Selective Transfer Hydrogenation of α,β-Unsaturated Carbonyl Compounds

Allylic alcohols are very versatile compounds which are used in a large variety of industrial processes. Transfer hydrogenation of α,β-unsaturated carbonyl compounds is a very appealing approach to obtain allylic alcohols. It avoids the use of stoichiometric and hazardous reagents such as NaBH₄ or LiAlH₄. Furthermore, compared to classical catalytic hydrogenations, these reactions do not need special equipment such as autoclaves or high-pressure reactors. Thus, protocols for transfer hydrogenation are cheaper and safer. One of the major problems in the reduction of unsaturated carbonyl compounds is to achieve high chemoselectivity. Free energy barriers for the reduction of carbonyl compounds and for the reduction of conjugated carbon-carbon double bonds are often very close in value. For that reason, mixtures of products as well as fully reduced products are often obtained making the scope of many catalysts limited. Herein we review the literature on selective transfer hydrogenation of α,β-unsaturated carbonyl compounds to the allylic alcohols using soluble transition metal complexes as catalysts. Ruthenium is the most employed metal in this field followed by iridium. In addition, some examples using complexes based on other transition metals including some first-row transition metals were found. This is a rapidly growing field. The review is structured according to the metals and to the hydrogen source used. In addition to these reductions catalysed by transition metal-based catalysts, there exists another type of transfer hydrogenation which follows a different mechanism which is known as the Meerwein-Ponndorf-Verley (MPV) reaction. This reaction uses metal alkoxide catalysts based on cheap metals such as aluminium. Whereas the original catalysts such as aluminium tri-isopropoxide were very slow, new variants have been developed that are much faster. The mechanisms reported for the MPV reaction and the transfer hydrogenation are briefly summarized, and the most interesting features of all references cited in this work are highlighted.