Insertion modes and origins of product selectivity in rare-earth-catalysed C–H functionalization with conjugated dienes

Abstract

Although rare-earth-catalysed C–H addition to unsaturated hydrocarbons has emerged as a powerful and atom-economical strategy, the mechanistic principles governing selectivity in reactions involving conjugated dienes remain poorly understood. Here, density functional theory (DFT) calculations are employed to elucidate the mechanistic origins of divergent selectivity in rare-earth-catalysed pyridine C–H alkylation and aldimine annulation reactions with conjugated dienes. The results indicate that, despite producing formal 1,4- and 1,2-insertion products, respectively, both transformations proceed through a preferred 1,4-cis-insertion pathway to generate η³-allyl intermediates. Comprehensive computational analyses reveal that the divergence arises from distinct post-insertion pathways: sterically favored protonation at the terminal carbon in pyridine alkylation, versus electronically and sterically driven cyclization at the internal carbon in aldimine annulation. These results demonstrate that product selectivity is dictated primarily by the site-selective evolution of the η³-allyl intermediate rather than by the initial insertion event. This work establishes a unified mechanistic framework for diene-enabled rare-earth-catalysed C–H functionalization and provides theoretical insights into selectivity control through post-insertion reactivity.