EXTREMELY FACILE LIGAND-EXCHANGE AND DISPROPORTIONATION REACTIONS OF DIARYL SULFOXIDES, SELENOXIDES, AND TRIARYLPHOSPHINE OXIDES WITH ORGANOLITHIUM AND GRIGNARD-REAGENTS

Diaryl sulfoxides undergo unusually rapid ligand-exchange reaction upon treatment with organolithium reagents at -95-degrees-C. Optically pure phenyl p-tolyl sulfoxide (4b) reacted with organolithium reagents at the range from -20 to -95-degrees-C to give facile ligand-exchange and disproportionation products, i.e., diphenyl sulfoxide (7), recovered 4b, and di-p-tolyl sulfoxide (8) in a statistical ratio of 1:2:1 in quantitative yields, and the recovered 4b was completely racemized. This facile ligand exchange was observed in the similar reactions using only diaryl selenoxides and triarylphosphine oxides. The reactions of O-18-labeled phenyl p-tolyl sulfoxide (4c) with organolithium reagents gave products in a statistical ratio without O-18 scrambling, indicating that only the C-S bond cleavage took place under low temperature. It is suggested that the ligand exchange reactions occur by the nucleophilic attack by organolithium reagent at the sulfinyl sulfur atom, giving sigma-sulfurane as an intermediate that collapses rapidly. These results suggest that the treatment of arylic sulfoxides, selenoxides, and phosphine oxides with strong bases should be effected with caution.