Mechanism of Enantioselective Hydrogenation

Abstract

John M. Brown IntroductionThe discovery of the first practical catalyst for homogeneous hydrogenation by Wilkinson, Osborn, Jardine and Young in 1965 [1] occurred at around the same time that others, especially Mislow and Horner [2], were demonstrating that chiral trivalent phosphorus compounds were capable of existing as stable, noninterconverting enantiomers. With suitable adaptation of Wilkinson's catalyst, a prostereogenic alkene could be hydrogenated with preferential formation of one enantiomer of the reduced product. The possibility of such enantioselective hydrogenation was recognized by both Horner [3] and Knowles [4], but it was Knowles who won the race to demonstrate the first example in 1968. This led rapidly to a period of seminal discoveries: the application of chelate biphosphine ligands came from Kagan [5], and the development of a full-scale enantioselective hydrogenation of a dehydroamino acid to provide a rhodium-complex-based catalytic synthesis of l-Dopa by Knowles' team at Monsanto [6]. For many years this provided a substantial part of the supply of the main drug active in the control of Parkinson's disease. Kagan was also the first to demonstrate, in his synthesis and application of DIOP (derived very simply from RR-or SS-tartaric acid), that the difficult synthesis of enantiomerically pure phosphines was unnecessary for effective enantioselective hydrogenation, since a suitable chelate backbone could provide the necessary level of stereochemical control. For many years the development of enantioselective hydrogenation converged on the preparation of enantiomerically pure chelate diphosphines and the application of their rhodium complexes to the hydrogenation of dehydroamino acids, enamides and closely related reactants [7]. Although both ruthenium [8] and iridium catalysts [9] for homogeneous hydrogenation were known at an early stage, the development of effective enantioselective hydrogenation in these two spheres occurred much later. For ruthenium enantioselective hydrogenation, the spectacular efficiency of BINAP catalysts [10] developed by Noyori's group was first demonstrated for alkenes, and then in rather greater depth for ketones [11]. The high efficiency of iridium complexes in hydrogenation had been demonstrated 1073The Handbook of Homogeneous Hydrogenation. Edited by J. G. de Vries and C. J. Elsevier

A Comparison of Phosphaferrocene and Phospharuthenocene Ligands in Rh+‐Catalysed Enamide Hydrogenation Reactions: Superior Performance of the Phospharuthenocene

“…those between 8 and 9, and an analysis quantifying these differences in Brunners terms [49] is given in Table 1. The phosphaferrocene complex (AE )-18 strongly resembles the homologue (+)-24, previously described by Tanaka et al [19] and shows a classical [50] ÀPPh 2 aryl group disposition having a pseudo l-configuration at the methylene bridge and an edge-on axial phenyl. The second "endo" phenyl lies equatorially, face-on to the diene and intercalates nicely between two methyl groups of the Cp* ligand.…”

A Local Pair Natural Orbital Coupled Cluster Study of Rh Catalyzed Asymmetric Olefin Hydrogenation

“…The field has gained additional momentum from the discovery that highly enantioselective olefin hydrogenation can be achieved not only by the classic Rh catalysts with bidentate phosphorus ligands but also by Rh complexes containing BINOL-based monodentate ligands such as phosphites, phosphonites, and phosphoramidites . For the classic catalysts with bidentate ligands, the major aspects of the mechanism appear to be well established by now, and it is generally accepted that enantioselectivity results from kinetic control. That is, the minor diastereomer of the initially formed catalyst−substrate adduct reacts faster and thus yields the major product (anti-lock-and-key principle).…”

Asymmetric Hydrogenation

“…Early studies using cationic rhodium complexes with C2-symmetric diphosphine ligands with two diaryl substituents at each phosphorus atom led to the so-called "unsaturated mechanism". This mechanism, proposed by Halpern and Brown, is based on the following experimental facts and considerations [25][26][27][28].…”