Metal-free catalytic reactions provide an attractive alternative to transition-metal catalysts and stoichiometric reagents, which can be toxic, expensive, and can generate excessive waste. Two research groups have now independently devised the first metal-free catalytic hydrogenations of ketones and aldehydes to make alcohols, one of the most fundamental reactions in chemistry.
The reactions leverage so-called frustrated Lewis pairs (FLPs), in which a sterically encumbered Lewis acid and Lewis base work as a cooperative catalyst system to activate small molecules such as H2 to functionalize unsaturated organic molecules.
“Catalytic reduction of carbonyl compounds by dihydrogen had for a long time been considered a domain of metal-containing systems,” comments Gerhard Erker of the University of Münster, in Germany, whose research includes FLP chemistry. “It is very remarkable that this important catalytic transformation has now been achieved metal-free. This significant development will help make frustrated Lewis pair chemistry increasingly useful and visible.”
In one of the new reports, Andrew E. Ashley and coworkers of Imperial College London coupled the bulky Lewis acid B(C6F5)3 with the Lewis base 1,4-dioxane, which also serves as the solvent. After adding H2, the researchers hydrogenated various alkyl and aryl ketones and aldehydes to make alcohols (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5088979). In a second report, Douglas W. Stephan of the University of Toronto and his group, who introduced the FLP concept in 2006, coupled B(C6F5)3 with diethyl ether and H2 to convert an array of alkyl and aryl ketones to alcohols (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja508829x ).
The secret to the new catalytic process, Stephan says, is the judicious choice of weakly basic ethers as solvents. The ethers optimize hydrogen bonding with the transient protonated carbonyl and promote hydride delivery to the C=O double bond. Even so, the approach isn’t ideal, researchers say. Transition-metal catalysts still provide better selectivity for performing asymmetric hydrogenations. And although the reactions do eliminate the need for transition metals, the fluorinated aromatic Lewis acids persist in the environment.