Medium-Sized Heterocycle Synthesis by the Use of Synergistic Effects of Ni-NHC and γ‑Coordination in Cycloisomerization

This paper describes a new approach in transition-metal-catalyzed unsymmetric cycloisomerization for medium-sized heterocycles. The steric and electronic effects of an NHC–NiH catalyst and γ-heteroatom chelation were used together as a basis for 1,n-diene termini differentiation and for n^γ-exo-trig (head-to-tail) product selectivity. Heterocycles bearing an exocyclic methylene such as oxepines, thiepines, siloxepines, and oxocanes were synthesized from the corresponding 1,n-dienes by a fine-tuning of the NHC properties. The implication of the underlying hypothesis was further demonstrated in a competition experiment in which strained oxepines were formed preferentially over other competing oxa-/carbocycles. Under more forcing physical conditions and the use of a suitable NHC ligand, the exocyclic methylene products were isomerized further into endocyclic olefin products regioselectively in one pot.