Reductions of Conjugated Carbonyl Compounds with Copper Hydride–Preparative and Mechanistic Aspects

The reaction of lithium trimethoxyaluminum hydride with 0.5 molar equiv of cuprous bromide produces a heterogeneous mixture referred to as the Li complex, while sodium bis(2-methoxyethoxy)aluminum hydride with 1.0 molar equiv of cuprous bromide gives a similar mixture, the Na complex. Both reagents are effective in selective reduction of the olefin unit in conjugated ketones and esters, including two examples of acetylenic esters. The Li complex is more efficient with cyclohexenones, while the Na complex gives better yields in reduction of acyclic enones and enoates, especially in the presence of 2-butanol. Deuterium labeling experiments show that the hydrogen which is transferred to the β position of the conjugated carbonyl compound originates from the hydridocuprate reagent; the 2-butanol appears to serve as a weak acid, inhibiting polymerization. In the absence of 2-butanol, reduction of methyl cinnamate produces dimethyl meso-3,4-diphenyladipate as a major product, apparently the result of radical anion intermediates. Aldehyde, ketone, and halide functionality are reduced at rates comparable to the rate of enone reduction, but nitrile and ester units are inert.