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-trimethylsilyl-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.