Catalyst for transfer hydrogenation

Transition-metal catalysts are, in general, more active than the MPVO catalysts in the reduction of ketones via hydrogen transfer. Especially, upon the introduction of a small amount of base into the reaction mixture, TOFs of transition-metal catalysts are typically five- to 10-fold higher than those of MPVO catalysts (see Table 20.7, MPVO catalysts entries 1-20, transition-metal catalysts entries 21-53). The transition-metal catalysts are less sensitive to moisture than MPVO catalysts. Transition metal-catalyzed reactions are frequently carried out in 2-propanol/water mixtures. Successful transition-metal catalysts for transfer hydrogenations are based not only on iridium, rhodium or ruthenium ions but also on nickel [93], rhenium [94] and osmium [95]. It has been reported that... 

In broad terms there are three types of catalyst for transfer hydrogenation dehydrogenases heterogeneous and homogenous metal catalysts. Here, the first two are mentioned for completeness, and the main focus of this chapter will be asymmetric transfer hydrogenation with homogenous metal catalysts. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Chowdhury and Backvall107 showed that RuCl2(PPh3)3 was an effective catalyst for transfer hydrogenation in the presence of 2% NaOH. This finding initiated the development of asymmetric processes that utilized chiral... 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

Scheme 1 A highly active iridium catalyst for transfer hydrogenation...

Nolan and coworkers have described the use of their cationic Ir complex 27 as an effective catalyst for transfer hydrogenations [73]. While complex 27 was found to be effective, the analog bearing the NHC ICy was found to be su-... 

The catalysts for transfer hydrogenations are usually late transition metal complexes with tertiary phosphine ligands or bidentate nitrogen ligands, and the donors are usually organic compounds whose oxidation potential is sufficiently low to tolerate hydrogen transfer under mild conditions. Suitable donors are secondary alcohols such isopropanol. This alcohol is the most convenient since it is stable, non-toxic, environmentally friendly, easy to handle (bp 82°C), inexpensive and dissolves many organic compounds. 

The use of liquid ammonia solutions of Eu or Yb enables the preparation of active catalysts for transfer hydrogenation in which ammonia is a preferred hydrogen donor. Catalytic transfer hydrogenation using hydrogen donors shows some interesting features which are of potential synthetic importance and use (Johnstone and Wilby 1985, Hartner 1980, Brieger and Nestrick 1974, Furst et al. 1965). 

However, some of the lanthanide elements form chiral complexes, which have already been applied asymmetric reactions. Samarium complexes are efficient catalysts for transfer hydrogenation (78). 

A range of NHCs and bis-NHC-ruthenium complexes have been characterized and tested as catalysts for transfer hydrogenation of ketones in basic isopropanol. ... 

In a subsequent work, Castarlenas and Esteruelas disclosed the synthesis of another cationic complex [(IPr)Os(OH)(p-cymene)][OTf] (45), which was an efficient pre-catalyst for transfer hydrogenation (Equation (7.11)). Several aliphatic and aromatic aldehydes could be reduced in high yields and high turnover numbers, whereas under these same experimental conditions, acetophenone reacted only very slowly. Regarding the activation of the pre-catalyst, the authors provided strong NMR evidence that, in the presence of isopropanol, compound 45 was converted into a cationic acetone-hydride complex [(IPr)0s(H)(0=C(CH3)2)][0Tf], which most likely acted as the active species for transfer hydrogenation. 

Ir(l) and Ir(lll) complexes containing a chelating bis(NHC) derived from both imidazole 557 and triazole have been prepared and used as efficient catalysts for transfer hydrogenation of ketones,aldehydes,including enolizable ones, and imines. Remarkably, in the case of ketones, a reverse activity (Ir > Rh) was found for these complexes. 

Xiao and coworkers [77] reported that complex 13 is a remarkably efficient catalyst for transfer hydrogenation of a wide range of aldehydes by HCOONa in neat water in air. The reduction can be carried out with S/C ratios as high as 5000 1, dehvering high yields (>90%) and an initial TOP up to 1.3 X 10 h h Selected examples are shown in Scheme 6.12. Bearing in mind that these substrates are water-insoluble, the hydrogenation takes place most likely on water. 

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