Facile one pot synthesis of 2-substituted benzimidazole derivatives under mild conditions by using engineered MgO@DFNS as heterogeneous catalyst

Abstract

Benzimidazole derivatives are considered as important heterocyclic motifs that show a wide range of pharmaceutical applications. In view of their wide-ranging bioactivities, it is imperative to direct research on the sustainable catalytic synthesis of benzimidazole. Therefore, herein, we report a novel approach for the synthesis of benzimidazole and its derivatives with engineered MgO supported on dendritic fibrous nano silica (MgO@DFNS) as a sustainable heterogeneous catalyst. The catalyst MgO@DFNS was thoroughly characterized to understand its physio-chemical properties using XRD, FE-SEM, XPS, FT-IR, zeta potential, HR-TEM, TGA, TPR and TPD. The obtained results suggested that the catalyst MgO@DFNS prepared well and have the desired characteristics in it. After the successful characterisation of the prepared catalyst MgO@DFNS, it was applied in the synthesis of benzimidazole derivatives via condensation of o-phenylenediamine, and various aromatic and aliphatic aldehydes under ambient temperature. The catalyst produced a clean reaction profile with excellent yields in a shorter time under the umbrella of green chemistry. The effect of reaction parameters such as the effect of time, catalyst dosage, loading of MgO, effect of solvents and effect of different homo and heterogeneous catalyst were also tested. Furthermore, to understand the scope of the catalyst different substituted diamines and substituted aldehydes were reacted and obtained desired products in good to efficient yield. In addition, a recyclability study was also conducted and it was observed that the catalyst could be recycled for up to six cycles without noticeable changes in the morphology and activity. We believe that the present methodology gave several advantages such as an eco-friendly method, easy work-up, good selectivity, high yields and quick recovery of catalyst. MgO@DFNS is highly stable for several cycles without significant loss of its activity, which possibly demonstrates its applicability at the industrial scale.

Fig. 1. FT-IR spectra of pure DFNS and 10 wt% MgO@DFNS catalyst.

Fig. 2. XRD pattern of MgO@DFNS catalyst.

Fig. 3. TGA analysis of pure DFNS and 10 wt% MgO@DFNS catalyst.

Fig. 4. FE-SEM analysis of SEM DFNS (a–d) and 10 wt% MgO@DFNS (e–h).

Fig. 5. EDX analysis of DFNS (a–d) and 10 wt% MgO@DFNS (e–i).

Fig. 6. H₂-TPR analysis of DFNS and 10 wt% MgO@DFNS.

Fig. 7. NH₃-TPD analysis of DFNS and 10 wt% MgO@DFNS.

Fig. 8. (a) Survey spectrum of pristine DFNS and MgO@DFNS, deconvoluted spectra of (b) Si 2p, (c) O 1s and (d) deconvoluted spectra of Mg 1s (e) Si 2p, (f) O 1s.

Fig. 9. N₂-adsorption of DFNS and 10 wt% MgO@DFNS.

Fig. 10. DLS and zeta potential analysis of DFNS and 10 wt% MgO@DFNS.

Fig. 11. CO₂-TPD of DFNS and 10 wt% MgO@DFNS.

Fig. 12. Plausible reaction mechanism for 2-phenyl-benzimidazole synthesis over 10% MgO@DFNS catalyst.

Fig. 13. Recyclability study of MgO@DFNS catalyst for the synthesis of benzimidazole product (a), XRD analysis of fresh and recycled catalyst (b), FE-SEM analysis of recycled catalyst (c) and EDX analysis and elemental mapping of recycled catalyst (d).