Pore chiral catalysts

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

The use of surface bound triflate ions has been exploited by Raja et al. to immobilize the complexes [Rh(COD) fSj-(-i-)-PMP ], [Pd(allyl) fSj-(-i-)-PMP ], [Rh(COD) fSj-(-)-AEP rand[Rh(COD) flR,2Rj-(-t)-DED ]"in the pores of silicas possessing various pore sizes with narrow distributions [128]. These constrained chiral catalysts were then tested for the asymmetric hydrogenation of methyl ben-zoylformate to its corresponding methyl mandelate (40°C, methanol, 2 MPa H2). In the homogenous form, only the catalysts [Rh(COD) fSj-(-i-)-PMP ], [Pd(allyl) (Sj-(-i-)-PMP ] exhibit any signiflcant e.e.s under the reaction conditions (53%... 

Our approach was to enlarge the intrazeolitic cavities in order to generate superior hosts for bulky homogeneous chiral catalysts. Mesopores created this way are completely surrounded by micropores and offer additional advantages. The entrapped metal complex can move freely and is more accessible during catalysis and even sterically demanding transition states can be formed within the individual pores. 

In particular, for the synthesis of optically pure chemicals, several immobilization techniques have been shown to give stable and active chiral heterogeneous catalysts. A step further has been carried out by Choi et al. [342] who immobilized chiral Co(III) complexes on ZSM-5/Anodisc membranes for the hydrolytic kinetic resolution of terminal epoxides. The salen catalyst, loaded into the macroporous matrix of Anodise by impregnation under vacuum, must exit near the interface of ZSM-5 film to contact with both biphasic reactants such as epoxides and water. Furthermore, the loading of chiral catalyst remains constant during reaction because it cannot diffuse into the pore channel of ZSM-5 crystals and is insoluble in water. The catalytic zeolite composite membrane obtained acts as liquid-liquid contactor, which combines the chemical reaction with the continuous extraction of products simultaneously (see Figure 11.28) the... 

The interesting structure of zeolites allows the incorporation and immobilization of suitable cations or chiral catalysts on the surface of zeolitic pores. These processes not only enhance the available surface area but also provide highly structured and confined cavities that can increase stereoselectivity. Cation-exchanged zeolites, such as KY and CsY, have received much attention as solid base catalysts owing to the following advantages (i) easy separation from the reaction mixture, (ii) reusabihty, (iii) easy modification of their surface and pore size, and (iv) use of nonpolluting natural minerals [4]. 

Electrochemical studies, in combination with EPR measurements, of the analogous non-chiral occluded (salen)Mn complex in Y zeoUte showed that only a small proportion of the complex, i.e., that located on the outer part of the support, is accessible and takes part in the catalytic process [26]. Only this proportion (about 20%) is finally oxidized to Mn and hence the amount of catalyst is much lower than expected. This phenomenon explains the low catalytic activity of this system. We have considered other attempts at this approach using zeolites with larger pore sizes as examples of cationic exchange and these have been included in Sect. 3.2.3. 

Ad(ii) On catalysts with pores and cavities of molecular dimensions, exemplified by mordenite and ZSM-5, shape selectivity provides constraints of the transition state on the S 2 path in either preventing axial attack as that of methyl oxonium by isobutanol in mordenite that has to "turn the comer" when switching the direction of fli t through the main channel to the perpendicular attack of methyl oxonium in the side-pocket, or singling out a selective approach from several possible ones as in the chiral inversion in ethanol/2-pentanol coupling in HZSM-5 (14). Both of these types of spatial constraints result in superior selectivities to similar reactions in solutions. 

The studies of Thomas and Raja [28] showed a remarkable effect of pore size on enantioselectivity (Table 42.3). The immobilized catalysts were more active than the homogeneous ones, but their enantioselectivity increased dramatically on supports which had smaller-diameter pores. This effect was ascribed to more steric confinement of the catalyst-substrate complex in the narrower pores. This confinement will lead to a larger influence of the chiral directing group on the orientation of the substrate. Although pore diffusion limitation can lead to lower hydrogen concentrations in narrow pores with a possible effect on enantioselectivity (see Section 42.2), this seems not to be the case here, because the immobilized catalyst with the smallest pores is the most active one. 

This tethered ferrocenyl-based Pd complex on MCM-41 (17) was then used for the catalytic amination reaction between cinnamyl acetate and benzylamine (40 °C, THF) [59]. In this case, confinement of the catalyst results in profound changes in regio- and enantioselectivity. When the homogeneous equivalent is used to catalyze the reaction, the straight chained derivative is the sole product. Similar results (only 2% of the branched product) were obtained when the catalyst was tethered to the surface of the non-porous silica Cabosil. When tethered inside the pores of MCM-41 a major change occurred in that now the branched product accounts for about 50% and a change in e.e. from 49% e.e. when anchored to the Cabosil support to +99% when anchored inside the MCM-41 pore could be observed. If the catalyst s chirality was reversed in the MCM-41 immobihzed case, so was the chirality of the product (measured at 93% e.e.) [60]. 

Figure 5.10 (a) The ligand (b) the catalytically catalyst constrained within a mesopore, active metal center bound inside the pores of indicating the space constraint and the mesoporous MCM-41, now with an extra diamine auxiliary functionality . (Modified nitrogen, indicating the anchoring point on from Thomas et al. [58].) the tether (c) schematic diagram of the chiral... 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

The SHB concept was expanded to chiral phosphine catalysts by de Rege et al., who reacted the trifluoromethanesulfonate (triflate) counter anion of the cationic complex [Rh(COD)((R,Rj-MeDuPhos)] with the surface hydroxyl groups of the silaceous mesoporous material MCM-41 [122]. The complex was loaded to a level of 1.03 wt% Rh. A decrease in support surface area and pore volume is consistent with the complex being located within the support pores. The counterion is very important in this process if the anion of the homogeneous catalyst precursor is altered to BArp no adsorption of the catalyst is observed. It is postulated that the mechanism of triflate binding is hydrogen bonding with the support, and that the... 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

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