Outer-sphere mechanisms metal-ligand bifunctional

Outer Coordination Sphere Catalysts. In the classical hydrogenation catalysis shown previously, the substrate must be coordinated to the metal prior to its insertion into a metal-hydrogen bond. However, in recent years, it has been found that unsaturated polar bonds can be hydrogenated without coordination of the substrate to the metal (37). Two well-known, nonclassical possibilities for the hydrogenation of unsaturated polar bonds, such as ketones, are the metal-ligand bifunctional mechanism (38) and the ionic mechanism (39). In the metal-ligand bifunctional mechanism discovered by Noyori (recipient of the Nobel Prize in 2001) for highly efficient ruthenium amine complexes, the hydridic RuH and... 

In the ionic mechanism, the proton and the hydride are sequentially transferred to a substrate, whereas in the metal-ligand bifunctional mechanism, the transfer occurs simultaneously. In both cases, the source of the H is a transition metal hydride, but the source of the proton can be a metal hydride or N—H or 0—H bonds. It seems likely that some previously reported catalysts could follow nonclassical outer-sphere mechanisms... 

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. ... 

Scheme 1.46 A revised catalytic cycle for the asymmetric transfer hydrogenation of aromatic ketones in propan-2-ol by the Noyori-Ikariya (pre)catalyst 2 demonstrates crossover of the reaction pathways the product is obtained via a H"/H+ outer-sphere hydrogenation mechanism and/or step-wise metal-ligand bifunctional mechanism (see text). Formation of the major enantiomeric product is shown. (Adapted from Dub, P. A. et al., /. Am. Chem. Soc., 135, 2604-2619. Copyright 2013 American Chemical Society.)...

Within the monohydridic route, apart from the already explained inner-sphere mechanisms, there is another possibility involving the concerted outer-sphere transfer of one hydride and one proton to the corresponding substrate (Scheme 4b). This mechanism is very common to the so-called bifunctional catalysts. This term was proposed by Noyori for those catalysts having one hydrogen with hydridic character directly bonded to the metal center of the catalyst, a hydride ligand, and another hydrogen with protic character bonded to one of the ligands of the metal complex (20). In Scheme 9, examples of bifunctional catalysts that are synthesized... 

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