W. Spillane, Andrew B. O'Byrne, C. McCaw
Aug 1, 2008
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Influential Citations
16
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Journal
European Journal of Organic Chemistry
Abstract
The kinetics of the reaction of 4-nitrophenyl sulfamate NH2SO2OC6H4NO2-4 (1a) in acetonitrile (ACN) with a series of pyridines (pKa range ca. 8 units) and alicyclic amines (pKa range ca. 3.6 units) has been studied in the presence of excess amine at various temperatures. The compounds 1a–1f are important as model substrates for the medicinally important sulfamate esters 667-coumate and emate and analogues. Pseudo-first-order rate constants (kobsd.) have been obtained mainly by the release of 4-nitrophenol/4-nitrophenoxide. Slopes of plots of kobsd. vs. [amine] gave second-order rate constants (k2), and Bronsted plots were biphasic for the aminolysis (with alicyclic amines) with an initial slope β1 = 0.53 and a subsequent slope β2 = 0.19. The change in slope occurs near the first pKa of 1a (17.9) in ACN. Leaving-group effects were probed by using the same series of phenyl sulfamates, i.e. 1a–f and the alicyclic amines N-formylpiperazine and pyrrolidine. The reactions were considered to be dissociative in nature involving E2- and E1cB- type mechanisms with the phenyl sulfamate anion 2 being involved in pyridine and in the weaker alicyclic amines (β1 segment) and a phenyl sulfamate dianion 3 being involved with the stronger alicyclic bases (β2 segment). The calculation of Leffler indices (α) for bond-forming (base···H+) and bond-breaking (S–OAr) steps allows fuller interpretation of the mechanisms occurring, which are seen as having the N-sulfonylamines, HN=SO2 and –N=SO2 on the reaction pathways leading to products. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)