Transfer hydrogenation in aqueous media

• An account of the work on transfer hydrogenation in aqueous media done by the Xiao group is given.
• Various polar bonds and heterocycles were reduced by aqueous transfer hydrogenation.
• New transfer hydrogenation catalysts, Iridicyles, were discovered.
• Water accelerated transfer hydrogenation reactions.
• Solution pH affects the rate and selectivity.

Transfer hydrogenation has become a versatile and practical method for reduction in organic synthesis. The development of aqueous transfer hydrogenation reactions is not only fundamentally interesting in terms of understanding enzymatic catalysis, but also offers economic and environmental benefits, as water is cheap and nontoxic. In this review paper, an account of the work on transfer hydrogenation in aqueous media done by the Xiao group is given. Aqueous transfer hydrogenation of ketones, aldehydes and heterocycles as well as reductive amination reactions, including extension into biomass-derived platform molecules, has been successfully developed employing the classical Noyori-type catalysts or the newly invented iridacycles, with most of the reactions taking place “on water”. Water is shown to be an enabling medium for transfer hydrogenation reactions of various features. Not only can it accelerate a reduction, it also provides a simple tool, the solution pH, to control a reaction.

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