Transfer hydrogenation rhodium catalysis

Gladiali, S., Pinna, L., Delogu, G., De Martin, S., Zassinovich, G., Mestroni, G. Optically active phenanthrolines in asymmetric catalysis. III. Highly efficient enantioselective transfer hydrogenation of acetophenone by chiral rhodium/3-alkyl phenanthroline catalysts. Tetrahedron Asymmetry 990, 1, 635-648. 

Polborn and Severin [23] recently reported ruthenium- and rhodium-based TSAs for the transfer hydrogenation reaction. These complexes were used as catalyst precursors in combination with molecular imprinting techniques. Phosphinato complexes were prepared as analogs for the ketone-associated complex. They demonstrated that the results obtained in catalysis were better in terms of selectivity and activity when these TSAs were imprinted in the polymer. This shows that organometallic complexes can indeed serve as stable TSAs (Figure 4.9). 

For a leading reference, see Willner, L, Maidan, R. and Shapira, M., Thermal and photochemical regeneration of nicotinamide cofactors and a nicotinamide model compound using a water-soluble rhodium phosphine catalyst, J. Chem. Soc., Perkin Trans. 2, 1990, 559. (b) For a recent reference, see Hollmann, F., Kleeb, A., Otto, K. and Schmid, A., Coupled chemoenzymatic transfer hydrogenation catalysis for enantioselective reduction and oxidation reactions. Tetrahedron Asymmetry, 2005, 16, 3512. 

The solvated ion pair [(C8Hi7)3NMe] [RhCl4]", formed from aqueous rhodium trichloride and Aliquat-336 in a two-phase liquid system, hydrogenates a,p-unsaturated ketones and esters selectively at the C==C double bond. The reduction of benzylideneacetone follows first-order kinetics in substrate below 0.2 M, and approaches second-order in hydrogen at partial pressures below 0.12 atm (1 atm = 101.3 kPa). The catalysis also depends on the nature of the solvent, the phase transfer catalyst and stirring rates. 

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