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This account describes studies on the catalytic hydrogenation of carbonyl groups that are not readily reduced. The development of well-defined chiral phosphine-diamine ruthenium catalysts for hydrogenation of ketones and esters is described. These chiral catalysts promote the hydrogenation of ketones functionalised with tertiary alkyl and gem-dimethyl groups with enantioselectivities of up to 98% ee. They also reduce a range of heterocyclic and nitrilefunctionalised ketones that react slowly with or inhibit other catalysts. Changing from ruthenium to iridium complexes gives highly selective catalysts that also hydrogenate (the more reactive) secondary alkyl functionalised ketones, including those with unprotected amines. These catalysts operate rapidly at room temperature, require low catalysts loadings, and deliver up to 98% ee. Achiral variants of these catalysts also promote the hydrogenation of esters under mild conditions. This account also describes our studies that shed light on the mechanism of action of these catalysts, and a stereochemical model is proposed. [ABSTRACT FROM AUTHOR]