Benzocyclobutadienyl anion: Formation and energetics of an antiaromatic molecule

Benzocyclobutadienyl diazirine (2) was synthesized and reacted with hydroxide ion in a Fourier transform mass spectrometer to afford the conjugate base of benzocyclobutadiene (1a). Authentication of the ion structure was carried out by a derivatization experiment (i.e., 1a was converted to benzocyclobutenone enolate, which has previously been studied), and its reactivity was explored. Thermochemical data for benzocyclobutadiene (1) were obtained (ΔH°(acid) (1) = 386 ± 3 kcal mol^-1, EA(1r) = 1.8 ± 0.1 eV, and C-H BDE (1) = 114 ± 4 kcal mol^-1), compared to MP2 and B3LYP calculations, and contrasted to a series of model compounds. Cyclobutadienyl radical appears to be quite different from benzocyclobutadienyl radical (1r) and worth further exploration.