The electron as a protecting group. 2. Generation of benzocyclobutadiene radical anion in the gas phase and an experimental determination of the heat of formation of benzocyclobutadiene

Benzocyclobutadiene radical anion (5) was synthesized in the gas phase by three independent approaches: collision-induced dissociation (CID) of 1,2-benzocyclobutenedicarboxylate, reaction of 2-tri-methylsilyl-1-benzocyclobutenyl anion with neopentyl nitrite followed by CID of the resulting β-nitroso carbanion intermediate, and reaction of the same β-silyl carbanion with molecular fluorine. The proton affinity and electron binding energy of 5 were measured (0.32 ± 0.05 eV and 368 ± 2 kcal mol^-1, respectively) and combined in a thermodynamic cycle to obtain the heat of hydrogenation (49 ± 4 kcal mol^-1) and the heat of formation (97 ± 4 kcal mol^-1) of benzocyclobutadiene (1). These results were compared to model compounds as well as MP2 and B3LYP calculations in order to assess the antiaromatic destabilization energy of 1. Based upon our data, a predicted heat of formation for cyclobutadiene (102 kcal mol^-1) was obtained. This work demonstrates the utility of dicarboxylates as radical anion precursors and the electron as a protecting group.