A New Technique for the Generation of Alkyl Metathiophosphates

Abstract

We have recently described a technique for the generation of alkyl metaphosphates, ROPO₂, by thermolysis of phosphoramidic acids of structure ROP(O)(OH)NHR (1) [1]. We have now successfully shown that alkyl metathiophosphates, ROP(S)(O), which have received but little previous consideration [2], can be generated by the same approach. Compounds 2 (Ad = 1-adamantyl) and 3 were synthesized for this study. Both were easily fragmented on heating in an inert solvent. When an alcohol trapping agent was present each gave EtO-P(S)(OR)(OH) consistent with EtOP(S)(O) as an intermediate. In the absence of a trap, 2 was cleanly converted to the pyrophosphate derivative 4 in a process established to follow first-order kinetics, thus proving that decomposition of 2 proceeded by elimination of EtOP(S)(O). Compound 3 decomposed by mixed first and second order kinetics, and gave a more complex mixture of products. EtOP(S)(O) generated from 2 phosphorylated the OH groups on the surface of silica gel, a process previously demonstrated for ROPO₂. With a monoester of phosphoric acid, the mixed monothiopyrophosphate EtOP(S)(OH)-O-P(O)(OH)(OR) was formed in a new synthetic approach to these valuable compounds.