Evidence for the Importance of Polarizability in Biomimetic Catalysis Involving Cyclophane Receptors.

Cyclophanes 1-6 catalyze the nucleophilic dealkylation of a simple sulfonium compound by potassium iodide. The cation-pi interaction is important in substrate binding, but the primarily electrostatic nature of this effect does not explain all observations concerning catalysis. As a series of substituents are placed on the cyclophane framework, a systematic variation in catalyst effectiveness is seen, such that more polarizable substituents produce more potent catalysts. This provides support for the notion that transition states are especially polarizable, and catalysis can be enhanced by maximizing London dispersion forces. The reactions studied here are very similar to the broad class of biological methylations mediated by S-adenosylmethionine, and the biological catalysts may use forces similar to those described here.