Journal of the American Chemical Society p. 3072 - 3084 (1980)
Update date:2022-08-03
Topics:
Ewing, Sheila P.
Lockshon, Daniel
Jencks, William P.
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 O,O- and O,S-N-n-butylcarbamates and by rapid acid-catalyzed exchange of the NH proton of n-BuNHCOS- with kexch = 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 β1g with more basic amines and upon substitution of sulfur for oxygen.The low pKa values of N-protonated carbamates and monothiocarbamates illustrate the strong electron-accepting ability of -COO- and -COS-.
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