NUCLEOPHILE-ASSISTED CLEAVAGE OF BENZYLTRIALKYLSILANE CATION RADICALS

The cation radicals of benzyltrialkylsilanes have been generated using photoinduced electron transfer and characterized using transient abso rption spectroscopy. Absolute rate constants for nucleophile-assisted C-Si bond cleavage have been determined, for different nucleophiles in different solvents and with different substituents on the phenyl ring . The short lifetime (<10(-9) s) of the parent benzyltrimethylsilane c ation radical in acetonitrile was unambiguously shown to be due to a r apid nucleophile-assisted bond cleavage, with the solvent acting as th e nucleophile. Even in less polar solvents, such as dichloromethane, t he lifetime of benzyltrimethylsilane cation radical is quite short (ca . 20 ns) unless trace amounts of water, which acts as an efficient nuc leophile, are removed. Consistent with the nucleophile-assisted cleava ge mechanism for the benzyltrialkylsilanes, sterically-demanding subst ituents on silicon decrease the rate constant for cleavage by as much as 4 orders of magnitude, depending upon the nucleophile. Similarly, i ncreasing steric crowding on the nucleophile also decreases the rate c onstant, although smaller changes in the rate constants are observed. Electron-donating substituents (4-methyl and 4-methoxy) on the phenyl group also lead to a substantial decrease in the rate constant for cle avage of the cation radicals, When measurements are performed in the l east nucleophilic solvent and under conditions that minimize contribut ions from adventitious nucleophiles, the lifetimes of the cation radic als of the benzyltrialkylsilanes can be so long that the rate of pseud o-first-order decay can not be accurately determined. If the cation ra dicals undergo unimolecular C-Si bond cleavage (i.e., not nucleophile- assisted) under these conditions, the rate constant for this process i s estimated to be less than 10(4) s(-1).