Alcohols are a good energy source from biomass, and can be converted to functional groups, such as amides, amines, carboxylic acids, esters and imines. One of the key transformations is the dehydrogenation of alcohols. The common method is to use traditional oxidants, or use catalysts based on precious metals. However, these two methods are either not “green” or sustainable. The 3d transition metals are cheap and relatively nontoxic, and they have a high abundance and a large annual production. Catalysts based on these metals could have a good performance on the dehydrogenation.
The acceptorless dehydrogenation of alcohols means that an alcohol, catalyzed by metal complexes, undergoes a formal oxidation to the corresponding carbonyl compound accompanied by the removal of H2 gas. The advantage of these dehydrogenative transformations is that alcohol oxidation occurs in the absence of an oxidizing agent, which leads to less waste and more environmentally friendly reaction conversions.
Two catalysts based on Earth-abundant transition metals for acceptorless alcohol dehydrogenation are described. Both catalysts mediate the dehydrogenative coupling of primary alcohols and amines to form imines. The first developed catalyst is chromium(III) tetraphenylporphyrin chloride (Cr(TPP)Cl), which is a well-known complex. This is the first time for Cr(TPP)Cl to be applied for acceptorless alcohol dehydrogenation. The second developed catalyst is vanadium(IV) salen dichloride. The mechanisms for two catalysts have been investigated by various practical experiments, and a metal-ligand cooperative pathway has been proposed.
The results show that chromium and vanadium complexes may be useful alternatives to other catalysts based on Earth-abundant metals when developing alcohol dehydrogenation reactions