Nucleophile-Assisted Cleavage of Benzyltrialkylsilane Cation Radicals

Abstract

The cation radicals of benzyltrialkylsilanes have been generated using photoinduced electron transfer and characterized using transient absorption 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 cation radical in acetonitrile was unambiguously shown to be due to a rapid nucleophile-assisted bond cleavage, with the solvent acting as the nucleophile. Even in less polar solvents, such as dichloromethane, the lifetime of benzyltrimethylsilane cation radical is quite short (ca. 20 ns) unless trace amounts of water, which acts as an efficient nucleophile, are removed. Consistent with the nucleophile-assisted cleavage mechanism for the benzyltrialkylsilanes, sterically-demanding substituents on silicon decrease the rate constant for cleavage by as much as 4 orders of magnitude, depending upon the nucleophile. Similarly, increasing steric crowding on the nucleophile also decreases the rate constant, 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 cleavage of the cation radicals. When measurements are performed in the least nucleophilic solvent and under conditions that minimize contributions from adventitious nucleophiles, the lifetimes of the cation radicals of the benzyltrialkylsilanes can be so long that the rate of pseudo-first-order decay can not be accurately determined. If the cation radicals undergo unimolecular C−Si bond cleavage (i.e., not nucleophile-assisted) under these conditions, the rate constant for this process is estimated to be less than 104 s-1.