Synthesis of 5‐Heteroaryl‐ and 5,7‐Bis(heteroaryl)azulenes by Electrophilic Substitution of 1,3‐Di‐tert‐Butylazulene with Triflates of N‐Containing Heterocycles

Abstract

AbstractAn efficient synthesis of 5‐heteroaryl‐ and 5,7‐bis(heteroaryl)azulene derivatives was established for first time. The reaction proceeded through electrophilic substitution of 1,3‐di‐tert‐butylazulene (1) with the triflates of N‐containing heterocycles, in the presence of excess heterocycles, in good yields. The presumed intermediates, 5‐(dihydroheteroaryl)azulene derivatives were also available by the reaction with several N‐containing heterocycles in the presence of Tf2O under milder reaction conditions. Treatment of the 5‐(dihydroheteroaryl)azulene derivatives with KOH readily gave the desired 5‐(heteroaryl)azulene derivatives. Unexpectedly, N‐(5‐azulenyl)pyridinium triflate 17 was also obtained by the reaction of 1 with (trifluoromethylsulfonyl)pyridinium trifluoromethanesulfonate (TPT). 5‐Heteroaryl‐ and 5,7‐bis(heteroaryl)azulene derivatives exhibited a significant color change in acetic acid compared to those in dichloromethane due to the development of intramolecular charge‐transfer (CT) absorption bands. The redox behavior of these new azulene derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV).