Conformational preferences and energetics of N–O heterolyses in aryl nitrenium ion precursors: ab initio and semiempirical molecular orbital calculations

Abstract

In the lowest energy conformations of N-hydroxyaniline and N-hydroxyacetanilide, ab initio HF/3-21G and semiempirical AM1 molecular orbital calculations predict that the NO bonds lie close to the planes of the aryl rings. However, the barriers to rotation about C_Ar–N bonds via species in which the NO bonds are approximately orthogonal to the aryl planes are small. According to semiempirical AM1 calculations in the N-acetoxy analogues of these, and more complex arylamines, the orthogonal conformations are among the most stable. According to a previous suggestion NO heterolysis to the corresponding nitrenium ions should proceed most favourably from such conformations. NO heterolysis is calculated to be energetically less favourable in the arylamides due to the loss of amide resonance in their precursors. Regardless of the nature of the nitrogen substituents the heterolysis becomes progressively more favourable as the aryl group is varied in the series: Ph < 2-naphthyl < 1-naphthyl ⩽ 2-fluorenyl.