Significant progress has been achieved in the last thirty years with homogeneous catalysts and the main emphasis has been on ligand design to control the reactant approach to the active centre, so that the reaction occurs at only one face of the approaching prochiral substrate
Heterogeneous enantioselective catalysts: strategies for the immobilisation of homogeneous catalysts.
CHEMICAL SOCIETY REVIEWS, no. 2 (2004): 108-122
Enantioselective formation of C-H, C-C, C-O and C-N bonds has been extensively studied using homogeneous asymmetric catalysts for many years. However, these catalysts have yet to make a significant impact in the industrial synthesis of fine chemicals. A central reason is that homogeneous asymmetric catalyst design requires relatively bulk...更多
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- The enantioselective formation of C–H, C–C, C–O and C–N bonds represents one of the biggest challenges in synthetic chemistry.
- Encapsulation is the only catalyst immobilisation process which does not require any interaction between the catalyst and the support, and because of this it is the only method which attempts to mimic the homogeneously catalysed reaction process.
- The enantioselective formation of C–H, C–C, C–O and C–N bonds represents one of the biggest challenges in synthetic chemistry
- Significant progress has been achieved in the last thirty years with homogeneous catalysts and the main emphasis has been on ligand design to control the reactant approach to the active centre, so that the reaction occurs at only one face of the approaching prochiral substrate
- In 2001, the significant achievements in the design and application of asymmetric homogeneous catalysts were recognised by the award of the Chemistry Nobel Prize to W
- The additional constraints placed upon the chiral catalyst can induce increased enantioselection compared with the homogeneous catalyst, and several examples have been given in this review
- It can be expected that the containment effect observed with both the tethering and electrostatic interaction can be used effectively to design a new generation of immobilised asymmetric catalysts capable of both high yields and enantioselection
- PDMS films have been used to immobilise Rh-MeDuPHOS for the enantioselective hydrogenation of methyl-2-acetamidoacrylate and methylacetoacetate. For both of these reactions high e.e.s were obtained with the heterogenised catalyst (90–96%), but this is still slightly reduced compared to the homogeneous reactions which typically give e.e.s of 99%.
- A combination of covalent attachment and electrostatic interaction has been used to immobilise Cr salen complexes for the enantioselective hydrolysis of epoxides with trimethylsilyl azide. The surface of silica, MCM-41 and ITQ-2 were initially functionalised with 3-aminopropyltriethoxysilane.
- The homogeneous equivalent reactions gave much higher conversions than those obtained for the immobilised catalyst (95 and 60–70% respectively).
- The authors have used this immobilisation strategy to design asymmetric heterogeneous catalysts for alcohol dehydration, aziridination, and epoxidation of alkene, and carbonyl and imino ene reactions. The design approach involves the ion exchange of suitable cations into the intracrystalline pores of zeolites, e.g. zeolite Y, or mesoporous solids, e.g. MCM-41 and the subsequent modification of the cation with a chiral ligand prior to reaction.
- The authors immobilised copper(II) bis(oxazoline) complexes with zeolite Y and Al-MCM-41.36 Initially, the zeolite or mesoporous solid was partially ion-exchanged to give the Cu2+ form with 3–5 wt% Cu. The aziridination of alkenes was investigated using [N-(p-tolylsulfonyl)imino]phenyliodinane (PhINNTs) and [N-(p-nitrophenylsulfonyl)imino]phenyliodinane (PhINNNs) as nitrene donors (Fig. 21).
- Of the four immobilisation strategies developed to date for the preparation of asymmetric heterogeneous catalysts, two, namely immobilisation by covalent tethering and electrostatic interaction, have been shown to form reasonably stable catalysts that are capable of re-use.
- Most importantly, is the observation by several research groups of the containment effect, i.e. that the immobilised asymmetric catalyst is constrained through interaction with the supporting matrix. The additional constraints placed upon the chiral catalyst can induce increased enantioselection compared with the homogeneous catalyst, and several examples have been given in this review.
- It can be expected that the containment effect observed with both the tethering and electrostatic interaction can be used effectively to design a new generation of immobilised asymmetric catalysts capable of both high yields and enantioselection.
- Table1: The asymmetric epoxidation of various alkenes catalysed by the (R,R)-(salen)Mn complex under homogeneous and heterogeneous conditions[<a class="ref-link" id="c15" href="#r15">15</a>]
- Table2: Reduction of acetophenone at room temperature[<a class="ref-link" id="c32" href="#r32">32</a>]
- Table3: Effect of styrene:PhINNs molar ratio on the asymmetric aziridination of styrene at 25 °Ca,[<a class="ref-link" id="c36" href="#r36">36</a>]
- Table4: Effect of bis(oxazoline) on the aziridination of styrenea[<a class="ref-link" id="c36" href="#r36">36</a>]
- The immobilised catalyst gave up to 72% e.e. for the hydrogenation of (Z)-a-acetamidocinnamic acid 4
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