Zeolites Supported Enantioselective Catalysis

Some of the most promising examples show the silicon atom binds irreversibly either to the metal or organic moiety of the chiral catalyst. The ions exchange of Na-i- or other ions during the zeolite formation along with the chiral catalyst results in the formation of stable catalyst. Recently, Corma and coworkers have demonstrated that zeolites bind directly to the metal of the heavy chiral catalyst such as Jacobsens catalyst [51]. 

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In the presence of the immobilized (2S,4S)-4-hydroxyproline Mo catalyst (13b), nerol and geraniol react selectively with r-BuOOH to form the 2,3-epoxide with ee values of 64 and 47%, respectively. Surprisingly, when Mo is complexed by the diastereomeric (2S,4R) form (13a), racemic epoxidation is observed. The enantioselective catalysis appears to be promoted by immobilization in the zeolite USY pores. Indeed, in the epoxidation of nerol, an ee of 10% was found for the homogeneous asymmetric Mo complex, whereas the supported catalyst favors the selective production of the (2S,3R)-epoxide (64% ee). 

Studies of the catalytic activity of MOFs are in their infancy with some encouraging results emerging in enantioselective catalysis. By contrast, meso-porous solids have already been studied extensively as catalytic supports, particularly of complexes too large to be encapsulated in zeolites. One of the most significant developments in this area is the observation that the constrained encapsulation of chiral catalysts in mesopores can raise the enantioselectivities of reactions well above those observed when the reaction is performed homogeneously. 

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