ESR investigation of the photochemical reaction of aroylsilanes with phosphorous compounds

Radicals of general structure Ar(R3Si)C-OP(O)Ph, have been observed by ESR upon reaction of aroylsilanes, Ar(R3Si)C0, with tetraphenylbiphosphine under UV irradiation. The same radicals could in fact be obtained in the reaction of aroylsilanes with diphenylphosphine oxide and di-tert-butyl peroxide (DBP). Addition of OP(OEt), to the carbonyl oxygen was also observed when photolyzing mixtures of aroylsilanes, diethyl phosphite, and DBP. In contrast, the photochemical reaction of the same silanes with tetraethyl pyrophosphite led to the detection of radicals, Ar[(EtO),(O)P]C-OSiR,, having the phosphonyl substituent a to the carbon radical center and the silyl group linked to the oxygen atom. Two possible mechanisms which may account for the formation of the observed species, one involving fragmentation of the aroyl-silicon bond and the other the intermediation of a siloxy carbene, are discussed, and experimental evidence is reported in favor of the former one.