Electrophilic aromatic N-substitution by ethoxycarbonylnitrenium ion generated from ethyl azidoformate in the presence of trifluoroacetic acid

Abstract

Thermal reactions of ethyl azidoformate with aromatic compounds in the presence of trifluoroacetic acid (TFA) gave ethyl N-arylcarbamates. Toluene, nitrobenzene, or naphthalene was selectively attacked on the ortho- and para-positions, on the ortho- and meta-positions, or on the 1- and 2-positions, respectively. The yields were ca. twice as high as those for thermolysis (a nitrene–azepine route) in the absence of TFA with the subsequent addition of TFA at room temperature. The positional selectivities for toluene and naphthalene were lower for the former thermolysis than for the latter. The reactivity of cyclohexane or toluene relative to benzene was greater in the latter thermolysis than the former. A Hammett plot with ρ–1.7 was obtained for the formation of ethyl N-arylcarbamates in the former thermolysis in TFA–nitrobenzene (8 : 2 v/v); the value was more negative than that (–1.32) for the disappearance of substituted benzenes in the reaction with ethoxycarbonylnitrene. The results suggest that the thermolysis in the presence of TFA involves an electrophilic aromatic N-substitution by ethoxycarbonylnitrenium ion.