Metal-ligand bifunctional catalysis

Catalytic Hydrogenations with Metal-Ligand Bifunctional Catalysis... 

The concerted delivery of protons from OH and hydride from RuH found in these Shvo systems is related to the proposed mechanism of hydrogenation of ketones (Scheme 7.15) by a series of ruthenium systems that operate by metal-ligand bifunctional catalysis [86]. A series of Ru complexes reported by Noyori, Ohkuma and coworkers exhibit extraordinary reactivity in the enantioselective hydrogenation of ketones. These systems are described in detail in Chapters 20 and 31, and mechanistic issues of these hydrogenations by ruthenium complexes have been reviewed [87]. 

Dehydrogenation of Imines and Alcohols by Shvo Complexes 191 Catalytic Hydrogenations with Metal-Ligand Bifunctional Catalysis 193... 

One of the systems was found to be very efficient catalyzing enantioface-selective hydrogen transfer reactions to aromatic and in particular to aliphatic ketones with up to 95% ee. Regarding the latter reaction these are unprecedented ee values. The reaction mechanism of these transformations is best described as a metal-ligand bifunctional catalysis passing through a pericyclic-like transition state. 

Figure 1.19. (5)-TolBINAP/(5,5 )-DPEN-RuH2 species and diastereomeric transition states in the metal-ligand bifunctional catalysis the equatorially oriented phenyl substituents in the DPEN ligands are omitted in the transition states 5 -19A and Re-19A (Ar = 4-CH3C6H4 O = Ru ax = axial, eq = equatorial).

Noyori, R., Yamakawa, M. and Hashiquchi, S. Metal-Ligand Bifunctional Catalysis A Nonclassical Mechanism for Asymmetric Hydrogen Transfer between Alcohols and Carbonyl Compounds. J. Org. Chem. 2001, 66, 7931-7944. 

A chiral Ru hydride 23 is formed and it is assumed that the hydrogen transfer occurs via metal-ligand bifunctional catalysis. The N-H linkage may stabilize a transition state 24 by formation of a hydrogen bond to the nitrogen atom. Stereochemistry is determined by formal discrimination of the enantiofaces at the sp2 nitrogen atom of the cyclic imine. 

Noyori R, Yamakawa M, Hashiguchi S (2001) Metal-ligand bifunctional catalysis a non-classical mechanism for asymmetric hydrogen transfer between alcohols and carbonyl compounds. J Org Chem 66 7931-7944... 

In this species, there is no direct coordination of the ketone to the Ru, but rather an outer-sphere association with an orientation of the ketone such that two H atoms can be transferred from the 18-electron hydride, one coming from the hydridic H and the other from the NHj group. This nonclassical mechanistic pathway is now widely accepted for this class of catalysts and is referred to as metal-ligand bifunctional catalysis. Theoretical work of Andersson and co-workers and Noyori et al. provided support for the mechanism and further details are discussed in a review by Noyori et al. ... 

There is general agreement that these systems also are proceeding via metal-ligand bifunctional catalysis, but the details of the reactive species remain somewhat obscure. A somewhat simplified version of the proposal by Noyori and co-workers for the system under basic conditions is shown in the following Scheme ... 

Noyori, R. Sandoval, C. A. Muniz, K. Ohkuma, T. Metal-ligand bifunctional catalysis for asymmetric hydrogenation. Philos. Trans. R. Soc. A-Math. Phys. Eng. Sci. 2005, 363,901-912. 

Yamakawa, M. Ito, H. Noyori, R. The metal-ligand bifunctional catalysis A theoretical study on the ruthenium(II)-catalyzed hydrogen transfer between alcohols and carbonyl compounds. /. Am. Chem. Soc. 2000,122,1466-1478. 

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