Metal-Free Olefin Hydrogenations

Adding hydrogen to a carbon-carbon double bond is one of the most widespread reactions used in all corners of the chemical industry. But typically, a metal catalyst is required. Chemists led by Jan Paradies of Karlsruhe Institute of Technology, in Germany, have now unveiled a metal-free hydrogenation strategy that uses a frustrated Lewis pair as an alternative catalyst (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201204007). A frustrated Lewis pair is a reactive complex made up of a Lewis acid and a Lewis base in which the two partners are so bulky they can’t completely neutralize each other. Since they were discovered in 2006, they have been used to reduce unsaturated bonds such as imines, silyl enol ethers, and nitriles. But they didn’t seem to work on olefins. Paradies and coworkers hydrogenated more than a dozen olefins using H₂ and a frustrated Lewis pair—for example, one made from B(C₆F₅)₃ and (C₆F₅)P(C₆H₅)₂. They propose, from experimental and computational evidence, that the reaction mechanism involves protonating the olefin with a subsequent hydride transfer. “This is the first and only metal-free process using molecular hydrogen for one of the oldest chemical transformations,” Paradies says.