Elimination–addition mechanism for the hydrolysis of carbamates. Trapping of an isocyanate intermediate by an o-amino-group

Abstract

The hydrolytic conversion of aryl carbamates to amines is base catalysed and evidence is presented that the process involves an E1cB elimination with the formation of isocyanate intermediates. Substituents in the leaving group (O-aryl ring) have a larger effect (ρ=+3·17) on the observed rate of hydrolysis than those in the N-aryl ring (ρ=+0·64) where the effects of a substituent are compensatory. Using N-(p-nitrophenyl)carbamates, it was possible to measure both the acidity of the carbamate (K_a1) and the rate of reaction of the carbamate anion (k₂); k₂ was sensitive to substituent effects (ρ=+2·90) for aryl carbamates indicating a high degree of acyl–oxygen bond cleavage in the transition state. A change in mechanism from E1cB to B_AC2 attack by hydroxide ion was noted for poorer leaving groups. The pH profile for the conversion of p-nitrophenyl N-(o-aminophenyl)carbamates to o-phenyleneurea was interpreted to show that the o-amino-group traps an isocyanate intermediate after the rate determining E1cB elimination. Substituent effects for NN-disubstituted carbamates which hydrolyse via a B_AC2 mechanism are also discussed.