Metal-catalysed selective transfer hydrogenation of α,β-unsaturated carbonyl compounds to allylic alcohols
Allylic alcohols are highly important compounds which are used in a variety of processes. They can be obtained from α,β-unsaturated carbonyl compounds via stoichiometric or catalytic reductions. Catalytic transfer hydrogenation is a safe and convenient method to use for this purpose since it circumvents the use of hydrogen gas or stoichiometric amounts of hydride-containing reducing agents. Apart from the benefits regarding safety and costs, it also provides an extra handle for the chemist for improving rate and selectivity as different reducing agents, such as secondary alcohols or formate salts can be used. In addition, there is a very wide range of catalysts, homogeneous and heterogeneous, to choose from. This review covers the literature of the past 30 years on metal-catalysed transfer hydrogenation of α,β-unsaturated ketones and aldehydes to unsaturated alcohols using homogeneous or heterogeneous catalysts.