Tailored SnO2@MWCNTs efficient and recyclable nano-catalyst for selective synthesis of 4, 5-dihydropyrrolo 1, 2-a quinoxalines via Pictet-Spengler reaction

摘要

Heterocyclic compounds have a wide range of applications, and Pictet-Spengler reaction is effectual in the synthesis of heterocyclic compounds such as quinoline and isoquinoline. The present work describes an effective Pictet-Spengler-type protocol for the selective synthesis of quinoxaline derivatives from substituted pyrroles and diverse aromatic aldehydes over tailored SnO2 nanoparticles anchored multi-walled carbon nanotubes (SnO2@MWCNTs) nano-catalyst under greener reaction condition. An elementary reaction process (chemical synthesis) of oxidation of MWCNTs followed by uniform dispersion of SnO2 nanoparticles is used to synthesize the SnO2@MWCNTs catalyst. The SnO2@MWCNTs catalyst was then characterized by using modern spectroscopic and analytical techniques. The activity of catalyst was investigated toward the formation of selective quinoxaline. The reaction progressed with 100% conversion and 93-80% yield for the desired product at ambient operating conditions in just 2.30-3.30 h of reaction time. The effect of different parameters such as reaction temperature, time, and loading of SnO2 on MWCNTs, SnO2@MWCNTs catalyst loading were also investigated and discussed in detail. The protocol displayed high tolerance to different functionalities with respect to different substituted aromatic aldehydes to form quinoxaline derivatives in efficient way and gave excellent yield. Plausible reaction mechanistic pathway for the selective formation of quinoxaline over SnO2@MWCNTs is also proposed. It is believed that high dispersion of SnO2 over MWCNTs provided sufficient Lewis acidic sites for the reaction to selectively obtain the final product. Additionally, recyclability of SnO2@MWCNTs catalyst was also studied which demonstrated that the catalyst could be efficiently reused for six successive cycles without significant loss in its activity. The present work provides greener approach for synthesis of quinoxaline derivative preparation with high selectivity (free from oxidized quinoxaline as a side product) and efficient yield. [GRAPHICS] .