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

The alkenylcopper adduct (1) from the conjugate addition of Me(2)CuLi . LiI (3) to methyl 4,4-dimethyl-2-pentynoate (2) forms a Ti-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 (13)C 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 TMSCl 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. (C) 1997 Elsevier Science S.A.