Synthesis of Azulene-substituted Tetraarylpyrroles by Reaction of 1-azulenyl Ketones with Benzoin and Ammonium Acetate

Tetraarylpyrroles with a 1-azulenyl substituent were prepared by the reaction of 1-azulenyl ketones, which have various aryl-substituents at their α-position, with benzoin in the presence of ammonium acetate as a nitrogen source of the pyrrole ring. Optical property of the tetraarylpyrroles obtained by the reaction was clarified by UV/Vis spectroscopy and/or time-dependent density functional theory (TD-DFT) calculations. INTRODUCTION A variety of synthetic methods for pyrrole derivatives have been developed so far, because the compounds having a pyrrole skeleton are found in numerous natural products and pharmaceuticals. In particular, the synthetic procedures for tetraarylpyrroles and their derivatives have been actively investigated in recent years, since the pyrrole derivatives with multiple aryl substituents have attracted the interest in the field of functional organic materials, such as luminescent materials, e.g., organic EL and LED, with high luminous efficiency and long emission lifetime. As a classical method, the dehydrative condensation reaction of 1,4-diketones with a primary amine, known as Paal-Knorr synthesis, has been employed for the synthesis of the pyrrole derivatives. However, there are some difficulties in the preparation of aryl-substituted 1,4-diketones, which become a good precursor for the arylpyrrole derivatives. As an alternative procedure, there is a dimerization method of aryl ketones in the presence of hydrazine via condensation reaction for the pyrrole synthesis, but this 1870 HETEROCYCLES, Vol. 94, No. 10, 2017