Past exam of the chemistry B course of the University of Cambridge / 2023 / Paper 2 / B7 / c / iii / Solution

The favoured insertion leaves nickel attached to the methyl-substituted alkyne carbon; the aryl group bonds to the SiMe${}_3$-substituted carbon. This is the connectivity leading to the illustrated ring-closed product: the carbon carrying methyl subsequently attacks the ester carbonyl.

The bulky trimethylsilyl substituent is kept away from the crowded phosphine-ligated nickel centre in the insertion arrangement. The alternative orientation would place Ni next to the more demanding SiMe${}_3$ end. regioselectivity can also reflect electronic polarization of a silyl-substituted alkyne and stabilization of the resulting alkenyl complex. The ortho ester helps capture the insertion product by oxygen coordination and subsequent ring closure. Steric discrimination at insertion, reinforced by the accessible intramolecular cyclization, accounts for the observed direction; the product drawing alone is not a quantitative separation of steric and electronic contributions.