Settling Down After It's All Over

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Science  23 Jun 2006:
Vol. 312, Issue 5781, pp. 1717
DOI: 10.1126/science.312.5781.1717a

Homogeneous catalysis maximizes the frequency and conformational flexibility of the collisions between catalyst and substrate, but a major shortcoming is the challenge of separating catalyst from product once the reaction is complete. Biphasic solvent systems can mitigate this problem, but often do so at the expense of reduced mixing efficiency.

Kim et al. demonstrate how to take advantage of molecular self-assembly in order to recover the catalyst in the dehydrogenative coupling of benzylic alcohols with olefins. The reaction is catalyzed by a phosphine-coordinated Rh complex and aminopyridine chelator (with one equivalent of olefin acting as the hydrogen acceptor). The phosphine and pyridine fragments are tethered to barbiturate derivatives that can assemble into a rigid network together with a third component—a triaminopyrimidine—by means of hydrogen bonding. When a dioxane suspension of the reagents and network-bound catalysts is heated to 150°C, the hydrogen bonds break, and a homogeneous solution forms. High yields are obtained in 2 hours, and cooling then regenerates the self-assembled network and precipitates the catalyst, allowing the product to be decanted. Catalysts were cycled eight times without significant loss in activity; moreover, switching substrates between cycles confirmed that none of the desired products partitioned into the solid phase. — JSY

Org. Lett. 8, 10.1021/ol0608045 (2006).

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