Systematic ranking of nucleophiles as electron donors

A systematic ranking of different nucleophiles (including enolates, phenola tes, thiophenolates, hydroxide, cyanide) with respect to their ability to s tabilise the transition state of substitution reactions has been carried ou t in acetonitrile and dimethyl sulfoxide. The method is based on a comparis on of the rate constant, k(SUB), for the substitution reaction between a gi ven nucleophile and benzyl chloride with the rate constant, ka(ET), for the corresponding electron transfer from an aromatic radical anion to benzyl c hloride. The ratio k(SUB)/k(ET) expresses the rate enhancement due to elect ronic interaction in the transition state, Delta G(sta), of the substitutio n reaction. In this study, k(SUB)/k(ET) ratios between 5 and 10(31) were de termined corresponding to Delta G(Sta)-values of 4-175 kJ mol(-1). These va lues cover substitution reactions going from outer-sphere electron transfer to S(N)2 with partial bond formation in the transition state. The k(SUB)/k (ET) values are round to be largest for nucleophiles such as OH- and CN- wi th very poor electron-donating abilities thigh oxidation potentials). When different. types of nucleophile with similar oxidation potentials are compa red, a decrease in k(SUB)/k(ET) is observed going from sulfur- to carbon- a nd further to oxygen-centred nucleophiles. The ranking of nucleophiles in r eactions involving electrophiles other than benzyl chloride is discussed wi th emphasis on steric hindrance and electronic effects.