The thermolysis and photolysis of ethyl azidoformate in acetic acid. The formation of ethoxycarbonylnitrenium ion intermediate

Abstract

The thermolysis and photolysis of ethyl azidoformate in acetic acid afforded O-acetyl-N-hydroxyurethane (I) as the major product, together with a trace of ethyl carbamate (II). The formation of (I) was suppressed in the presence of cyclohexane-1,2-dione and cyclohexene which gave 3-(ethoxycarbonylimino)-2-hydroxycyclohexanone (III) and trans-1-acetoxy-2-(ethoxycarbonylamino)cyclohexane (IV), respectively. The yield of (I) levelled off at ca. 20%. The addition of the dione and the olefin also caused a remarkable increase in the yield of (II). The addition of 9,10-anthraquinone or 9,10-phenanthraquinone resulted both in a decrease in the yield of (II) and a slight increase in the yield of (I). The relative reactivity of the addition to cyclohexene [the formation of (IV)] with respect to C–H insertion of the nitrene into cyclohexane in acetic acid is greater than that of the addition of the nitrene with respect to insertion showing that the intermediate in acetic acid is more electrophilic than the nitrene. Further, the rate data obtained from thermolysis in acetic acid or toluene indicate that the nitrene is primarily formed in acetic acid. This suggests that the reaction in acetic acid proceeds via an ion pair intermediate, CH₃CO₂^–+HCO₂Et, which is formed as the result of the attack of the nitrene on acetic acid, and which either collapses to produce (I) or dissociates to form (I) on reaction with acetic acid. The ethoxycarbonyl-nitrenium ion which escapes from the solvent cage is trapped with cyclohexane-1,2-dione or cyclohexene to give (III) or the stereospecific isomer (IV), along with (II).