A linear Br�nsted-type behavior in the aminolysis of substituted naphthyl acetates
摘要:
The reactions of 4-acetyl-1-naphthyl acetate (I) and 6-acetyl-2-naphthyl acetate (2) with a series of amines of varying pK(a),viz, morpholine, ammonia, ethanolamine, glycine n-butylamine, piperidine. hydrazine, imidazole, and hydroxylamine, are subjected to a kinetic investigation in aqueous medium. 30 degreesC, ionic strength 0.1 M (KCl). Pseudo-first-order rate coefficients (k(obs)) are found under amine excess. The plots of (k(obs) - k(H)) against free amine concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (k(N)) are obtained as the slopes of these plots and found to be pH-independent for all the amines employed. The Bronsted-type plots obtained (log k(N) against amine pK(a)) for the aminolysis of both esters 1 and 2 are linear with slope values of beta = 0.74 and beta = 0.94, respectively. From these values, the kinetic law and the analysis of products. it is deduced that for both esters aminolysis proceed through a zwitterionic tetrahedral addition intermediate (T+/-) whereby its dissociation into products is rate-limiting (k(2) step). Comparison of k(N) values among them shows that both esters follow an identical mechanistic pathway with 1 having higher reactivity than 2, the reasons for which are discussed. (C) 2001 John Wiley & Sons Inc.
An atom-efficient and powerful method for direct esterification of silyl ethers catalyzed by HClO4–SiO2
摘要:
An efficient and convenient procedure for direct esterification of alkyl and aryl silyl ethers with Ac2O and a catalyst system of perchloric acid immobilized on a silica gel (HClO4 SiO2) has been developed. The silyl protecting groups are directly replaced by acetyls and the protecting groups themselves are transformed into acetates as the sole byproducts, which can be readily recovered and converted back to silylchlorides, the original protecting agents, thus minimizing wastes. (C) 2010 Elsevier Ltd. All rights reserved.