AlCl3-Promoted Conjugate Reduction of α,β-Unsaturated Carbonyl Compounds with 1,3-Dimethyl-2-phenylbenzimidazoline

Article abstract of DOI:10.1246/bcsj.59.1747

The reduction of a variety of α,β-unsaturated carbonyl compounds, except for α,β-unsaturated aldehyde, to the corresponding saturated carbonyl compounds was effectively performed by using 1,3-dimethyl-2-phenylbenzimidazoline (DMBI) as a reducing agent with the aid of AlCl3.The reduction is discussed on the basis of the hydride-donating ability of DMBI, together with the role of AlCl3 as an electrophilic activator for the substrates.The catalytic efficiency of Lewis acids was found to be proportional to the efficiency to form a complex with a carbonyl group.The reduction of 2-cinnamoylpyridine with 2-deuterated DMBI revealed that in the reduction product, a deuterium atom was located at the β-position with respect to the carbonyl group.On the other hand, the reduction of the same substrate with DMBI followed by quenching with D2O gave a reduced product which contained a deuterium label at the α-position.These results were interpreted in terms of a mechanism involving an enolate intermediate produced by either a one-step hydride transfer or a sequential transfer of an electron (e) and a hydrogen atom (H^.) from the reducing agent to the substrate.