π-Complexation between a mixed methylalkenylcuprate and an alkyne. Further insight into the mechanism of organocopper additions to ynoates

Abstract

The alkenylcopper adduct ( 1) from the conjugate addition of Me 2CuLi · LiI ( 3) to methyl 4,4-dimethyl-2-pentynoate ( 2) forms a π-complex when exposed to an excess of the starting acetylene. The first adduct and most stable intermediate in the reaction between 2 and 3 is an alkenylcopper from a syn-addition as revealed by low temperature 13C NMR measurements. The rate of isomerization of the initially formed E-adduct into the Z-analogue through the short-lived lithium allenolate ( 5) was retarded by the removal of lithium iodide. Addition of TMSC1 produces the O-silylated allenolate ( 6) exclusively. Neither double alkyl transfer from the cuprate to produce di-homovinyl cuprates nor a possible alkenyl transfer is observed.