Microcapsule-enabled dual-catalyst systems

One-pot multistep reactions in which one catalyst facilitates formation of a transient intermediate that can be trapped and converted into the desired product by a second catalyst are becoming popular in organic synthesis. The utility of this approach has now been extended by D. Tyler McQuade and his group at Cornell University, who have developed an improved strategy for preventing problematic catalyst-catalyst interactions (J. Am. Chem. Soc., 2006, 128, 15586). Typically, the two catalysts are kept isolated by immobilizing them on solid supports. But this method often is not very efficient. The Cornell researchers instead encapsulate one of the catalysts, in their case a polymeric amine, in emulsion droplets. The emulsified polymer is then cross-linked, and the resulting microcapsules are isolated as a dry powder. The researchers tested the catalyst in a two-step addition reaction in which the second catalyst was a nickel-based Lewis acid. The amine and Lewis acid catalysts normally are incompatible, but the microcapsules permitted efficient formation of the desired addition product that can't be obtained when the steps are run as separate reactions.