Aminolysis of sulfamate esters in non-aqueous solvents. Use of Brønsted coefficients (βnuc) to assign E2 and E1cB mechanisms

Abstract

A series of Hammett ρacyl values for the aminolysis in chloroform and acetonitrile at 37 °C of the sulfamate esters RNHSO2OC6H4NO2-4 (R = XC6H4-, X = 4-MeO, 4-Me, 3-Me, H, 4-Br and 3-Cl) have been measured using piperidine and a set of five pyridines. Hammett ρpyr values have also been measured for various esters using a set of pyridines with varying substituents in the pyridine ring. A series of Bronsted βnuc values have been measured for the sulfamate esters. All the available βnuc values for sulfamate ester aminolysis have been compared and from this it has been possible to make mechanistic assignments to E1cB (βnucca. 0.7 and ca. 0) and E2 (βnuc 0.17 to 0.54) pathways. In the case of the E2 mechanism those reactions with lower βnuc values are thought to have little Nβ–H bond-breaking but this process is well advanced giving carbanionic E1cB-like characteristics to the E2 mechanism for reactions with larger βnuc values. Thermodynamic data (ΔH ‡ and ΔS ‡) support these interpretations.