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

Abstract

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.