An Expeditious Green Synthesis of Benzimidazoles using VO (acac)2 as Catalyst Under Microwave Irradiation

Abstract

Benzimidazoles are considered as a ubiquitous
heterocyclic motif and its analogs are used extensively for diverse
pharmacological and biological activities. Based on this a series of
substituted benzimidazoles has been synthesized from ophenylenediamines
and aromatic aldehydes / acids under
microwave condition using vanadyl acetylacetonate, VO(acac)2 as
catalyst. The condensations have been carried out under solvent-free
conditions in compliance with Green Chemistry norms. The
heterogeneous reactions conditions provided a very simple,
environmentally friendly, clean, economical and selective protocol
for the preparation of a number of substituted benzimidazole
derivatives. The synthesized benzimidazoles were characterized by
comparing their melting points and spectral data with authentic
samples. The products were obtained in excellent yields and high
purity. The metal catalyzed syntheses of substituted benzimidazoles
reaction may proceed through the formation of Schiff’s base by
condensation of aldehydes with o-phenylenediamine and their
subsequent oxidative ring closure to give the respective
benzimidazole derivatives. Best results in respect of the catalyst
were obtained when VO(acac)2 was taken in 5mol%. Lower loading
of the catalyst resulted in lower yields and longer reaction time,
while higher amount of catalyst did not have much impact on the
yields of the products. The absence of either the catalyst or the
microwave radiation produced benzimidazoles for some of the
products but such reactions required longer reaction times and the
yields of the products obtained were also found to be very low.