Alkyne insertion at a nickel-aluminum heterometallic complex: implications for pyridine alkenylation.

Cooperative Ni-Lewis acid catalysis has emerged as an effective strategy for the selective C-H functionalisation of heteroarenes, yet the elementary steps following C-H activation remain poorly understood. In particular, direct experimental insight into the migratory insertion step, which governs C-C bond formation and potentially product regioselectivity, has been lacking. Herein, we demonstrate that a heterobimetallic {Ni-Al} complex derived from the C-H activation of a pyridine undergoes efficient and selective insertion of alkynes under mild conditions. This reaction provides direct access to structurally well-defined intermediates relevant to cooperative pyridine functionalisation catalysis. A broad range of internal alkynes undergo rapid insertion to furnish five-membered {Ni-Al} metallacycles featuring alkenyl fragments that are σ-bound to aluminum and π-bound to nickel. Kinetic and mechanistic studies establish that alkyne insertion is non-reversible and that both steric and electronic effects govern the rate of insertion and the coordination mode of the bridging pyridyl ligand. Oxidatively-induced reductive elimination affords a corresponding vinylpyridine product, establishing the competence of these species in C-C bond formation. These findings provide the first direct experimental characterisation of migratory insertion intermediates relevant to cooperative {Ni-Al}-catalysed pyridine functionalisation and define mechanistic principles governing substrate coordination, insertion, and product formation.