Non-covalent catalysts

It remains to conclude that non-covalent catalyst immobilization is an interesting alternative to covalent attachment and this growing field will certainly influence catalytic processes. 

As this area continues to progress, it is inevitable that alternative, efficient non-covalent catalyst architectures that utilize recognition motifs borrowed from supramolecular chemistry will be developed and widely applied in synthetic chemistry. 

Horn J, Michalek F, Tzschucke CC, Bannwarth W (2004) Non-Covalently Solid-Phase Bound Catalysts for Organic Synthesis. 242 43-75 Houseman BT, Mrksich M (2002) Model Systems for Studying Polyvalent Carbohydrate Binding Interactions. 218 1-44 Hricovinlova Z,see Petrus L (2001) 215 15-41... 

Non-covalent Immobilization of Catalysts Based on Chiral Diazaligands... 

Non-covalently Immobilized Catalysts Based on Chiral Salen Ligands. . 152... 

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

Fontecave M, Hamelin O, Menage S (2005) Chiral-at-Metal Complexes as Asymmetric Catalysts. 15 271-288 FraUe JM, Garcia JI, Mayoral JA (2005) Non-covalent Immobilization of Catalysts Based on Chiral Diazaligands. 15 149-190 Frenking G, see Deubel D (2005) 12 109-144 Fu GC, see Netherton M (2005) 14 85-108... 

Layered inorganic solids have been used for site isolation, for example, nickel phosphine complexes confined within the interlayer spaces of sepiolite have been used as olefin hydrogenation catalysts [63], and similarly there has been the encapsulation of metal complexes into zirconium phosphates [64], The principal idea is illustrated in Figure 5.8. The metal complex can be encapsulated by covalent means (a) or by non-covalent interactions (b). 

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