Synthesis of 1,6′-Bi- and 1,6′:3,6-Terazulenes from 1-Pyridyl- and 1,3-Di(pyridyl)azulenes by the Ziegler-Hafner Method

The synthesis of 1,6 '-biazulenes 3a and 3b and 1,6 ':3,6 ''-terazulenes 7a and 7b was established by one-pot reactions via the Zincke-type ring-opening of the corresponding pyridinium salts with diethylamine, followed by the reaction with cyclopentadiene in the presence of sodium methoxide. The intramolecular charge-transfer characters between azulene rings were investigated by UV-vis spectroscopy and theoretical calculations.