Aminolysis of aryl chlorothionoformates with anilines in acetonitrile: effects of amine nature and solvent on the mechanism.

The aminolysis of aryl chlorothionoformates (7, YC(6)H(4)OC(=S)Cl) with anilines (XC(6)H(4)NH(2)) in acetonitrile at 5.0 degrees C has been investigated. The rates are slower than those for the corresponding reactions of aryl chloroformates (6, YC(6)H(4)OC(=O)Cl). This rate sequence is a reverse of that for alkyl chloroformates (1-4) in water, for which rate-limiting formation of a tetrahedral intermediate, T(+/-), is predicted. On the basis of the large negative cross-interaction constant, rho(XY) = -0.77, failure of the reactivity-selectivity principle, normal k(H)/k(D) values involving deuterated nucleophiles (XC(6)H(4)ND(2)), and low DeltaH(not equal) with large negative DeltaS(not equal) values, a concerted mechanism with a four-membered hydrogen bonded cyclic transition state (11) is proposed for the title reaction series. It has been shown that the solvent change from water to acetonitrile for the aminolysis of 6 and 7 causes a mechanistic change from stepwise to concerted.