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-tolyt sulfoxide afforded beta-(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 a-butyl and cx-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. (C) 1999 Elsevier Science Ltd. All rights reserved.