Gel Formation Induces Organocatalysis

An L-proline construct that forms supramolecular gels can also act as an organocatalyst, reports Francisco Rodríguez-Llansola, Beatriu Escuder, and Juan F. Miravet of James I University, in Spain (J. Am. Chem. Soc., DOI: 10.1021/ja902589f). The compound, formed by attaching L-proline groups to the ends of an L-valine-based scaffold, aggregates in solvents through multiple hydrogen-bonding interactions to form a temperature-dependent catalytic gel. The researchers found that the proline moieties in nitroethane catalyze the Henry nitroaldol reaction, which they demonstrated by converting the aldehyde group of 4-nitrobenzaldehyde to a nitroaldol group. The reaction proceeded with 99% yield at 5 °C when the system was gelled but didn't work at 25 °C when the system was in liquid form. The researchers propose that the close proximity of the proline groups in the gel allows them to work cooperatively to deprotonate nitroethane, which then adds to the aldehyde group. The tunable nature of such supramolecular catalysts, as well as their easy recovery by filtration, should make them useful for a variety of chemical transformations, the researchers say.