Complexes Anchored by Covalent Bonds

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 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180 

Zeolite-anchored complexes may exhibit a greater activity and selectivity in chiral catalysis. This has been demonstrated by Corma and colleagues.181-184 They prepared a chiral ligand from L-proline (1 in Fig. 7.3). 

On silica, the rate was lower than in the case of the free complex. The zeolite, however, shows a cooperative effect on the rate and on enantioselectivity of the reaction. The positive effect on the rate can be related to the strong capacity of the zeolite to adsorb H2. These zeolite-anchored complexes can also be recycled without loss of activity. 

Chiral dioxomolybdenum complexes were synthesized from (25,4R)-4-hydroxyproline and connected to the surface of USY zeolite by covalent bonding (2 in Fig. 7.4). 

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