Production of α-Siloxycarbenium Ions by Protonation of Photochemically Generated α-Siloxycarbenes. Formation Mechanism and Reactivities with Nucleophiles

Abstract

The acyltrimethylsilanes 4-RC6H4C(O)SiMe3 (R = H, Me, MeO) and β-naphthylC(O)SiMe3, upon photolysis in acetonitrile with 20 ns pulses of 248 nm light from an KrF* excimer laser, give rise to the corresponding α-siloxycarbenes ArC:OSiMe3, whose absorption spectra (λmax between 270 and 310 nm), lifetimes (between 130 and 260 ns), and reactivities with proton donors (ROH, mainly alcohols) are reported. With highly acidic ROH, such as 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP), the reaction is of simple second order, rate constants being in the 109 M-1 s-1 range and virtually independent of the nature of the aromatic moiety of the carbene. Isotopic substitution of H in ROH by D has no effect on the rate constant for reaction with carbene. For less acidic alcohols such as, e.g., methanol, the reactivity of the carbenes increases with increasing [ROH]. This behavior is interpreted in terms of reversible adduct formation between carbene and alcohol followed by reaction with further alcohol molecule(s) to giv...