Stabilization of Vinyl Cations by β-Silicon: A Quantitative Mass Spectrometric Study

The stabilization afforded a vinyl cation by a β-(CH3)3Si substituent has been determined by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of three alkynes (RCCR‘) with (CH3)3Si+ and with the proton. The measured −ΔH° (kcal mol-1) and −ΔS° (in parentheses, cal K-1 mol-1) values for the reaction (CH3)3Si+ + RCCR‘ ⇄ (CH3)3Si·C(R)CR‘+ are as follows: 1-hexyne (R = H, R‘ = n-C4H9) 25.9 ± 1.5 (19.1 ± 0.2), 2-hexyne (R = CH3, R‘ = n-C3H7) 28.8 ± 1.4 (25.5 ± 0.3), and phenylacetylene (R = H, R‘ = C6H5) 28.2 ± 2.8 (16.5 ± 0.4). By comparison the values for 1-hexene which forms an alkyl cation are 38.2 ± 0.5 kcal mol-1 (48.2 ± 0.1 cal K-1 mol-1). The deduced stabilizations (A) for all the substituents (R, R‘ and (CH3)3Si) obtained from the isodesmic reaction (CH3)3Si·C(R)CR‘+ + CH2CH2 → (CH3)3Si·C(R)C(H)R‘ + CH2CH+ are (kcal mol-1) as follows: 1-hexyne 55, 2-hexyne 58, and phenylacetylene 58. The deduced stabilization for the (CH3)3Si+ adduct of 1-hexene relative to the ethyl cation is 60 kcal mol-1. The measured proton affinities are (kcal mol-1) as follows: 1-hexyne 194.5 ± 0.5, 2-hexyne 195.8 ± 0.2, phenylacetylene 198.6 ± 0.2, and 1-hexene 194.0 ± 0.5. The stabilizations (B) due to R and R‘ in the vinyl cations RC(H)CR‘+ produced by protonaton are calculated from the isodesmic reactions RC(H)CR‘+ + CH2CH2 → RC(H)C(H)R‘ + CH2CH+ and are (kcal mol-1) as follows: 1-hexyne 44, 2-hexyne 46, and phenylacetylene 50. The comparable value for the alkyl cation from the protonation of 1-hexene is 34 kcal mol-1. The stabilizations of the vinyl cations RC(H)CR‘+ due to the presence of a β-(CH3)3Si (A − B) are (kcal mol-1) as follows: 1-hexyne 11, 2-hexyne 12, and phenylacetylene 9. For the alkyl cation formed from 1-hexene, the value is 26 kcal mol-1. The stabilization of a vinyl cation by an α-alkyl or α-aryl substituent is subtantially greater than that afforded by the same substituent in an alkyl cation. The total stabilization afforded by both an α-alkyl or α-aryl substituent and a β-(CH3)3Si substituent appears to be approximately the same in both alkyl and vinyl cations and hence the β-silicon effect is considerably smaller for the vinyl cation.