The role of solvent reorganization in proton transfer and nucleophilic addition reactions

Abstract

The factors believed to be of primary importance in determining the intrinsic barrier and affecting transition state structure in proton transfers at carbon are critically reviewed. They include the absence of strong hydrogen bonding between carbon and solvent and the occurrence of extensive structural and solvent reorganization in going from reactants to products. Structural and solvent reorganization depend strongly on the activating substituen's capabilities in delo-calizing or dispersing negative charge. Our recent work on nucleophilic additions to olefins and preliminary work on the addition of carbanions to aldehydes is then discussed. There exist both similarities and differences between nucleophilic additions and proton transfers. Similarities are expected since carbanions of similar structure are involved. Differences exist because in the olefin reactions the carbon bearing the activating groups remains essentially sp2-hybridized while in the proton transfers and the aldehyde reactions there is a change from sp3 to sp2 or vice versa. Our results indicate that solvent reorganization indeed plays a qualitatively similar role as in proton transfers. The influence of structural reorganization is also similar to that in proton transfers for the aldehyde reactions but in the olefin reactions it is affecting the kinetic barriers in the opposite direction. © 2013, Walter de Gruyter. All rights reserved.