Transfer Hydrogenation Using Heterogeneous Catalysts

Especially worth mentioning in a green context is the use of mesoporous materials and zeolites, as stable and recyclable catalysts for MPV reductions. High activity was obtained by using zeolite-beta catalysts. Beta zeolites have a large pore three-dimensional structure with pores of size 7.6 x 6.4 A2 which makes them suitable for a large range of substrates. Al, Ti- and Sn-beta zeolite have all been used as catalysts for the selective reduction of cyclohexanones [40-42]. The 

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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]. 

We recently reported that a heterogeneous copper catalyst prepared with a non-conventional chemisorption-hydrolysis technique is able to promote a hydrogen transfer reduction using a donor alcohol. In this case, the role of copper is cricial, both for activity and selectivity [20]. 

Heterogeneous copper catalysts prepared with the chemisorption-hydrolysis technique are effective systems for hydrogen transfer reactions, namely carbonyl reduction, alcohol dehydrogenation and racemization, and allylic alcohol isomerization. Practical concerns argue for the use of these catalysts for synthetic purposes because of their remarkable performance in terms of selectivity and productivity, which are basic features for the application of heterogeneous catalysts to fine chemicals synthesis. Moreover, in all these reactions the use of these materials allows a simple, safe, and clean protocol. 

In contrast, continuous flow reactors are already being used for hydrogenation reactions industrially (Licence el al., 2003). They are simple to construct and modify, and possess excellent mass- and heat-transfer properties. In academia, flow reactors have been used in conjunction with a variety of heterogeneous catalysts to carry out many reactions, including hydrogenations, dehydrogenations, hydroformylations, Friedel-Crafts acylations and alkylations, etherifications and oxidations (Hyde et al., 2001). 

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