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

The kinetics and mechanism of the anilinolysis (XC6H4NH2) of dithio esters, RC(=S)SC(6)H(4)Z with R = C2H5 and C6H5CH2 are investigated in acetonitril e at 45.0 degreesC. By,application of various structure-reactivity correlat ions, selectivity parameters rhox, betax, rhoz, betaz and rho xz are determ ined. The reactions are predicted to proceed stepwise with rate-limiting ex pulsion of the ArS- group. The dithio ester with R=C2H5 exhibits the fastes t rate and the largest positive rho xz value; this is interpreted to result from the strongest electron donating ability of the ethyl group in the int ermediate and a crowded tetrahedral intermediate and transition state in wh ich 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 (C=S) rather than carbonyl (C=O) esters in the stepwise nucleophilic substi tution reactions, which may be ascribed to the lower pi*(C-S) and sigma*(C- LG) levels than those of the corresponding antibonding levels in the carbon yl esters. The normal kinetic isotope effects, k(H)/k(D) > 1.0, involving d euterated anilines suggest concurrent proton transfer with the expulsion of the ArS- leaving group in a four-center hydrogen bonded transition state.