Outer-sphere mechanisms bifunctional catalysts

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

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... 

Bifunctional rhenium complexes related to the Shvo catalyst have been used in TH reactions, including tests on three non-prochiral imines, with TOFs up to 79 h obtained for imines. In common with the Shvo catalysts, DFT calculations have indicated the operation of an outer-sphere mechanism for the reaction [118]. 

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... 

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.)...

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... 

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