Metal-Free Catalytic Hydrogenations

Hydrogenation, the addition of hydrogen to unsaturated organic compounds, is broadly used in chemical production and usually mediated by precious-metal catalysts. Now, adding to a developing trend to go metal-free, Douglas W. Stephan and colleagues at the University of Windsor, in Ontario, report the first catalysts that eschew costly metals and can use H₂ directly to reduce imines, nitriles, and aziridines to primary and secondary amines under mild reaction conditions (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200702908).

Organocatalysts exist that can hydrogenate enones and imines, but these catalysts use a surrogate source of H₂, such as a Hantzsch ester (a pyridine diester). The Canadian researchers have instead created a phosphonium borate system that activates H₂ for direct hydrogenations.

For example, they use their catalyst to hydrogenate imines. The phosphonium group of the catalyst zwitterion initially protonates an imine, leading to formation of an intermediate that subsequently undergoes nucleophilic attack by the borohydride anion. Hydride transfer yields the amine.