First examples of enantioselective palladium-catalyzed thiocarbonylation of prochiral 1,3-conjugated dienes with thiols and carbon monoxide:: Efficient synthesis of optically active β,γ-unsaturated thiol esters

A catalyst system based on [Pd(OAc)(2)]/(R,R)-DIOP has been found to effect asymmetric thiocarbonylation of certain prochiral 1,3-dienes to produce good yields of optically enriched,beta,gamma -unsaturated thiol esters. The reaction was performed under an atmosphere of carbon monoxide (400 psi) at 110 degreesC in methylene chloride for 60 h. The asymmetric thiocarbonylation proceeded with good to excellent enanotioselectivities (up to 89% ee). The stereoselectivity is strongly influenced by the structure of the chiral phosphine ligands and substrates, as well as by the reaction conditions. The enantiodetermination step is assumed to be CO insertion to a pi -allylpalladium intermediate.