REVERSIBLE OXIDATION OF PHOSPHYLTHIONATES AND PHOSPHYLSELENONATES WITH TRIFLUOROACETIC-ANHYDRIDE

Trifluoroacetic anhydride oxidizes a variety of phosphylthionates and -selenonates into corresponding oxo products at room temperature. In the case of phosphine sulfide 1, the reaction proceeds with complete racemization, while phosphine selenide 2 is oxidized with a net inversion and a high degree of racemization. The extent of epimerization during the oxidation of diastereomeric phosphoroselenonates is much lower. The variable-temperature 31P NMR spectra show the existence of two intermediates: a phosphonium salt 12 and the pentacoordinated compound 13, both originating from the acylation of the product at phosphoryl oxygen. Two analogous intermediates containing sulfur or selenium, occurring earlier on the reaction pathway, are also postulated. The entire process is fully reversible as evidenced by the conversion of ethylmethylphenylphosphine oxide into the corresponding sulfide during the desulfurization of methyl-n-propylphenylphosphine sulfide. The equilibrium is gradually shifted into the oxidized product by the decomposition processes of trifluorothio- or trifluoroselenoacetic anhydride. © 1990 American Chemical Society.