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

Abstract

The aqueous reaction mechanisms of methyl N-(substituted phenoxycarbonyl)sulfamates 4a–d were examined in the entire pH range 0–14 at 50 °C. The pH–rate profiles indicate a rate law that includes two pH-independent terms, ka (s−1) in acid and kp (s−1) around neutral pH, with ka > kp, and a hydroxide-ion dependent term, kOH (dm3 mol−1 s−1), at high pH. In acid, product analysis reveals that two competitive reactions are involved for hydrolysis with ka = kCO + kMe: a general acid-catalysed acyl–oxygen bond cleavage reaction of anions 4−−− (kCO) and a methyl–oxygen bond cleavage reaction resulting from water attack at the methyl carbon of neutral compounds 4 (kMe). For all compounds 4 investigated the kMe reaction is the dominant pathway (i.e., >79%) in 1.0 mol dm−3 HCl solution. In contrast to ka, the spontaneous hydrolysis reaction of 4−−−, kp, takes place exclusively by acyl–oxygen bond cleavage and leads to the formation of methoxysulfonyl isocyanate as free intermediate. As observed in acid for the kMe reaction, the kOH reaction of 4−−− is best rationalized by a SN2Al mechanism in which hydroxide ion attacks anions 4−−− at the saturated methyl carbon leading to methyl–oxygen bond cleavage.