NMR SPECTROSCOPIC INVESTIGATION OF THE ADDUCTS FORMED BY ADDITION OF CUPRATES TO YNOATES AND YNONES - ALKENYLCUPRATES OR ALLENOLATES

The adducts obtained by reaction of Me3CuLi . LiI, tBuCu(CN)Li, and tB u(2)CuLi . LiCN with ynoates and ynones were characterized by determin ation of their C-13 NMR chemical shifts and C-13, C-13 coupling consta nts. Alkenyl-copper adducts were formed from ynoates and Me2CuLi . LiI or tBuCu(CN)Li, whereas allenolates are obtained from ynoates and tBu (2)CuLi . LiCN, as well as from ynones and Me2CuLi . LiI. The transfor mations can therefore be regarded as carbocupration of an alkyne rathe r than a Michael addition. The equilibrium between alkenylcuprates and allenolates can be shifted towards lithium allenolates by addition of organolithium compounds. In the case of adducts formed from Me2CuLi . LiI, isomerization of cis- to trans-alkenylcuprates via the correspon ding allenolate can be prevented by removal of LiI or by the use of TH F as solvent. Whereas the protonation of alkenylcuprates to the corres ponding alkenes proceeds stereospecifically, it is difficult to contro l the steric course of the protonation of allenolates. Addition of chl orotrimethylsilane to the adducts gives silyl ketene acetals or enol e thers in all cases.