Mechanism of biphosphine disulphide formation in reactions of thiophosphoryl and phosphonothioic halides with Grignard reagents

Abstract

The factors favouring formation of biphosphine disulphides are identified and evidence is presented (a) for attack by the Grignard reagent on chlorine, and (b) that phosphorus–phosphorus bond formation does not occur when two organic groups are attached to phosphorus. A mechanism is proposed, involving simultaneous removal of two halogen atoms from the thiophosphorus halide yielding a phosphinidene sulphide, R–PS, which forms the phosphorus–phosphorus bond either via a thiophosphorus magnesium halide or by insertion in a phosphorus–halogen bond. The mechanism accounts satisfactorily for the restrictions on the organic groups which can be involved and for the higher yields of biphosphine disulphides from alkylmagnesium bromides than from alkylmagnesium chlorides.