Dearomatization-induced transannular cyclization: Synthesis of electron-accepting thiophene- S, S -dioxide-fused biphenylene

The transannular cyclization of dehydroannulenes bearing several alkyne moieties in close proximity is a powerful synthetic method for producing polycyclic aromatic hydrocarbons. We report that the reactivity can be switched by the aromaticity of the ring skeletons fused with the dehydroannulene core. Thus, while thiophene-fused bisdehydro[12]annulene 1 was handled as a stable compound in the air at room temperature, the oxidation with m-chloroperbenzoic acid from the aromatic thiophene rings to the nonaromatic thiophene-S,S-dioxides induced the transannular cyclization, even at room temperature, which was completed within 1 day to produce the formal [2 + 2] cycloadduct 3. This is in stark contrast to the fact that the thermal cyclization of 1 itself required heating at 80 °C for 9 days for completion. Experimental and theoretical studies indicate that the oxidation of even one thiophene ring in 1 sufficiently decreases the activation barrier for the transannular cyclization that proceeds through the 8π and 4π electrocyclic reaction sequence. The thiophene-S,S-dioxide-fused biphenylene 3 thus produced exhibits a set of intriguing properties, such as a higher electron affinity (E_1/2 = -1.17 V vs Fc and Fc⁺) and a stronger fluorescence (φ_F = 0.20) than the other relevant biphenylene derivatives, which have electron-donating and nonfluorescent characteristics.