International audienceA library of organometallic compounds derived from primary and secondary amines cyclometalated by ruthenium(II), rhodium(III) and iridi- um(III) was tested in the asymmetric transfer hydrogenation of a number of ketones and imines. All compounds displayed high catalytic activity for the reduction of ketones under mild conditions. The most enantioselective catalysts were based on secondary amines containing two asymmetric carbon atoms bound to the nitrogen atom. For the reduction of aryl alkyl ketones [Ar(C[DOUBLE BOND]O)R where R=CH3 or CH2R′] the cyclometalated ruthenium and rhodium derivatives of the (2R,5R)-2,5-diphenylpyrrolidine ligand displayed the best results with respect to activity and selectivity (ees up to 97%). However, for the reduction of aryl tert-alkyl ketones [Ar(C[DOUBLE BOND]O)R′′ in which R′′ is a tertiary alkyl group] the best catalyst was a ruthenium compound derived from bis[(R)-1-phenylethyl]amine, allowing the reduction of isobutyrophenone and cyclohexyl phenyl ketone which were both reduced with high enantioselectivities (ees up to 98%). This shows that the cyclometalated compounds have a high substrate specificity. In addition, acyclic and cyclic imines were reduced with good selectivities by both rhodium(III) and iridium(III) metalacycles built up with (2R,5R)-2,5-diphenylpyrrolidine
The copper‐catalyzed asymmetric hydrosilylation of ketones is an efficient method for the synthesis of chiral enantiopure secondary alcohols. Herein, we present a detailed computational study (DFT/B3LYP) of the copper(I)‐catalyzed reaction. In particular, the two transition states involved in the catalytic cycle have been determined. The insertion of the ketone into the Cu–H bond was found to have a lower activation barrier than the reaction of the copper alkoxy intermediate with the silane, which regenerates Cu–H along with the silyl ether product. Our findings also reveal the importance of the copper hydride dimer in controlling the reactivity toward the ketone. The conclusions are supported by experimental mechanistic investigations including kinetic studies, kinetic isotope effect, and isotope labeling measurements.
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