Metal-catalyzed hydrogenations diastereoselective hydrogenation

Dynamic Resolution of Chirally Labile Racemic Compounds. In ordinary kinetic resolution processes, however, the maximum yield of one enantiomer is 50%, and the ee value is affected by the extent of conversion. On the other hand, racemic compounds with a chirally labile stereogenic center may, under certain conditions, be converted to one major stereoisomer, for which the chemical yield may be 100% and the ee independent of conversion. As shown in Scheme 62, asymmetric hydrogenation of 2-substituted 3-oxo carboxylic esters provides the opportunity to produce one stereoisomer among four possible isomers in a diastereoselective and enantioselective manner. To accomplish this ideal second-order stereoselective synthesis, three conditions must be satisfied (1) racemization of the ketonic substrates must be sufficiently fast with respect to hydrogenation, (2) stereochemical control by chiral metal catalysts must be efficient, and (3) the C(2) stereogenic center must clearly differentiate between the syn and anti transition states. Systematic study has revealed that the efficiency of the dynamic kinetic resolution in the BINAP-Ru(H)-catalyzed hydrogenation is markedly influenced by the structures of the substrates and the reaction conditions, including choice of solvents. 

Another significant development in oxazoline chemistry is the application of oxazoline-containing ligands for asymmetric catalysis, such as palladium-catalyzed allylic substimtions, Heck reactions, hydrogenations, dialkylzinc additions to aldehydes, and Michael reactions. The discovery of diastereoselective metalation of chiral ferrocenyloxazolines has further expanded the availability of chiral ligands for metal-catalytic reactions. 

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