Origin of Enantioselectivity in the Ru(arene)(amino alcohol)-Catalyzed Transfer Hydrogenation of Ketones

Mechanistic aspects of transition metal-catalyzed hydrogen transfer reactions

In this tutorial review recent mechanistic studies on transition metal-catalyzed hydrogen transfer reactions are discussed. A common feature of these reactions is that they involve metal hydrides, which may be monohydrides or dihydrides. An important question is whether the substrate coordinates to the metal (inner-sphere hydrogen transfer) or if there is a direct concerted transfer of hydrogen from the metal to substrate (outer-sphere hydrogen transfer). Both experimental and theoretical studies are reviewed.

An outstanding catalyst for asymmetric transfer hydrogenation in aqueous solution and formic acid/triethylamine

A Rh/tetramethylcyclopentadienyl complex containing a tethered functionality has been demonstrated to give excellent results in the asymmetric transfer hydrogenation of ketones in both aqueous and formic acid/triethylamine media.

Asymmetric transfer hydrogenation: chiral ligands and applications

Hydrogen transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational simplicity and high selectivity. In this tutorial review the most significant advances recently achieved in the stereoselective reduction of unsaturated organic compounds catalyzed by homogeneous transition metal complexes are critically reviewed. A sharp growth of the synthetic applications of this technique in the synthesis of fine chemicals is predictable as the use of transition metal catalyzed reactions will become more familiar to synthetic chemists.

Mechanistic Insights into the Phosphine-Free RuCp*-Diamine-Catalyzed Hydrogenation of Aryl Ketones: Experimental and Theoretical Evidence for an Alcohol-Mediated Dihydrogen Activation

The commercially available chiral diamine quincorine-amine, originally derived from quinine, was found to be a highly active catalyst for ruthenium-catalyzed hydrogenation of ketones. The complex formed between the quincorine-amine, containing both a primary and a quinuclidine amino function, and RuCp*Cl catalyzes the hydrogenation of aromatic and aliphatic ketones in up to 90% ee approximately 24 times faster than previously reported Ru-diamine complexes. The pseudo-enantiomer of the quincorine-amine, i.e., quincoridine-amine, also showed high activity; however, the enantioselectivities obtained with this catalyst were lower. The reason for the lower, but opposite stereoselectivity seen with the quincoridine-amine, as compared to the quincorine-amine, was rationalized by a kinetic and computational study of the mechanism of the reaction. The theoretical calculations also revealed a significantly lower activation barrier for the alcohol-mediated split of dihydrogen, as compared to the nonalcohol-mediated process, a finding of utmost implication also for the diphosphine/diamine-mediated enantioselective hydrogenation of ketones.

A Class of Ruthenium(II) Catalyst for Asymmetric Transfer Hydrogenations of Ketones

Ruthenium dimer 6 (readily available in two steps from TsDPEN) is converted directly to monomeric asymmetric transfer hydrogenation catalyst 3 in situ under the conditions employed for ketone reduction. Catalyst 3 is a significantly more active catalyst for this application than the untethered derivative, exhibits higher enantioselectivities across a range of substrates, and appears to be highly stable to the reaction conditions. It is active at loadings of as low as 0.01 mol %, and reductions at the 0.1 mol % level are complete within 20 min at 80 degrees C without significant loss of enantioselectivity.

Artificial metalloenzymes based on biotin-avidin technology for the enantioselective reduction of ketones by transfer hydrogenation

Most physiological and biotechnological processes rely on molecular recognition between chiral (handed) molecules. Manmade homogeneous catalysts and enzymes offer complementary means for producing enantiopure (single-handed) compounds. As the subtle details that govern chiral discrimination are difficult to predict, improving the performance of such catalysts often relies on trial-and-error procedures. Homogeneous catalysts are optimized by chemical modification of the chiral environment around the metal center. Enzymes can be improved by modification of gene encoding the protein. Incorporation of a biotinylated organometallic catalyst into a host protein (avidin or streptavidin) affords versatile artificial metalloenzymes for the reduction of ketones by transfer hydrogenation. The boric acid.formate mixture was identified as a hydrogen source compatible with these artificial metalloenzymes. A combined chemo-genetic procedure allows us to optimize the activity and selectivity of these hybrid catalysts: up to 94% (R) enantiomeric excess for the reduction of p-methylacetophenone. These artificial metalloenzymes display features reminiscent of both homogeneous catalysts and enzymes.

“Tethered” Ru(II) Catalysts for Asymmetric Transfer Hydrogenation of Ketones

Stereochemically well-defined ruthenium(II) catalysts have been applied to the asymmetric transfer hydrogenation of a series of ketones. In one case, statistical experimental design was employed to optimize the enantiomeric excess of the product. In the case of the TsDPEN-based systems, the replacement of trans-1,2-diphenyl substitution with cis-, or deletion of one of the phenyl groups, results in significant deterioration of the enantiomeric excess. A new method is described for the synthesis of tethered amino alcohol-containing catalysts.

A remarkably effective catalyst for the asymmetric transfer hydrogenation of aromatic ketones in water and air

A Rh(III) complex generated in situ from [Cp*RhCl2]2 and (1R,2R)-N-(p-toluenesulfonyl)-1,2-cyclohexanediamine (TsCYDN) serves as a remarkably effective, robust catalyst for the asymmetric transfer hydrogenation of aromatic ketones by HCOONa in water in air, affording alcohols in up to 99% ee.