Chiral Catalyst Immobilization Using Organic Polymers

Chiral Catalyst Immobilization Using Organic Polymers 

Despite the well-known advantages of insoluble supports, there are several drawbacks in using these solids as supports for chiral catalysts due to the heterogeneous nature of the reaction conditions. The catalyst resides in the solid phase while reactants are in solution, which can often result in a decreased reaction rate owing to diffusion problems. Furthermore, the matrix effect of the solid support, though sometimes favorable, is difficult to predict and can often lead to lower enantioselectivities for the immobilized chiral catalysts than those for their 

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Chiral Catalyst Immobilization Using Organic Polymers... 

Figure 5.6 Schematic representation of the classical methods for immobilization of chiral catalysts using organic polymers as the supports.

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mnm-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and in their use as recyclable chiral catalysts. Techniques of copolymerization of appropriate functional monomers have been utilized to prepare these polymers, and both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes.103... 

Under homogeneous conditions [73], various chiral Schiff base-Cu(II or I) or Ru(II) complexes have been used as chiral catalyst precursors and from all of them the most efficient systems are formed with oxazolines and bis-oxazolines. Thus after their first use by Masamune [74], oxazoline-derived ligand have been extensively employed in homogeneous enantioselective catalytic reaction particularly in cyclopropanation [75]. Thus nitrogenanchoring ligands on organic polymers are mainly formed by immobilized oxazolines. 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

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