摘要:

IntroductionThe ability to synthesise pure enantiomers is of crucial importance for the preparation of modern pharmaceuticals and agrochemicals. Most early progress has been focussed on the design of molecular catalysts operating as homogeneous catalysts, work which culminated in the award of the Nobel Prize for Chemistry in 2001 to Noyori, Sharpless and Knowles.1However, industry has been slow to take up these synthetic methodologies since separation of the catalyst from the reaction mixture can be difficult and hence catalyst recycle and reuse can be very difficult. For this reason, attention has recently focussed on the design of immobilised catalysts which can overcome these problems.2,3In general, progress in the design of robust heterogeneous enantioselective catalysts has proved to be much more elusive, and they tend to be very specific for particular reactions and classes of substrates, and there remain very few examples of heterogeneously catalysed reactions giving high enantioselection. Unfortunately, almost invariably, poorer enantioselection is achieved with the immobilised complex compared to the comparable homogeneous catalysts.4However, there are a number of studies indicating that improved enantioselectivity can be achieved by immobilisation. In 1991, this was pioneered by the Corma and co-workers.5In 2000 Raynoret al.6showed that a carefully planned immobilisation strategy could result in enhanced enantioselection being observed with the immobilised catalyst when it is constrained within the ordered mesopores of MCM-41. They demonstrated this for the allylic amination of cinnamyl acetate using a catalyst based on 1,1′-bis(diphenylphosphino)ferrocene anchored to the inner wall of MCM-41 and coordinated to Pd2+. The approach was subsequently extended to the enantioselective reduction of ethyl nicotinate. The improved effect of immobilisation is noted in other studies7–9and is considered to be due to a containment effect observed for the immobilised systems.10We have used a different immobilisation strategy in which catalytically active cations are electrostatically immobilised within microporous and mesoporous materials by ion exchange and they are subsequently modified by chiral ligands.2Until now, we have concentrated our attention on the heterogeneous asymmetric aziridination of alkenes11–14using Cu2+ion-exchanged into zeolite H-Y modified by chiral bis(oxazoline) ligands, but we have also used this approach to design heterogeneous enantioselective dehydration,15epoxidation,16Diels–Alder17and imino–ene reactions.18We have now used this immobilisation strategy to synthesise stable reusable catalysts for enantioselective hydrogenation.Asymmetric hydrogenation reactions continue to demand attention using a whole library of chiral phosphine ligands, however, in many cases the enantioselectivity achieved with the immobilised complex can be significantly lower than those observed for the non-immobilised catalyst. It should be noted that other strategies for the stabilisation of homogeneous catalysts are being considered, for example the use of ionic liquids is producing interesting results,19–21but most research effort has been directed towards immobilisation strategies. Progress can only considered to be made if the enantioselection or overall turnover numbers achieved with the stabilised or immobilised catalyst is comparable to the homogeneous catalyst, and to date there are only few examples of such improvement.6,18There have been numerous previous studies concerning the design of immobilised asymmetric hydrogenation catalysts, due primarily to the central importance of hydrogenation as a unit process in industrial organic synthesis.22,23Noyoriet al.24prepared an immobilised Ru-BINAP-diamine system that compared favourably with its homogeneous counterpart. Recent attention has focussed on the immobilisation of phosphine containing catalysts onto inorganic supports since their rigid structure prevent intermolecular aggregation of the active catalyst species which can lead to loss of activity in the polymer supported catalysts. Robust asymmetric catalysts using inorganic supports, such as MCM-41 have been reported by Raynoret al.,6Hölderich and co-workers,25de Regeet al.26and by Augustineet al.for heteropolyacids.27These researchers have shown that chiral Rh diphosphine complexes can be immobilised on MCM-41. However, these catalysts, although initially effective, tend to give lower enantioselection compared with their homogeneous counterparts and also typically required long reaction times. For example, Hölderich and co-workers25immobilised Rh diphosphine complexes by adsorption onto MCM-41 and, although the catalysts were stable and could be reused, the ee obtained with the immobilised catalyst (85–92%) was significantly lower than that observed with the homogeneous catalyst (99%) and these catalysts required 24 h to achieve complete conversion. In this paper we demonstrate immobilisation strategies for enantioselective hydrogenation based on immobilisation of rhodium complexes into mesoporous materials. We demonstrate that comparably high enantioselection can be achieved with these supported catalysts using two immobilisation strategies using electrostatic forces. Furthermore, we demonstrate that the manner in which the immobilisation is achieved is of crucial importance in achieving high sustained enantioselectivity and reusability for these catalysts.

ISSN:1477-0520    

DOI:10.1039/b501359c

出版商:RSC    

年代:2005

数据来源: RSC