Competition between SN2 and (general acid-catalysed) E1cB reactions in the aqueous decomposition of methylN-(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, k_a (s⁻¹) in acid and k_p (s⁻¹) around neutral pH, with k_a > k_p, and a hydroxide-ion dependent term, k_OH (dm³ mol⁻¹ s⁻¹), at high pH. In acid, product analysis reveals that two competitive reactions are involved for hydrolysis with k_a = k_CO + k_Me: a general acid-catalysed acyl–oxygen bond cleavage reaction of anions 4⁻−− (k_CO) and a methyl–oxygen bond cleavage reaction resulting from water attack at the methyl carbon of neutral compounds 4 (k_Me). For all compounds 4 investigated the k_Me reaction is the dominant pathway (i.e., >79%) in 1.0 mol dm⁻³ HCl solution. In contrast to k_a, the spontaneous hydrolysis reaction of 4⁻−−, k_p, 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 k_Me reaction, the k_OH reaction of 4⁻−− is best rationalized by a S_N2_Al mechanism in which hydroxide ion attacks anions 4⁻−− at the saturated methyl carbon leading to methyl–oxygen bond cleavage.