Macrocyclic cyclooctene-supported AlCl-salen catalysts for conjugated addition reactions: Effect of linker and support structure on catalysis

AlCl-salen (salen = N.N′ bis(salicylidene)ethylenediamine dianion) catalysts supported onto macrocyclic oligomeric cyclooctene through linkers of varying length and flexibility have been developed to demonstrate the importance of support architecture on catalyst activity. The role played by the support and the linkers in dictating catalyst activity was found to vary for reactions with contrasting mechanisms, such as the bimetallic cyanide and the monometallic indole addition reactions. While the flexible support significantlyenhanced the cyanide addition reaction, most likely by improving salensalen interactions in the transition state, it lowered the reaction rate for the monometallic indole reaction. For both reactions, significant increase in catalytic activity was observed for cata lysts with the longest linkers. The effect of the flexible macrocyclic support on catalysis was further exemplified by the enhanced activity of the supported catalyst in comparison with its unsupported analogue for the conjugate addition of tetrazoles, which is known to be catalyzed by dimeric μ-oxo-salen catalysts. Our studies with the cyclooctene supported AlCl-salen catalysts provides significant insights for rationally designing highly efficient AlCl-salen catalysts for a diverse set of reactions.