Silylene-transfer reactions of cyclic organosilanes induced by phenanthraquinone triplet

On irradiation cyclic organosilanes 1a–b, 7 and 8 react with phenanthraquinone (PQ) to afford corresponding dioxasilolenes 2a–b as silylene-transfer products. The quenching experiment by anthracene reveals that PQ should act as triplet (3PQ*) to undergo radical displacement at the silicon atom of 1a–b. In polar solvents, electron-transfer from 1a–b to 3PQ* would generate PQ radical anion and the coupling of the radical ion pair formed would cause the SiSi bond cleavage followed by intramolecular OSi bond formation to give silylene transfer product 2a–b. Since the photolysis of PQ with 1a–b in the presence of CCl4 gives dichlorooligosilanes, intermediacy of oligosilanyl radicals is highly probable. The quenching rate constant kq of 3PQ* by cyclic organosilane 1b has been determined by laser flash photolysis. In a mixed solvent of CH3CN and CH2Cl2, 3PQ* is readily quenched by the addition of 1b with the formation of PQ radical anion. On the other hand, since PQ radical anion has not been observed on the laser photolysis, the quenching of 3PQ* with 1b in benzene should be less efficient.