Structure–reactivity correlations in the aminolysis of aryl dithiomethyl- and dithiophenylacetates with anilines in acetonitrile

Abstract

The kinetics and mechanism of the anilinolysis (XC₆H₄NH₂) of dithio esters, RC(S)SC₆H₄Z with R = C₂H₅ and C₆H₅CH₂ are investigated in acetonitrile at 45.0 °C. By application of various structure–reactivity correlations, selectivity parameters ρ_X, β_X, ρ_Z, β_Z and ρ_XZ are determined. The reactions are predicted to proceed stepwise with rate-limiting expulsion of the ArS⁻ group. The dithio ester with R = C₂H₅ exhibits the fastest rate and the largest positive ρ_XZ value; this is interpreted to result from the strongest electron donating ability of the ethyl group in the intermediate and a crowded tetrahedral intermediate and transition state in which the nucleophile (X) and leaving group (Z) are in close proximity due to the bulky C₂H₅ group. Much faster rates are observed for the thiocarbonyl (CS) rather than carbonyl (CO) esters in the stepwise nucleophilic substitution reactions, which may be ascribed to the lower π*_CS and σ*_C–LG levels than those of the corresponding antibonding levels in the carbonyl esters. The normal kinetic isotope effects, k_H/k_D > 1.0, involving deuterated anilines suggest concurrent proton transfer with the expulsion of the ArS⁻ leaving group in a four-center hydrogen bonded transition state.