Solid support catalysts immobilization

Up to now a broad variety of common organic and inorganic polymer systems have been used as a solid support for immobilized metal complex catalysts. During the first period of the development work the need for a tailor- made support to meet the requirements of this application became apparent, e. g., with respect to general and structural stability, nature and degree of functionalization, functional group distribution and density, and accessibility of the functional sites [17]. 

Due to their cost, instability, and limited longevity, enzymes are not widely employed in production-scale syntheses however, through their immobilization and incorporation into flow reactors, biocatalysts have the potential to be employed in the synthesis of high-value products. Although the use of microfabricated reactors for the screening of biocatalysts for organic synthesis is a relatively new area of research, the field has been quick to employ those techniques developed for the use of solid-supported catalysts under continuous flow, a feature that is illustrated by the diverse array of immobilization techniques reported to date. 

Solid supported catalysts have been utihzed in ATRA and ATRC reactions in order to facilitate reaction work-up and enable catalyst recycling. In one example, V-alkyl-2-pyridylmethanimines were immobihzed on sihca and in conjunction with Cu Br or Cutl utilized in ATRC reactions of trichloro-, dichloro- and monobromo-substrates. Efficient 5-exo and 5-endo cyclizations could be mediated using this immobilized system, however, as coimnonly encountered, the solid supported catalyst was much less active than its homogeneous counterpart. The decrease in the catalytic activity was not induced by catalyst leaching, but rather accumulation of copper(II) deactivator. The accumulation of deactivator during ATRA or ATRC process was result of irreversible and often diffusion controlled radical-radical couphng reactions (A 1.0xl0 M- s- ). 

Keywords Immobilized metal catalysts Heterogenized homogeneous catalysts Solid supported catalysts Industrial catalysis ... 

Although an enantioselective reaction carried out on solid phase, in principle, also includes the reactions wherein a chiral catalyst or a chiral auxiliary is polymer bound, in this chapter we have mainly focused on reactions wherein a substrate is bound to a solid support and the chemical transformation is carried out by chiral reagents. However, we have included few examples of employing solid-supported catalyst and auxiliaries to give readers an idea of these alternative synthesis strategies. Readers are, however, advised to refer to Refs 6 and 7 for a review on the applications of immobilized chiral catalysts and solid-phase-bound chiral auxiliaries, respectively, and Refs 1,5, and 8 for diverse solid-phase synthesis approaches directed toward small molecules and natural products. ... 

The ability of iron(III) chloride genuinely to catalyze Friedel-Crafts acylation reactions has also been recognized by Holderich and co-workers [97]. By immobilizing the ionic liquid [BMIM]Cl/FeCl3 on a solid support, Holderich was able to acetylate mesitylene, anisole, and m-xylene with acetyl chloride in excellent yield. The performance of the iron-based ionic liquid was then compared with that of the corresponding chlorostannate(II) and chloroaluminate(III) ionic liquids. The results are given in Scheme 5.1-67 and Table 5.1-5. As can be seen, the iron catalyst gave superior results to the aluminium- or tin-based catalysts. The reactions were also carried out in the gas phase at between 200 and 300 °C. The acetylation reac-... 

Immobilization of catalysts is an important process design feature (see Chapter 9.9). A recent example of catalyst immobilization is the biphasic approach which seems superior to immobilization on solids, as successfully proven in the Ruhrchemie/Rhone Poulenc process for the hydro-formylation of olefins.286 Supported liquid phase catalysis was devised as a method for the immobilization of homogeneous catalysts on solids. When the liquid phase is water, a water-soluble catalyst may be physically bound to the solid. 

Recyclability can be achieved by heterogenization of the reaction mixture, by binding the catalyst and products to different phases. This can be achieved by (i) immobilization of the catalyst on a solid inorganic or polymeric support (solid-liquid protocols) or (ii) partitioning the catalyst and reagents/products in different liquid phases (liquid-liquid protocols) (see Chapter 9.9 for more details on supported catalysts). 

Another method for generating an epoxidation catalyst on a solid support is to simply absorb or non-covalendy attach the catalyst to the solid support <06MI493>. Epoxidation of olefin 6 with mCPBA and catalyst 8 provides 7 in quantitative yields and with 89% ee. The immobilization of 8 on silica gel improves the enantioselectivity of the reaction providing 7 with 95% ee. Recycling experiments with silica-8 show a decrease in both yield and the enantiomeric excess for each cycle (45% ee after 4 cycles). This is attributed to a leaching of the catalyst from the silica gel. Two other solid supports, a Mg-Al-Cl-LDH resin (LDH) and a quaternary ammonium resin (Q-resin) were also examined. It was expected that ionic attraction between 8 and the LDH or Q-resin would allow the catalyst to remain immobilized through multiple cycles better than with silica gel. Both of these resins showed improved catalytic properties upon reuse of the catalyst (92-95% ee after 4 cycles). 

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