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

Abstract

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.

Manifestation of the β-Silicon Effect in the Reactions of Unsaturated Systems Involving a 1,2-Silyl Shift

“…aryl groups, the participation of the silicon in the stabilization is lower when compared to -alkyl substituents (Scheme 3). 31 It was discovered that ,-silyl disubstituted vinyl cations 41 are surprisingly stable in solution and as anhydrous salts with tetrakis(pentafluorophenyl)borate as counterion. Their NMR, 32 X-ray diffraction, 33 and IR spectroscopy 34 studies revealed further correlation between cation stabilization from an -aryl substituent and hyperconjugation from the -silyl group (Scheme 4).…”

Radiolytic Silylation of Alkenes and Alkynes by Gaseous R3Si+ Ions. Stereochemical Evidence for the β-Silyl Effect

“…Once again, the formation of neutral silylated products is in favor of an ionic silylated intermediate which can be depicted either as a bridged species or as a β−silyl substitued vinyl cation. The formation of the silylation product of acetylene itself, the parent alkyne, is remarkable, in view of the inability to observe the silylated adduct ion in high-pressure mass spectrometry . This contrasting behavior is probably the result of the different pressure range used by the two techniques.…”

Borane-Catalyzed Selective Hydrosilylation of Internal Ynamides Leading to β-Silyl (Z)-Enamides

“…This geometrical bias can be attributed to the β-silicon effect to overlap the C(1)–Si and empty p-orbital of a carbocationic C(2) atom . This alignment will accordingly decrease the dihedral angle of Si–C(1)–C(2) relative to that of C(3)–C(1)–C(2), thus favoring the borohydride attack to ketene iminium species via the sterically more accessible anti -addition path.…”