Noyori catalytic asymmetric hydrogenation mechanism

The role of base in the Noyori s asymmetric hydrogenation of ketone catalysed by trans-RutdiphosphinejCMdiamine) has been evaluated. Several catalytic intermedi- ates have been characterized and a mechanism (Scheme 17) has been presented.338... 

More recently Noyori developed asymmetric hydrogenation of simple ketones with BlNAP/diamine-ruthenium complexes.In this system the catalytic process contrasted with the conventional mechanism of asymmetric hydrogenation of unsaturated bonds which requires metal-substrate 7t-complexation. In fact with BlNAP/diamine-ruthenium neither the ketone substrate nor the alcohol product interacted with the metallic centre during the catalytic cycle. The enantiofaces of the prochiral ketones were differentiated on the molecular surface of the coordinatively saturated RuH intermediate. 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

Scheme 1.45 A revised catalytic cycle for the asymmetric hydrogenation of aromatic ketones in propan-2-ol by Noyori s (pre)catalyst 1 based on a computed MEp253 follows a H /H" " outer sphere hydrogenation mechanism (see text). KO-f-C4H9 free conditions X = Y = H. Under high KO-f-C4H9 concentration X = Y = K and/or H. Formation of the major enantiomeric product is shown. (Adapted from Dub, P. A. et al., /. Am. Chem. Soc., 136, 3505-3521. Copyright 2014 American Chemical Society.)...

The side-on (r)2) bonding in M r 2-H2 and other a-complexes has been termed non-classical, on analogy to the 3-center, 2-electron bonding in non-classical carbocations and boranes (Fig. 2). One of the first questions raised when H2 complexes were discovered is whether they would be important in catalytic reactions. As will be shown below the answer is an emphatic yes, as exemplified by the elegant asymmetric catalytic hydrogenation systems of Nobel-laureate Ryoji Noyori. Also, the mechanism of catalytic silane alcoholysis directly involves two different a complexes M(r 2-Si-H) and M(r 2-H2). In both of these systems, the crucial step is heterolytic cleavage of the H H and/or Si-H bond, the primary subject of this review. 

A general mechanism for the Wilkinson catalytic hydrogenation method, adapted with permission of John Wiley 8e Sons, Inc. from Noyori, Asymmetric Catalysis in Organic Synthesis, p. 17. Copyright 1994. 

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