Mechanism of Cleavage of Carbamate Anions

Article abstract of DOI:10.1021/ja00529a033

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 k_D/k_H = 3.6-4.8 for O,O- and O,S-N-n-butylcarbamates and by rapid acid-catalyzed exchange of the NH proton of n-BuNHCOS^- with k_exch = 5 <*> 10⁷ M^-1 s^-1.The lifetimes of substituted N-protonated carbamates have been estimated to range down to <10^-10 s.It is concluded that general acid catalysis of the cleavage of carbamates of weakly basic anilines (α = 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 α) and a smaller β_1g with more basic amines and upon substitution of sulfur for oxygen.The low pK_a values of N-protonated carbamates and monothiocarbamates illustrate the strong electron-accepting ability of -COO^- and -COS^-.