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

摘要

The photochemical reaction of alpha,alpha,alpha-trifluoroacetophenone (TFA), benzophenone, and p-cyanoacetophenone with benzyltrimethylsilane in MeCN involves hydrogen transfer from the benzylic position. Desilylation occurs as a minor process only in the case of TFA (but it increases greatly in the presence of MeOH or LiClO4). The final products result from the statistical recombination of the benzyl and ketyl radicals. Further cases of intramolecular selectivity studied are p-methylbenzyl- and p-methoxybenzyltrimethylsilane (the latter substrate undergoes mainly C-Si bond cleavage in the reaction with TFA) as well as benzyltributylstannane (only destannylation observed with all ketones). Product studies are complemented by the determination of relevant kinetic parameters through steady-state and flash-photolysis experiments. The results are explained in terms of hydrogen transfer proceeding from an exciplex and desilylation from the solvent separated radial ion pair. The latter species predominates when Delta G(et) < -10 kcal mol(-1). [References: 53]