Abstract
The Rh-catalyzed enantioselective hydrogenation of N-acylated dehydroamino acids or their esters, initiated by Kagan, constitutes now a standard tool for the synthesis of natural and unnatural amino acids of high enantiomeric purity. The reaction proceeds via the unsaturate-dihydride mechanism in which the oxidative addition of hydrogen to Rh-enamide complex is the rate-limiting and stereodifferentiating step. In addition to this standard pathway, alternative dihydrideunsaturate mechanisms have also been reported. The formation of a RhH2 may precede the olefin-Rh interaction, depending on the chiral ligands and reaction conditions. Reactions using BINAP-Ru (II) complexes (BINAP = 2, 2'-bis (diphenylphosphino) -1, 1'-binaphthyl), find a wider synthetic scope, allowing asymmetric hydrogenation of a range of functionalized olefins and ketones. Interestingly, the Rh and Ru complexes with the same BINAP ligand exhibit an opposite sense of asymmetric induction in hydrogenation of dehydroamino acid derivatives. This review summarizes some mechanistic studies in this hydrogenation catalyzed by diphosphine-Rh and-Ru complexes.
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