posted on 2014-12-19, 00:00authored byChun-Yu Ho, Lisi He
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 nγ-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.