Continuous Hydrogenation Reaction Yields Chiral Product Without Solvent

The ideal reaction for an industrial chemist, Nobel Laureate Sir John W. Cornforth said humorously in 1975, “is something to be carried out in a disused bathtub by a one-armed man who cannot read, the product being collected continuously through the drain hole in 100% purity and yield.” Rare is the reaction that clears that bar, but a team at the University of St. Andrews, in Scotland, is giving it their best shot. They’ve developed a continuous hydrogenation that requires no solvent and yields a nearly enantiomerically pure product (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201302718). Similar hydrogenations require supercritical carbon dioxide solvent. Solventless reactions, however, can reduce costs and waste. Rubén Duque, Peter J. Pogorzelec, and David J. Cole-Hamilton anchored a chiral rhodium catalyst to alumina using phosphotungstic acid. They packed the alumina into a 10-mL steel column and pumped substrate and hydrogen gas through it. At ambient temperature and roughly 5-atm pressure, they collected (S)-(–)-dibutyl 2-methylsuccinate directly from the reactor in 98% enantiomeric excess. The product elutes with 45-ppb rhodium, which leaches from the reactor. The reaction’s enantioselectivity decreases after 23 hours. The team is working on making the catalyst more stable.