Nucleophilic substitution at phosphorus. Phosphorothioates

Abstract

The stereochemistry of nucleophilic substitutions at phosphorus has been a subject of intense study. Most studies have made use of optically active reactants and products and have been hampered by the ambiguities normally associated with-such systems. In our laboratory, we introduced a new diagnostic tool which enables us to distinguish between possible stereochemical pathways. The tool, 2-substituted- 5-chloromethyl-5-methyl-2-oxo-1,3,2-dioxaphosphorinan, allows us to study substitutions at phosphorus without resorting to optically active substrates. The object of our study was to determine the mechanism of nucleophilic substitutions at thiophosphoryl centers. To accomplish this objective, a series of 2-substituted-5-chloromethyl-5-methyl-2- thio-1,3,2-dioxaphosphorinans were prepared and employed as substrates. The stereochemical consequences due to a change in the basicity of nucleophiles, stability of leaving groups, polarity of solvents and added salts was determined. A comparison of the effects upon thiophosphates and phosphates was made.