Excited-State Reactions of an Isolable Silylene with Aromatic Compounds

The first intermolecular reactions of the excited state of a silicon divalent compound (silylene) with benzene derivatives were discovered. Typically, when a benzene solution of an isolable silylene is irradiated with light of wavelengths longer than 420 nm at room temperature, the corresponding silacyclohepta-2,4,6-triene (silepin) is yielded quantitatively. The photochemical insertion of the silylene toward substituted benzenes occurs in general to give the corresponding substituted silepins. The insertion reaction is highly sensitive to the steric hindrance at a reacting C-C double bond in benzene; during the reactions of the silylene with substituted benzenes, only unsubstituted C-C double bonds in the benzene ring reacted selectively. The irradiation of the silylene in the presence of mesitylene afforded the insertion product to a benzylic C-H bond, indicative of the biradical nature of the excited-state silylene.