The design of tailor-made specific catalysts for a given organic reaction was investigated. Dibenzamide or N-benzoylbenzenesulfonamide, transition state analogs of benzoic anhydride on transacylation, was found to act as a template upon aluminium ion-doped silica gel and to imprint specific adsorption sites, “footprints,” on the surface. The footprints were particular Lewis acid sites with a complementary structure to the template. They demonstrated an enzyme-like specific catalytic function on the butanolysis of benzoic anhydride through their specific affinities for substrate molecules in the transition state.
Chiral amines, (R)- and (S)-N-benzyl-α-methylbenzylamine, served as templates in a molecular imprinting we developed. They formed “molecular footprint” cavities on a silica (alumina) gel surface, which showed catalytic activities toward transacylation of benzoic anhydride. The catalyses were susceptible to enantioselective inhibitions by the rebinding of the chiral template amines. This finding proved that the cavities could show chiral recognition through their chirally marked cavity structures.
Chiral catalytic cavities were imprinted on a silica gel surface using a chiral template, Nbe n zo y I -( Na-b e n z y I ox y ca r b o n y I -L-a I a n i n e a m id e , by a n i m p r i n t i n g p r o ce d u re ; t h es e ca v i t i e s d i s p I a y enantioselective catalysis in 2,4-dinitrophenolysis of the corresponding substrate, benzoic N-benzyloxycarbonyl-L-alanine anhydride.
Footprint catalysts are silica(alumina)gel catalysts with tailored specificities. Their catalytic sites are “molecular footprint-like” cavities formed by a molecular imprinting procedure with templates, which are referred to as transition-state or reactive-intermediate analogs of the catalyzed reactions. To clarify the relationship between the structural feature of template molecules and catalytic behavior, seven footprint catalysts were prepared by imprinting with templates of several types closely related to the substrates, benzoic anhydride and acetic anhydride; their catalytic activity and thermodynamic parameters for 2,4-dinitrophenolysis were then estimated. Among these catalysts, a catalyst imprinted with N,N′-dibenzoylbenzenephosphonediamide, a tetrahedral intermediate analog for benzoic anhydride substrate, showed a 10-fold higher catalytic activity than did the other imprinted catalysts.
Imprinting using chiral 5-phenyhydantoins ((5R)- and (5S)-5-pheny-2,4-imidazolinedione) as templates could mark chiral molecular footprint-like cavities on a silica (alumina) gel surface. These cavities displayed evident enantioselective catalyses in reactions of (R)- and (S)-N-carboxyphenylglycine anhydrides, and (R)- and (S)-5-phenylhydantoins with 2,4-dinitrophenolate, respectively. A proposed mechanism and temperature effects on catalysis suggested that the enantioselectivities depended on the binding step of the catalysis.
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