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-nitropben yl N-methylsulfamate (NPMS) with a series of pyridines and a series of alic yclic amines and of 4-nitrophenyl N-benzylsulfamate (NPBS) with pyridines, alicyclic amines, and a series of quinuclidines have been investigated in a cetonitrile (ACN) in the presence of excess amine at various temperatures. Pseudo-first-order rate constants (k(cbsd)) have been obtained by monitorin g the release of 4-nitrophenol/4-nitrophenoxide. From the slope of a plot o f k(cbsd) vs [amine], second-order rate constants (k ' (2)) have been obtai ned for the pyridinolysis of NPMS, and a Bronsted Plot of log k ' (2) vs pK (a) of pyridine gave a straight line with beta = 0.45. However, aminolysis with alicyclic amines of NPMS gave a biphasic Bronsted plot (beta (1) = 0.6 , beta (2) approximate to 0). Pyridinolysis and aminolysis with alicyclic a mines and quinuclidines of NPBS also gave similar biphasic Bronsted 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 Bronst ed plot for NPMS with pyridines occurs because the later bases are not stro ng enough to substantially remove the substrate proton and initiate the mec hanistic change observed in the reaction of NPMS with the strong alicyclic amines and quinuclidines. An entropy study supports the change from a bimol ecular to a unimolecular mechanism. This is the first clear demonstration o f this E1cB mechanistic changeover involving a nitrogen acid substrate.