397
nSepopulation	1.799	1.727	1.606
F(i,j) (a.u.)	0.037	0.046	0.083

^a pK_R+ values for the corresponding non-substituted carbenium ions.

4. Conclusions

Selenium substituents interact with electron deficient orbitals at the β-position by a combination of C-Se hyperconjugation, in which the electrons in the σ_C-Se bonding orbital mix with the electron deficient orbital, and a through space interaction between the selenium p-type lone pair orbital and the electron deficient centre, this latter interaction is also referred to as homo-conjugation. For cations with low electron demand, there is very little distortion of the Se-C-C(+) bond angle, and the most important mode of stabilisation is by σ_C-Se–π hyperconjugation. However as the electron demand of the cation increases, then closing of the Se-C-C(+) bond angle occurs, this increases the orbital overlap between the selenium p-type lone pair orbital and the cabocation p-orbital and this becomes the predominant mode of stabilisation.