Exceptional reactivity and selectivity of lower-order cyanocuprates in the SN2′-substitution of propargyl acetates

The SN2′-substitution reaction of various propargyl acetates with lower-order cyanocuprates RCu(CN)Li was examined. In the case of pyridyl-substituted substrates, good to excellent chemical yields of the desired pyridylallenes were obtained without chelate formation, which often hampers the corresponding reactions of magnesium cuprates. In the analogous SN2′-substitutions of chiral secondary propargyl acetates, lower-order cyanocuprates gave the desired allenes with higher enantioselectivities than magnesium cuprates or cyano-Gilman reagents. Thus, their high reactivity and low tendency to racemize allenes make lower-order cyanocuprates the reagents of choice for these SN2′-substitution reactions.