Carbamates and monothiocarbamates of basic aliphatic amines undergo rate-determining C-N cleavage after a rapid equilibrium protonation step, as shown most directly by inverse solvent deuterium isotope effects of kD/kH = 3.6-4.8 for 0.0-and 0,s-N-n-butylcarbamates and by rapid acid-catalyzed exchange of the N H proton of n-BuNHCOS-with k e x c h = 5 X lo7 M-I s-I. The lifetimes of substituted N-protonated carbamates have been estimated to range down to <1O-Io s. It is concluded that general acid catalysis of the cleavage of carbamates of weakly basic anilines ( a = 0.84) occurs through an enforced preassociation mechanism with hydrogen bonding to the leaving protonated nitrogen atom and C-N cleavage in the rate-determining step. There is more proton transfer in the transition state (larger a ) and a smaller / ?I, with more basic amines and upon substitution of sulfur for oxygen. The low pK, values of N-protonated carbamates and monothiocarbamates illustrate the strong electron-accepting ability of -COO-and -COS-.
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