Solvent-Free and Catalyst-Free Annulation of Aryl-2-carbonitriles with Cyclic Amines: A Convenient Pathway to Imidazole-Fused Heterocycles and Carboxamides

Cyclic fused imidazoles such as pyrrolo[1,2-c]imidazoles are biologically significant and novel aza-heterocycles, and very few examples are available in the literature. Accordingly, the present study describes a sustainable and one-step solvent-free protocol for the thermal annulation of aryl-2-carbonitriles 1a-d with cyclic amines 2a-e. This thermal-promoted reaction is simplified for facile access to imidazole-fused polycyclic frameworks 3aa-dc via a three-component dimerization-coupling sequence in low to moderate yields (up to 29%). Concurrently, competitive hydrolysis pathways result in the formation of the corresponding tertiary carboxamides 4aa-dc (17–45% yields) and primary carboxamides 5a-d (5–34% yields). The structures of all the synthesized bis-aryl cyclic[1,2-c]imidazoles and carboxamides have been established through complete NMR and mass spectral data. The X-ray crystal structures of compounds 3aa and 3da were confirmed through XRD crystallographic studies. In addition, a conceivable mechanistic approach has been established for the annulation of cyclic fused imidazoles and their carboxamides. The reaction proceeds through a complex, competing pathway to simultaneously afford three distinct classes of products without any external catalyst or solvent. The substrate scope was explored using various cyclic amines 2a-e including pyrrolidine, piperidine, morpholine, thiomorpholine, and piperazine, as well as the electronically distinctive derivatives quinoline (1a,b,d) and naphthalene carbonitrile (1c). The present study demonstrates the distinct reactivity profile of heterocyclic nitriles under thermal conditions and provides valuable insights into atom-economical tandem operations and amide-driven functional group transformations.