An efficient and versatile synthesis of 2, 2'-(alkanediyl)-bis-1H- benzimidazoles employing aqueous fluoroboric acid as catalyst: Density Functional Theory calculations and fluorescence studies
Chhanda Mukhopadhyay,*a Sabari Ghosh,a and Ray J. Butcherb
aDepartment of Chemistry, University of Calcutta, 92 APC Road, Kolkata-700009, India bDepartment of Chemistry, Howard University, Washington DC 20059, USA
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Abstract
2,2'-(Alkanediyl)-bis-1H-benzimidazoles (simple and mixed) with variable methylene spacers were synthesized in excellent yields with aqueous fluoroboric acid (45%) (0.1 ml) as catalyst under solvent-free conditions. Their optimized structures were obtained using DFT calculations where it was seen that the s-trans orientation of the two imidazole rings was preferred for all types of bis-benzimidazole systems. The X-ray crystal structure of one such bis-benzimidazole further corroborated this fact. Finally, photophysical studies were carried out to get insight into the fluorescence characteristics of the newly synthesized bis-1H-benzimidazoles.
Keywords: 2,2´-(alkanediyl)bis-1H-benzimidazoles, 45% aqueous fluoroboric acid, Solvent-free conditions, DFT, photophysical studies
Introduction
The development of efficient approaches to chemically and biologically important products from readily available inexpensive starting materials has been an active topic in modern organic chemistry.1 The main objective of this research was to use easily available reagents and their application in an environment-friendly way to the synthesis of functionalized heterocycles that are of great interest in organic synthesis, as such motifs are ubiquitous in both natural products and biologically active pharmaceutical agents. Development of a more practical pathway is important to synthesize new heterocyclic compounds that are otherwise difficult to synthesize by conventional methodologies. We became particularly interested in the synthesis of bis-1H- benzimidazoles by solvent-free techniques that have gradually replaced the use of volatile, hazardous and toxic organic solvents over the past few decades.2 Such ecofriendly chemical processes have attained substantial interest both in the industry and in academia.3 Bis- benzimidazoles are known to offer lead inhibition of the activity of M1-RNA, the inhibitions being caused by the unusual mechanism of the binding of these organic ligands (whose structures
