STABILIZATION OF VINYL CATIONS BY BETA-SILICON - A QUANTITATIVE MASS-SPECTROMETRIC STUDY

The stabilization afforded a vinyl cation by a beta-(CH3)(3)Si substit uent has been determined by measuring in a high-pressure mass spectrom eter the thermodynamic data for the association of three alkynes (RCCR ') with (CH3Si+ and with the proton. The measured -Delta H degrees (kc al mol(-1)) and -Delta S degrees (in parentheses, cal K-1 mol(-1)) val ues for the reaction (CH3Si+ + RCCR' reversible arrow (CH3)(3)Si . 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 ar e 38.2 +/- 0.5 kcal mol(-1) (48.2 +/- 0.1 cal K-1 mol(-1)). The deduce d stabilizations (A) for all the substituents (R, R' and (CH3)(3)Si) o btained from the isodesmic reaction (CH3)(3)Si . C(R)CR'(+) + CH2CH2 - -> (CH3)(3)Si . C(R)C(H)R' + CH2CH+ are (kcal mol(-1)) as follows: 1-h exyne 55, 2-hexyne 58, and phenylacetylene 58. The deduced stabilizati on for the (CH3Si+ adduct of l-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, phenylacetyle ne 198.6 +/- 0.2, and 1-hexene 194.0 +/- 0.5. The stabilizations (B) d ue to R and R' in the vinyl cations RC(H)=CR'(+) produced by protonato n 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 alky l cation from the protonation of 1-hexene is 34 kcal mol(-1). The stab ilizations of the vinyl cations RC(H)=CR'(+) due to the presence of a beta-(CH3)(3)Si (A - B) are (kcal mol(-1)) as follows: 1-hexyne 11, 2- hexyne 12, and phenylacetylene 9. For the alkyl cation formed from 1-h exene, the value is 26 kcal mol(-1). The stabilization of a vinyl cati on by an alpha-alkyl or alpha-aryl substituent is substantially greate r than that afforded by the same substituent in an alkyl cation. The t otal stabilization afforded by both an a-alkyl or a-aryl substituent a nd a beta-(CH3)(3)Si substituent appears to be approximately the same in both alkyl and vinyl cations and hence the beta-silicon effect is c onsiderably smaller for the vinyl cation.