title: Electrophilic alkylations of vinylsilanes: a comparison of α- and β-silyl effects.
abstract: Kinetics of the reactions of benzhydrylium ions (Aryl2CH(+)) with the vinylsilanes H2C=C(CH3)(SiR3), H2C=C(Ph)(SiR3), and (E)-PhCH=CHSiMe3 have been measured photometrically in dichloromethane solution at 20 °C. All reactions follow second-order kinetics, and the second-order rate constants correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, thus allowing us to include vinylsilanes in the benzhydrylium-based nucleophilicity scale. The vinylsilane H2C=C(CH3)(SiMe3), which is attacked by electrophiles at the CH2 group, reacts one order of magnitude faster than propene, indicating that α-silyl-stabilization of the intermediate carbenium ion is significantly weaker than α-methyl stabilization because H2C=C(CH3)2 is 10(3)  times more reactive than propene. trans-β-(Trimethylsilyl)styrene, which is attacked by electrophiles at the silylated position, is even somewhat less reactive than styrene, showing that the hyperconjugative stabilization of the developing carbocation by the β-silyl effect is not yet effective in the transition state. As a result, replacement of vinylic hydrogen atoms by SiMe3 groups affect the nucleophilic reactivities of the corresponding C=C bonds only slightly, and vinylsilanes are significantly less nucleophilic than structurally related allylsilanes.
authors.affiliation: Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377 München (Germany), Fax: (+49) 89-2180-77717.
authors.forename: Hans A
authors.initials: HA
authors.lastname: Laub
authors.forename: Herbert
authors.initials: H
authors.lastname: Mayr
keywords: electrophilic substitution; hyperconjugation; kinetics; linear free-energy relationships; organosilanes