Synthesis of 4‑Chalcogeno-1-aza-1,3-butadiene Derivatives by Intramolecular Cyano-Diels–Alder Reaction and Borane-Coordination-Induced Fluorescence Enhancement

4-Thio-1-aza-1,3-butadiene derivatives 3 incorporating a rigid dibenzobarrelene skeleton were synthesized by the intramolecular cyano-Diels–Alder reaction of 9-anthryl cyanoethenyl sulfides 6. The thermal reaction of 6 afforded an equilibrium mixture of 3 and 6, but the cyclization was effectively promoted by the addition of BF3·Et2O or B­(C6F5)3 to yield imine–borane adducts 8 and 9. The imine–borane adducts emit intense blue fluorescence both in solution and in the crystalline state. This is in stark contrast to free imines 3, which are weakly fluorescent. Selenium analogue 4 and N-oxide 12 of 3a were synthesized, along with their B­(C6F5)3 adducts.