Free-Energy Profile for a Host-Accelerated Diels-Alder Reaction: The Sources of cxo Selectivity

摘要

The use of free-energy profiles for enzyme-catalyzed reactions has transformed the study of enzyme mechanisms, because the dependence of the height of the energy barrier on substrate variation and on enzyme mutation allows one to dissect out rationally the important contributions to binding and catalysis. We have now applied this approach to enzyme mimics as exemplified by the porphyrin irimer 1 and have examined its exo-sclective acceleration of the Diets. Alder reaction shewn in Scheme 1. There areas yet few effective enzyme mimics for bimolecular reactions, because the design rules governing their operation are not understood; in particular the importance of factors such as substr ite strain, host flexibility, solvation, and quality of fit is not yet clear. We have begun to uncover the rules for our hosts by elucidating some of the binding and kinetic parameters for the reactions in Scheme 1. and in doing so have acquired some insight into the exo selectivity of 1. We also demonstrate the accelerated conversion of endo adduct to exo adduct by 1.