Evaluation of initial and transition state solvation effects in aprotic media by the competetive preferential solvation theory

Abstract

A chemical reaction taking place in aprotic media is frequently submitted also to the influence of specific solvent effects. The problem is even more complicated in mixed aprotic solvents since preferential solvation may contribute to the observed effects. A recent proposed model [1] called the Competitive Preferential Solvation Theory (COPS theory) accounts quite naturally for these phenomena. Based essentially on five postulates, COPS theory allows the quantitative evaluation of solut—solvent interactions both in initial and transition states. The affinity constants obtained are perfectly transferable and the lead immediately to a new type of linear free energy relationship [2] connecting the interacting power of solvents with their catalytic activity. COPS theory accepts neither ‘inert’ solvents nor ‘free’ solutes and it implies a virtual microscopic partitioning in homogeneous media. Beside kinetics COPS theory have been successfully applied to NMR and UV spectroscopies, to nuclear relaxation, to fluorescence proving its general validity. The kinetic application to COPS theory is illustrated by two reactions (aminolysis of imides and 2 + 4 cycloaddition reaction) in about sixty different media. The results provide a kinetic confirmation of the existence of solvent classes discovered previously [3] in spectroscopy: halogen containing, n-donor and aromatic solvents. All these classes influence differently the chemical reactivity. No close bonding contacts are apparent between the cation and BPh 4 or CH 3CN. Upon heating the complex, as a solid or when dissolved in acetonitrile, we observe the loss of ethylene. Reaction of other donor ligands such as PMe 3, CO, O 2..are investigated and the results will be discussed.