Change in rate-determining step in an E1cB mechanism during aminolysis of sulfamate esters in acetonitrile

The kinetics of the reactions of the nitrogen-sulfur(VI) esters 4-nitrophenyl N-methylsulfamate (NPMS) with a series of pyridines and a series of alicyclic amines and of 4-nitrophenyl N-benzylsulfamate (NPBS) with pyridines, alicyclic amines, and a series of quinuclidines have been investigated in acetonitrile (ACN) in the presence of excess amine at various temperatures. Pseudo-first-order rate constants (k_obsd) have been obtained by monitoring the release of 4-nitrophenol/4-nitrophenoxide. From the slope of a plot of k_obsd vs [amine], second-order rate constants (k′₂) have been obtained for the pyridinolysis of NPMS, and a Brønsted plot of log k′₂ vs pK_a of pyridine gave a straight line with β = 0.45. However, aminolysis with alicyclic amines of NPMS gave a biphasic Brønsted plot (β₁ = 0.6, β₂ ≅ 0). Pyridinolysis and aminolysis with alicyclic amines and quinuclidines of NPBS also gave similar biphasic Brønsted plots. This biphasic behavior has been explained in terms of a mechanistic change within the E1cB mechanism from an (E1cB)_irrev (less basic amines) to an (E1cB)_rev (more basic amines), and the change occurs at approximately the pK_a's (in ACN) of NPMS (17.94) and NPBS (17.68). The straight line Brønsted plot for NPMS with pyridines occurs because the later bases are not strong enough to substantially remove the substrate proton and initiate the mechanistic change observed in the reaction of NPMS with the strong alicyclic amines and quinuclidines. An entropy study supports the change from a bimolecular to a unimolecular mechanism. This is the first clear demonstration of this E1cB mechanistic changeover involving a nitrogen acid substrate.