Silylene-transfer reactions of cyclic organosilanes induced by phenanthraquinone triplet

On irradiation cyclic organosilanes 1a-b, 7 and 8 react with phenanthraquin one (PQ) to afford corresponding dioxasilolenes 2a-b as sitylene-transfer p roducts. The quenching experiment by anthracene reveals that PQ should act as triplet ((3)PQ*) to undergo radical displacement at the silicon atom of 1a-b. In polar solvents, electron-transfer from 1a-b to (3)PQ* would genera te PQ radical anion and the coupling of the radical ion pair formed would c ause 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 olig osilanyl radicals is highly probable. The quenching rate constant k(q) of ( 3)PQ* by cyclic organosilane lb has been determined by laser flash photolys is. In a mixed solvent of CH3CN and CH2Cl2, (3)PQ* is readily quenched by t he 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 (3)PQ* with lb in benzene should be less efficient. (C) 2001 Elsevier Science B.V. All rights reserved.