Reductive Claisen Rearrangements Catalyzed by Copper Hydrides

Abstract

Copper is a useful and desirable metal for the catalysis of reactions in chemistry, because of its low cost and toxicity in comparison with other metals. In our continuing interest and exploration of transformations mediated by copper hydrides, we discovered that in the presence of boranes, ligated copper hydrides catalyzed the reductive Claisen rearrangement reaction of allyl acryloyl esters to give alpha,beta-disubstituted carboxylic acids diastereoselectively. Propargyl acryloyl esters also underwent the analogous reductive Claisen rearrangement. We found that the ligand and the stoichiometric reductant critically affect the outcome of the reaction. Interestingly, only the reduction, but not the rearrangement, occurs when stoichiometric copper hydride is used. This indicates that it is the boron enolate that undergoes the Claisen rearrangement, and may be providing experimental evidence that intermediate obtained from copper hydride reduction is not a copper enolate as is often protrayed and depicted, but may instead be an alpha-cuprio ester.