A kinetic separation method for the stereoselective preparation of (Z)- and (E)-monofluoroenynes from E/Z mixtures of 1-bromo-1-fluoroolefins

Reaction of E/Z mixtures of 1-bromo-1-fluoroolefins with 1-alkynes and catalytic Pd(PPh3)2Cl2 and CuI in triethylamine at room temperature gave (after 16–24 h) predominately the (Z)-monofluoroenyne (Z/E>92/8) in good yields. Pure (Z)-monofluoroenyne could generally be obtained by chromatographic separation of the crude Z/E mixture. Pure (Z)-1-bromo-1-fluoroolefin could he recovered and reacted with 1-alkynes under similar conditions and longer reaction times (48 h) to give pure (E)-monofluoroenynes in excellent yields (78–89%). Thus, E/Z mixtures of 1-bromo-1-fluoroolefins could be kinetically separated into (Z)- and (E)-monofluoroenynes. This methodology provides a simple one-step unequivocal route to the isomerically pure (Z)- and (E)-monofluoroenynes from the readily available 1-bromo-1-fluoroolefins.