title: Regioselectivity and the nature of the reaction mechanism in nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates with primary amines.
abstract: Second-order rate constants have been measured for the reaction of 2,4-dinitrophenyl X-substituted benzenesulfonates with a series of primary amines. The nucleophilic substitution reaction proceeds through competitive S-O and C-O bond fission pathways. The S-O bond fission occurs dominantly for reactions with highly basic amines or with substrates having a strong electron-withdrawing group in the sulfonyl moiety. On the other hand, the C-O bond fission occurs considerably for the reactions with low basic amines or with substrates having a strong electron-donating group in the sulfonyl moiety, emphasizing that the regioselectivity is governed by both the amine basicity and the electronic effect of the sulfonyl substituent X. The apparent second-order rate constants for the S-O bond fission have resulted in a nonlinear Brønsted-type plot for the reaction of 2,4-dinitrophenyl benzenesulfonate with 10 different primary amines, suggesting that a change in the rate-determining step occurs upon changing the amine basicity. The microscopic rate constants (k(1) and k(2)/k(-)(1) ratio) associated with the S-O bond fission pathway support the proposed mechanism. The second-order rate constants for the S-O bond fission result in good linear Yukawa-Tsuno plots for the aminolyses of 2,4-dinitrophenyl X-substituted benzenesulfonates. However, the second-order rate constants for the C-O bond fission show no correlation with the electronic nature of the sulfonyl substituent X, indicating that the C-O bond fission proceeds through an S(N)Ar mechanism in which the leaving group departure occurs rapidly after the rate-determining step.
authors.affiliation: Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea. ihum@mm.ewha.ac.kr
authors.forename: Ik-Hwan
authors.initials: IH
authors.lastname: Um
authors.forename: Jin-Young
authors.initials: JY
authors.lastname: Hong
authors.forename: Jung-Joo
authors.initials: JJ
authors.lastname: Kim
authors.forename: Ok-Mi
authors.initials: OM
authors.lastname: Chae
authors.forename: Sun-Kun
authors.initials: SK
authors.lastname: Bae