Ruthenium catalytic hydrogenation mechanism

The review proper begins with an assessment of the current status of reaction mechanisms and then recent synthetic advances in both rhodium and ruthenium catalytic hydrogenation. In view of the vast literature generated it has been necessary to be selective, particularly in synthesis where advances in ligand design can render even recent work on similar substrates obsolete. Reference to reviews should alleviate the ensuing deficiencies. 

An interesting catalytic ruthenium system, Ru(7/5-C5Ar4OH)(CO)2H based on substituted cyclopentadienyl ligands was discovered by Shvo and coworkers [95— 98]. This operates in a similar fashion to the Noyori system of Scheme 3.12, but transfers hydride from the ruthenium and proton from the hydroxyl group on the ring in an outer-sphere hydrogenation mechanism. The source of hydrogen can be H2 or formic acid. Casey and coworkers have recently shown, on the basis of kinetic isotope effects, that the transfer of H+ and TT equivalents to the ketone for the Shvo system and the Noyori system (Scheme 3.12) is a concerted process [99, 100]. 

The second and final example of a computational study of a reaction mechanism that will be considered here is drawn from work carried out by the author s group and serves to illustrate some of the points discussed in the previous section. The reaction in question is the catalytic hydrogenation of ketones by ruthenium(bisphosphine) (diamine) complexes. This reaction was developed by the group of Professor Ryoji Noyori20 and was also studied by the group of Professor Robert Morris. The initial computational work discussed here was a collaboration with Professor Morris. It was motivated by the desire to test the feasibility of a proposed mechanism, involving a key ruthenium dihydride complex, that would transfer a hydride (from Ru) and a proton (from N) in a concerted step to the ketone (Figure 10.9). 

The exploration of transition metals as redox catalysts is a large and very active area of research. Transition metal mechanisms are quite different from those discussed so far. Catalytic hydrogenation of pi bonds is an important reaction for organic synthesis, and will serve as a good example. The mechanism proposed for catalytic hydrogenation by platinum, palladium, nickel, rhodium, iridium, and ruthenium is shown in Figure 8.10. 

The asymmetric catalytic hydrogenation of 2-(6 -methoxy-2 -naphthyl)propenoic acid to (S)-naproxen was monitored in both methanol- and C02-expanded methanol.f The catalyst used was [dichloro-(S)-(-)-2,2 -bis(diphenyl-phosphino)-1, l -binapthyl]ruthenium(II). Addition of CO2 to the methanol produced strong retardation of the reaction rate. An average reduction in enantioselectivity of 6%i was observed compared to that in methanol. The enantioselectivity was found to increase as the temperature decreased in both neat methanol and C02-expanded methanol systems, indicating that the underlying mechanisms were similar. Insufficient oxygen removal may have prevented the... 

In aqueous hydrochloric add solutions, ruthenium(II) chloride catalyzed the hydrogenation of water-soluble olefins such as maleic and fumaric acids [6], After learning so much of so many catalytic hydrogenation reactions, the kinetics of these simple Ru(II)-catalyzed systems still seem quite fascinating since they display many features which later became established as standard steps in the mechanisms of hydrogenation. The catalyst itself does not react with hydrogen, however, the mthenium(II)-olefin complex... 

In most catalytic hydrogenation reactions of the keto functional group by rhodium and iridium complexes, Hj is cleaved homolytically by the metal which then assists the reductive elimination of the alcohol. In contrast, ruthenium catalysts generally reduce ketones to alcohols with ionic mechanisms involving the heterolytic splitting of H2 [60] (Scheme 12). Obviously this is only a general... 

In Section 5.9, we saw that alkenes can be converted to alkanes by catalytic hydrogenation by a variety of catalysts, such as palladium and platinum. These are heterogeneous catalysts. We also noted that homogeneous catalytic hydrogenation can be carried out by Wilkinsons catalyst, Ru[(PPh3)3Cl. We now return to that subject to discuss the reaction mechanism. We will find that hydrogenation by Wilkinson s catalyst occurs in a catalytic cycle that is strikingly similar to the catalytic cycles of the reactions we have discussed thus far in this chapter. The transition metal in the Wilkinson catalyst, however, is ruthenium, not palladium. 

Ruthenium catalysts, such as Ru(CHCH(Ph))Cl(CO)(PCy3)2, have been found to be active for catalyzing the hydrogenation of various diene-based polymers. The catalytic mechanism for the hydrogenation of NBR, SBR and PB has been investigated [68]. 

Kinetic results show that the hydrogenation reaction rate exhibits a first-order dependence on both hydrogen concentration, [H2], and the total ruthenium concentration, [Ru]t and an inverse dependence on the nitrile concentration, [CN]. The catalytic mechanism proposed for polymer hydrogenation is illustrated in Scheme 19.5 and the main points of the mechanism are outlined below ... 

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]. 

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