SYNTHETIC AND MECHANISTIC ASPECTS OF THE REACTION OF TRIALKYLSILYL HALIDES WITH THIO AND SELENO ESTERS OF PHOSPHORUS

A study is described of the reaction of trimethylbromosilane and trimethyliodosilane with various esters of phosphorothioic and phosphoroselenoic acids of general structures (RO)2(RY)P=O (I) and (RO)3P=Y (II) (Y = S or Se, R = alkyl). The reaction results in replacement of the R groups by trimethylsilyl groups. No S-trimethylsilyl and Se-trimethylsilyl substituted esters were found among the products. Thus the displacement of the O-R group in II as well as the replacement of S-organyl or Se-organyl groups in I both lead to the corresponding O-trimethylsilyl isomer containing thiophosphoryl or selenophosphoryl groups. The following sequence of reactivity of the ester groups was observed: {A figure is presented}. The process involves initially the reversible formation of a phosphonium salt intermediate in a fast step, followed by slow dealkylation. It was shown by low temperature 31P FT-NMR studies that in the reaction of trimethyliodosilane with an ester bearing a PO phosphoryl group, the equilibrium for the phosphonium salt formation lies well over towards the intermediate and so the intermediate can be observed directly. These mechanistic proposals were confirmed by conductance studies on the reaction system and by investigation of the stereochemical course of the corresponding reaction of an optically-active model thiophosphonate. In some cases the reaction of the halosilanes with the phosphates takes a selective course which allows the reaction to be used as a convenient route to O-silyl substituted esters of phosphorothioic or phosphoroselenoic acids. © 1979.