PRODUCTION OF ALPHA-SILOXYCARBENIUM IONS BY PROTONATION OF PHOTOCHEMICALLY GENERATED ALPHA-SILOXYCARBENES - FORMATION MECHANISM AND REACTIVITIES WITH NUCLEOPHILES

The acyltrimethylsilanes 4-RC(6)H(4)C(O)SiMe(3) (R = H, Me, MeO) and b eta-naphthylC(O)SiMe(3), upon photolysis in acetonitrile with 20 ns pu lses of 248 nm light from an KrF excimer laser, give rise to the corr esponding alpha-siloxycarbenes ArC:OSiMe(3), whose absorption spectra (lambda(max) between 270 and 310 nm), lifetimes (between 130 and 260 n s), and reactivities with proton donors (ROH, mainly alcohols) are rep orted. With highly acidic ROH, such as 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP), the reaction is of simple second order, rate constant s being in the 10(9) M(-1) s(-1) range and virtually independent of th e 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 reactiv ity of the carbenes increases with increasing [ROH]. This behavior is interpreted in terms of reversible adduct formation between carbene an d alcohol followed by reaction with further alcohol molecule(s) to giv e product. On the basis of experiments in the acidic and only weakly n ucleophilic solvents 2,2,2-trifluorethanol (TFE) and HFIP, protonation of the carbenes leads to the corresponding carbenium ions, whose abso rption spectra (lambda(max) between 305 and 355 nm), Lifetimes (100 ns -5 mu s in TFE), and reactivities with nucleophiles (halides, alcohols , and ethers) are reported. In the solvent HFIP, the reactivities of t he carbenium ions with the alcohols and ethers increase with their con centration, in a way analogous to that observed in the reaction of the carbenes with the alcohols. This is explained as resulting from rever sible formation of a cation-nucleophile complex followed by reaction o f the complex with a second nucleophile molecule which acts as a base. In solvents more basic than HFIP, it is presumably the solvent which serves this function.