Marked effect of the shape of solvent molecules on molecular recognition in the oxidation of associating thiols

Abstract

Oxidation of a pair of associating thiols (1 and 2), each having a binding site [–C(O)NHC(O)NH–] and a recognition site (R¹ or R²), is examined in binary solvent mixtures of a propanol with water and organic co-solvents. The selectivity (r)—a measure of the degree of recognition of (1) by (2)[or of (2) by (1)]—in the oxidation is represented by the logarithmic ratio of the yield of an unsymmetrical disulphide to twice that of a symmetrical one. It is found that (i) higher selectivity is achieved when the non-polar group of a propanol in a mixed solvent resembles a given non-polar group of one of the reacting molecules in three-dimensional shape and (ii) the above ‘solvent shape effect’ on the selectivity is produced more markedly in aqueous propanols than in the corresponding non-aqueous ones. Correlation of the observed selectivity with physico-chemical properties of (aqueous) propanols, reactivity difference between thiols, and so on is discussed together with a possible explanation of the solvent shape effect on the degree of recognition of non-polar groups.