Mechanism of addition of organocuprates to alkynyl carbonyl compounds. A mechanistic bridge between carbocupration and conjugate addition

The reaction pathway of the addition of a lithium dialkylcuprate to an alkynyl carbonyl compound was examined by the use of hybrid density functional calculations for Me2CuLi.LiCl and two carbonyl substrates, 3-butyn-2-one and methyl propynoate. Both substrates react in essentially the same manner to give first a 1,2-carbocupration product. This alpha-cupriocarbonyl product is thermodynamically stable in the case of methyl propynoate. On the other hand, the a-cupriocarbonyl product obtained from 3-butyn-2-one is in equilibrium with the corresponding lithium allenolate with a low activation barrier. Comparison of the calculated IR frequencies of the putative products with those of the experimentally obtained product mixture suggested that the product of the ynone reaction is a mixture of an alpha-cuprioketone and an allenolate, which may be in mobile equilibrium with each other in solution.