Ekambaram Balaraman, Eugene Khaskin, Gregory Leitus and David Milstein
We’ve discussed ruthenium-catalysed oxidative cross-coupling before, and the area appears to developing rapidly. Milstein and co-workers have reported dimerisation of alcohols (to esters)1 and coupling of alcohols with amines (to amides)2 previously, and now report a remarkable oxidation of alcohols to acids using water as the terminal oxidant.
The reported reaction is operationally straightforward, requiring only the alcohol substrate, catalyst, a slight excess of hydroxide and water at reflux. Alcohols are oxidised with high tolerance and high yield, though any present alkenes are subsequently hydrogenated.
The ruthenium catalyst operates with a bipyridyl ligand that can oxidise the metal centre through dearomatising loss of a proton. The proposed mechanism of the reaction cycles this dearomatising/aromatising process with a bound alochol and water to give the acid and two equivalents of hydrogen.
The authors comment on the important role hydroxide plays in the mechanism. Without it, only trace product is observed. They conclude that hydroxide is necessary to scavenge the acid (as the salt) in order to regenerate the active catalyst.
Direct oxidations from alcohols to acids are uncommon, yet, obviously, very useful. Not only does this report address a general synthetic problem, it does so with an incredibly mild and accessible method.
1. J. Am. Chem. Soc. 2005, 127, 10840
2. Science 2007, 317, 790