Highly Enantioselective Synthesis of Monofluoroalkenes by Rhodium-Catalyzed Asymmetric Arylation/Defluorination of Allyl Difluorides.

The reaction of readily available E-allyl difluorides (R1CH = CHCF2COR2) containing a CF2COR2 moiety with arylboronic acids catalyzed by a chiral diene-rhodium complex gives high yields of chiral monofluoroalkenes (R1ArC*HCH = CFCOR2) with perfect enantioselectivity (enantiomeric excess (ee) of > 99% in all cases) and high level of Z/E selectivity (up to 20:1). The reaction is proposed to proceed through enantioselective arylrhodation of the E-allyl difluorides forming a β,β-difluoroalkylrhodium intermediate, followed by stereoselective β-fluoride elimination. The CF2COR2 moiety plays a key role in activating the E-allyl difluorides toward arylrhodation/defluorination in executing the catalytic cycle, and it serves as a handle for further functionalization.