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

Abstract

A series of Hammett ρ_acyl 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 ρ_pyr values have also been measured for various esters using a set of pyridines with varying substituents in the pyridine ring. A series of Brønsted β_nuc values have been measured for the sulfamate esters. All the available β_nuc values for sulfamate ester aminolysis have been compared and from this it has been possible to make mechanistic assignments to E1cB (β_nucca. 0.7 and ca. 0) and E2 (β_nuc 0.17 to 0.54) pathways. In the case of the E2 mechanism those reactions with lower β_nuc values are thought to have little N_β–H bond-breaking but this process is well advanced giving carbanionic E1cB-like characteristics to the E2 mechanism for reactions with larger β_nuc values. Thermodynamic data (ΔH ^‡ and ΔS ^‡) support these interpretations.