Photochemistry of some trialkylsilyloxybenzylidenemalonic acid derivatives. C–Si bond fragmentation in a polarized excited state

Abstract

Photolysis of 3-(tert-butyldimethylsilyloxyphenyl)methylenemalonodinitrile (2) in a polar medium leads to C–Si cleavage via the polarized singlet excited state. The thus formed tert-butyl radicals diffuse out of cage and are trapped by further molecules of 2. In contrast, the photochemical alkylation of benzylidenemalonodinitrile by tert-butyldimethylsilyl phenyl ether occurs via the radical cation and requires phenanthrene sensitization. In an apolar solvent, 2 undergoes only a slow dimerization and gives two (one main) cyclobutanes. Photoinduced cleavage is not observed neither with the corresponding diester nor with the 2-silyloxy analogue of 2 (the lowest excited singlet of the latter compound is an ICT state).