ALPHA-PHOSPHORYL SULFOXIDES .8. STEREOCHEMISTRY OF ALPHA-CHLORINATION OF ALPHA-PHOSPHORYL SULFOXIDES

Chlorination of alpha-phosphoryl sulfoxides 1 with iodobenzene dichloride and sulfuryl chloride in the presence and in the absence of pyridine and stereochemical investigations of this reaction using (+)-(S)-alpha-dimethoxyphosphorylmethyl p-tolyl sulfoxide 1a are described. It was found that conversion of 1 to the corresponding alpha-chloro-alpha-phosphoryl sulfoxides 2 occurs under the stereochemical control of the sulfinyl group and leads to diastereomeric mixtures. The extent of asymmetric induction at the alpha-carbon atom and racemization of the chiral sulfinyl centre in 1 depend on the reaction conditions. The structure of the major diastereomer formed in the chlorination of (+)-(S)-1a i.e. (+)-(S)(C)(S)(S)-alpha-chloro-alpha-dimethoxyphosphorylmethyl p-tolyl sulfoxide 2a was determined by X-ray analysis. The structure was solved by direct methods and refined to R = 0.081. The experimental data on chlorination of alpha-phosphoryl sulfoxides 1 point to retention of the configuration at both chiral centres (C and S) and this stereochemistry can be rationalized assuming addition-elimination mechanism involving the formation of a positively charged ylide as intermediate.