Thermal 8π electrocyclic reaction of heteroarene tetramers: new efficient access to π-extended cyclooctatetraenes†
Bromo-substituted acyclic thiazole tetramers selectively underwent an 8π thermal electrocyclic reaction at high temperatures of 280–300 °C without solvent, producing π-extended thiazole-fused cyclooctatetraenes (COTs) in high yields. A quantum chemical study provided a rationale for the lower activation barrier and high selectivity of the 8π electrocyclic reaction mode over a competitive 6π electrocyclic reaction. The terminal bromine atom acts as a leaving group to trigger the rearomatization of the electrocyclized intermediate, which leads to the formation of the energetically stable COT products. This synthetic protocol enables us to synthesize a series of thiazole-fused COTs, including a derivative that can form loosely stacked columnar arrays in the crystalline state and shows fluorescence in the solid state.