PI-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 Me2CuLi . LiI (3) to methyl 4,4-dimethyl-2-pentynoate (2) forms a Ti-complex whe n exposed to an excess of the starting acetylene. The first adduct and most stable intermediate in the reaction between 2 and 3 is an alkeny lcopper from a syn-addition as revealed by low temperature C-13 NMR me asurements. The rate of isomerization of the initially formed E-adduct into the Z-analogue through the short-lived lithium allenolate (5) wa s retarded by the removal of lithium iodide. Addition of TMSCl produce s the O-silylated allenolate (6) exclusively. Neither double alkyl tra nsfer from the cuprate to produce di-homovinyl cuprates nor a possible alkenyl transfer is observed. (C) 1997 Elsevier Science S.A.