Benzocyclobutadienyl anion: Formation and energetics of an antiaromatic molecule

Benzocyclobutadienyl diazirine (2) was synthesized and reacted with hydroxi de ion in a Fourier transform mass spectrometer to afford the conjugate bas e of benzocyclobutadiene (1a). Authentication of the ion structure was carr ied out by a derivatization experiment (i.e., 1a was converted to benzocycl obutenone enolate, which has previously been studied), and its reactivity w as explored. Thermochemical data for benzocyclobutadiene (1) were obtained (Delta H degrees(acid) (1) = 386 +/- 3 kcal mol(-1), EA(1r) = 1.8 +/- 0.1 e V, and C-H BDE (1) = 114 +/- 4 kcal mol(-1)), compared to MP2 and B3LYP cal culations, and contrasted to a series of model compounds. Cyclobutadienyl r adical appears to be quite different from benzocyclobutadienyl radical (1r) and worth further exploration.