SET AND EXCIPLEX PATHWAYS IN THE PHOTOCHEMICAL-REACTIONS BETWEEN AROMATIC KETONES AND BENZYLSILANE AND STANNANE DERIVATIVES

The photochemical reaction of alpha,alpha,alpha-trifluoroacetophenone (TFA), benzophenone, and p-cyanoacetophenone with benzyltrimethylsilan e in MeCN involves hydrogen transfer from the benzylic position. Desil ylation occurs as a minor process only in the case of TFA (but it incr eases greatly in the presence of MeOH or LiClO4). The final products r esult from the statistical recombination of the benzyl and ketyl radic als. Further cases of intramolecular selectivity studied are p-methylb enzyl- and p-methoxybenzyltrimethylsilane (the latter substrate underg oes mainly C-Si bond cleavage in the reaction with TFA) as well as ben zyltributylstannane (only destannylation observed with all ketones). P roduct studies are complemented by the determination of relevant kinet ic parameters through steady-state and flash-photolysis experiments. T he results are explained in terms of hydrogen transfer proceeding from an exciplex and desilylation from the solvent separated radial ion pa ir. The latter species predominates when Delta G(et) < -10 kcal mol(-1 ).