ENHANCED CYCLIZATION RATES OF LARGE RINGS INDUCED BY A MICELLAR ENVIRONMENT

The kinetics of ring formation of a series of o-(omega-bromoalkoxy)phenoxides, where the ring size varies from 7 to 20, have been studied in a cationic micellar environment (CTAB). Compared to a homogeneous reaction environment, where the ring closure rate constants decrease by ca. 500-1500-fold on going from the 7-membered ring [7] to the 14-membered ring [14] 1, only a 6-fold decrease in the micelle-localized reaction is observed. In addition, micellar effective molarities (EM)m for these reactions are found to be 7-1900-fold greater than the homogeneous solution EM's. These results constitute a significant example of true micellar catalysis, where the catalysis is reasoned to originate from an increase in the ground-state free energy of the substrate induced by localization of the phenolate oxygen and alkyl bromide moieties at the polar micelle interface. This localization of the "chain ends" leads to a significant decrease in the ground-state entropy compared with the homogeneous situation and establishes that the concept of interfacial templating of chemical reactions is valid. Furthermore, these rate enhancements establish that a micellar environment can affect reactivity in a more complex way than via simple medium and/or concentrating effects.