Mechanisms of Decomposition of Aryl N-(Methoxycarbonyl)sulfamates in Aqueous Media

Abstract

Rate constants and products are reported for the decomposition of aryl N-(methoxycarbonyl)sulfamates 1a − h (range) in aqueous solutions (pH 0−14) at 50 °C. The pK a values at 25 °C of 1a − h are 0.46−2.40. The pH-rate profiles of 1 indicate a rate law that includes three terms: two pH independent terms, k a in acid and k p around neutral pH, with k a > k p, and a hydroxide ion dependent term, k OH. In acid, product analysis reveals that both S−O (k SO2) and C−O (k CO) bond cleavage reactions are involved. From an analysis of βlg, solvent isotope effects, and solvent isotopic labeling of the products, it is concluded (a) that the C−O cleavage reaction involves protonation of the leaving group methanol and its expulsion from the dipolar intermediate ArOSO2N-CO−O+HCH3 and (b) that the S−O cleavage reaction may involve either an intra- or an intermolecular general acid-catalyzed decomposition of 1 or 1 - , respectively. In contrast to k a, the spontaneous hydrolysis reaction of 1 - , k p, takes place exclusively via S−O bond fission. The k OH reaction of 1 - is rationalized by OH- attack either at the carbonyl center (1b − h) or at the aromatic ring (1a). It is concluded that the −SO2NHCO− group may be viewed as an attractive phosphate analogue.

A General Buffer-Acid-Catalyzed C−O Cleavage Reaction in the Hydrolysis of PhenylN-(Phenoxycarbonyl)sulfamate Ester

“…[11] However, hydrolysis of N-(oxycarbonyl)sulfamate esters may occur with SÀO bond-breaking if the leaving group on sulfur is much better than that on carbon, as previously observed with certain aryl N-(methoxycarbonyl)sulfamates, ArOSO 2 NH-CO 2 Me. [8] In the latter case, no buffer catalysis was detected in acid for SÀO cleavage, while the analogous CÀO breaking reaction previously reported for phenyl N-(phenylsulfonyl)-carbamate anion (PhSO 2 N À CO À OPh) was unambiguously demonstrated to be general buffer-acid-catalyzed. [7] The different behaviors observed in acid between the C À O and SÀO cleavage reactions of N-(oxycarbonyl)sulfamate esters are discussed.…”

“…The increased reactivity of the title compound compared to phenyl N-(methoxycarbonyl)sulfamate, [8] by a factor of ca. 4 at 50 8C, is as expected for a mechanism involving a rate-limiting nucleophilic attack at the carbonyl center with different leaving groups such as phenol and methanol.…”

“…In fact, extrapolation of the Brùnsted plot log k p vs pK lg obtained for the uncatalyzed decomposition of the anions of aryl N-(phenylsulfonyl)carbamates, [7] as indicated in ref. [8], suggests that the zwitterionic species PhSO 2 N À COÀO (H)Ar (I) do not exist as discrete species for leaving groups of pK a`c a.13.4 (at 50 8C), that is, for all leaving phenols. [21] Similarly, taking into account the stabilizing effect (ca.…”

“…Indeed, the aqueous decompositions of weakly basic anions proceeding with C À O bond breaking are notable by comparison with those proceeding with SÀO (Schemes 7 and 9 below) or PÀO (Scheme 8) bond cleavage, in that where there is evidence for an enforced general acid ± base catalysis (because of the nonexistence of a putative intermediate), these other bondbreaking reactions (i.e., SÀO and PÀO cleavages) are not buffer-catalyzed. From our earlier study of aryl N-(methoxycarbonyl)sulfamate esters [8] it was concluded that the zwitterionic species in Scheme 7 probably do not exist as discrete species (i.e., k z b 10 13 s…”

Competition between SN2 and (general acid-catalysed) E1cB reactions in the aqueous decomposition of methyl N-(substituted phenoxycarbonyl)sulfamate esters †

“…2 Recently we have been looking at the reaction mechanisms involved in the aqueous decomposition of unusually weak nitrogen anions of type G 1 -SO 2 N Ϫ CO-G 2 (with G 1 and/or G 2 = alkoxy or aryloxy leaving groups). [2][3][4] Our study of aryl N-(phenylsulfonyl)carbamate esters 1 3 (pK a = 2-3) and phenyl N-(phenoxycarbonyl)sulfamate 2 2 (pK a = 1.18) allowed us to demonstrate that a general acid catalysis may occur in an E1cB process to assist leaving group departure, as observed for the C-O bond cleavage reaction of anions 1 ؊ and 2 ؊ at pH < 4 (Scheme 1). In addition, our kinetic results showed that in the entire pH range both 1 and 2 hydrolyse exclusively by way of their anionic form with 100% C-O bond cleavage.…”