Characterization of Two Chloro-Substituted m-Benzyne Isomers: Effect of Substitution on Reaction Efficiencies and Products

Abstract

Two isomeric chloro-substituted N-phenylpyridinium-type m-benzyne analogues were generated and examined in a Fourier transform ion cyclotron resonance mass spectrometer with the goal of comparing the reactivity and singlet−triplet gaps of the chlorinated and nonchlorinated benzynes. Theoretical calculations predict that the chloro isomers should have singlet−triplet gaps that differ by about 4 kcal/mol. Indeed, experiments confirm that the two singlet benzynes are separated in energy by 1.9 kcal/mol. Despite this difference, both isomers react at nearly identical rates that are as much as 2 orders of magnitude greater than the rates of the corresponding unchlorinated m-benzyne analogue. An interesting finding is that all of the N-phenylpyridinium m-benzyne ions react with tert-butyl isocyanide to form a strained, cyclic allene (although to different extents) that is almost 70 kcal/mol less stable than one of its hydrogen-shift isomers. The cyclic allene was characterized and quantified by collision-activa...