Studies on the Pauson-Khand reaction of alkynyl sulfoxides. Unexpectedly easy racemization of their dicobalt hexacarbonyl complexes

The scope and limitations of the Pauson-Khand reactions of alkynyl sulfoxides with strained alkenes have been studied. (S)-1-Hexynyl p-tolyl sulfoxide and (S)-2-phenylethynyl p-tolyl sulfoxide afforded β-(p- tolylsulfinyl)enones in moderate yields and low diastereoselectivities. The diastereomeric mixtures were easily separated by column chromatography and the sulfinyl group could be cleaved by a sequence involving reduction to sulfide, carbonyl reduction and enol thioether hydrolysis to afford 1,3- rearranged enantiomerically enriched α-butyl and α-phenyl enones. The absolute configuration of the Pauson-Khand adducts was determined by X-ray diffraction. The decreased enantiomeric excess of the final products uncovered an unprecedented low temperature racemization of the hexacarbonyl dicobalt complexes of alkynyl sulfoxides.