J. Am. Chem. Soc. 134, 4561–4564 (2012)

Asymmetric hydrogenation of alkenes is arguably the most successful method for the production of single enantiomer products from prochiral starting materials. The majority of catalysts for these reactions, however, are based on precious metals such as ruthenium, rhodium or iridium. Furthermore, many of these reactions reported so far require the presence of coordinative groups in the substrates to 'lock' the conformation of the intermediate and ensure high selectivity. Now, Paul Chirik and co-workers from Princeton University have reported a series of cobalt complexes that can catalyse the highly enantioselective hydrogenation of unfunctionalized alkenes.

Non-precious metal catalysts for asymmetric hydrogenation have been described previously, but these have had problems with poor selectivity or activity. Chirik and co-workers, among others, have reported highly active cobalt and iron catalysts based on coordination of tridentate bis(imino)pyridine ligands — but so far these ligands have involved symmetric systems not suitable for asymmetric catalysis. In the present work, a redesign of the ligand allows incorporation of a single chiral amine. Initially a series of methyl-substituted catalysts were targeted, but the route Chirik and co-workers used for the synthesis also led to the formation of cyclometallated complexes.

The methylated complexes catalysed the hydrogenation of a series of styrene derivatives with generally good selectivities and activities. The cyclometallated compounds, however, although poor as precatalysts for the standard substrates, were found to be useful for hydrogenation of one very unhindered substrate where the methylated catalyst had performed badly. This intriguing pattern of reactivity will help to inform the design of future catalysts.