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

Chlorination of alpha-phosphoryl sulfoxides 1 with iodobenzene dichlor ide and sulfuryl chloride in the presence and in the absence of pyridi ne and stereochemical investigations of this reaction using (+)-(S)-al pha-dimethoxyphosphorylmethyl p-tolyl sulfoxide 1a are described. It w as 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 as ymmetric induction at the alpha-carbon atom and racemization of the ch iral sulfinyl centre in 1 depend on the reaction conditions. The struc ture of the major diastereomer formed in the chlorination of (+)-(S)-1 a i.e. S)(S)-alpha-chloro-alpha-dimethoxyphosphorylmethyl p-tolyl sulf oxide 2a was determined by X-ray analysis. The structure was solved by direct methods and refined to R = 0.081. The experimental data on chl orination of alpha-phosphoryl sulfoxides 1 point to retention of the c onfiguration 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.