Insertions of unsymmetric alkynes into the metal-carbon bonds of nickelacycles: What determines the regiochemistry?

Although the insertion of alkynes into transition-metal - carbon bonds plays an important role in synthesis, the regioselectivities observed with unsymmetric alkynes have usually been interpreted on the basis of steric effects. In this perspective paper, we review the available data for such reactions with nickelacycles and present the results of some preliminary theoretical (DFT) calculations. These suggest that, even for unactivated alkynes, the regiochemistry may also be controlled by electronic factors such as frontier-orbital interactions between the triple-bond of the alkyne and the polarized metal - carbon bond.