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Web Date: September 6, 2007

Metal-Free Catalytic Hydrogenations

Phosphonium borate organocatalysts are a first for direct hydrogenation of unsaturated nitrogen compounds
Department: Science & Technology | Collection: Green Chemistry
News Channels: Environmental SCENE
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HYDROGENATION CYCLE
Phosphonium borate catalysts (R = 2,4,6-(CH3)3C6H2 or tert-butyl) activate H2 for direct hydrogenation of imines (Ph = phenyl, R??? = various groups) to amines.
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HYDROGENATION CYCLE
Phosphonium borate catalysts (R = 2,4,6-(CH3)3C6H2 or tert-butyl) activate H2 for direct hydrogenation of imines (Ph = phenyl, R??? = various groups) to amines.

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.

 
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