Hydrogenation outer-sphere" mechanism

NMR and UV-visible techniques have been used in the characterization of intermediates in the [Fe (edta)]" -promoted decomposition of hydrogen peroxide7 Fe complexes of edta, nta, and dtpa react with FISOs by an inner-sphere one-electron transfer mechanism with transient production of S04, in contrast to Cu, which reacts by an outer-sphere mechanism to give S04 and hydroxy radicalsFe -edta redox properties are relevant to Fe /Cu /H202 systems. ... 

Scheme 4. (a) Inner-sphere and (b) outer-sphere mechanisms for ketone hydrogenation. 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

Scheme 10. Concerted outer-sphere mechanism for the transfer hydrogenation using the Ru(r 6-arene)TsDPEN complex.

Fig. 4. Energy profiles in THF for both concerted pathways at B3LYP level for the hydrogenation of ketones by the Shvo s catalyst. Inner-sphere mechanism dashed fine outer-sphere mechanism solid line.

Fig. 5. Transition-states of the concerted outer-sphere mechanism for the hydrogenation of ketones in both the model (left) and complete (right) Shvo s 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. 

The rate of oxidation of acetophenoximes with bismuth(V) fluoride in a mixture of hydrogen fluoride and perchloric acid follows first-order kinetics in both the oxime and Bi(V). The reaction is acid catalysed. A bridged outer-sphere mechanism, involving formation of an iminoxy radical, has been suggested.81... 

For the latter reason atom or group transfer may sometimes also take place in outer-sphere processes, and it has even been suggested that atom transfer can be part of an outer-sphere mechanism, if only for the case of hydrogen atom transfer. Such a case is the Fe(II)—Fe(III) self-exchange reaction in water where hydrogen bonding between two ligands in the transition state [2] would... 

The identity of active catalytic species for the TH of ketones with our iron carbonyl [6.5.6]-P-N-N-P complexes was still unclear. Did the imine or imines on the ligand get reduced in situ, allowing catalysis to occur through a bifunctional outer sphere mechanism, as seen with the analogous ruthenium systems This question drove us to further investigate the mechanism of transfer hydrogenation with our first generation [6.5.6]-P-N-N-P systems. 

The activity of complex [lT2(CH3CN)(H)3(p-H)(P Pr3)2(p-Pz)2] as a catalyst for the hydrogenation of diphenylacetylene and ethylene contrasts with its inactivity when employed in the hydrogenation of A -benzylideneaniline. However, when transformed into its protonated derivative, for example, [lr2(CH3CN)(H)2(H2) ( 4-H)(P Pr3)2(p-Pz)2]BF4 by reaction with HBF4, the new complex becomes a very active catalyst for C=N hydrogenation [111]. The catalytic cycle involves fast elementary steps of hydride and proton transfer according to an ionic outer sphere mechanism that takes place at one of the iridium centers of the binuclear complex (Scheme 27). 

Outer-Sphere Mechanism for the Hydrogenation of Ketones and Imines... 

More recently, Morris reported the hydrogenation of benzonitrile catalyzed by a ruthenium complex containing a P-NH-NH-P tetradentate ligand (Equation 15.121). The presence of an amine N-H and metal hydride make it likely that the reaction occurs by an outer-sphere mechanism involving two sequential simultaneous transfers of the hydride and the N-H proton, first to the nitrile and second to the imine (Scheme 15.27). This proposal was supported by DPT calculations. The hydrogenation of aryl and heteroaryl nitriles catalyzed by a combination of [Ru(COD)(2-methylallyl) J and DPPF has also been reported to occur in high yields. 

The Concerted Bifunctional Mechanism. At the end of the 20th century, Noyori and co-workers reported a new hydrogen transfer mechanism in which a metal-hydride participates together with a primary or secondary amine NH bond (38,278,283,298). The hydrogen transfer catalytic activity of the isolated hydride intermediate allowed the authors to propose the outer-sphere mechanism outlined in Figure 93. The key intermediate is the six-membered metallacyle represented in the figure in which the metal-hydride and the NH functionality are interacting with the carbonyl bond. Theoretical studies support this mechanism and explain the encountered enantioselectivity (301,363-365). 

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