Graphitic Carbon Nitride (g-C3N4)-Catalysed Green Synthesis of Heterocycles: Progress, Mechanistic Insights, and Future Perspectives

Graphitic carbon nitride (g-C3N4) has proved to be an excellent and versatile catalyst, devoid of any metal, for the sustainable production of heterocycles. Owing to its nitrogen-rich conjugated framework, which provides Lewis-basic and hydrogen-bonding sites and enables tuneable electronic properties, together with its excellent thermal and chemical stability and recyclability, graphitic carbon nitride (g-C3N4) has emerged as a versatile metal-free heterogeneous catalyst for the synthesis of heterocycles. In this review, a critical analysis of recent trends in g-C3N4-catalysed multicomponent reactions (MCRs) and their related approaches towards the synthesis of pharmaceutically important heterocycles is provided. This review highlights current developments in g-C3N4-catalysed multicomponent reactions (MCRs) for the synthesis of biologically and pharmaceutically relevant heterocyclic frameworks, including pyrimidines, pyridines, pyrans, chromenes, tetrazoles, quinolines, imidazoles, triazoles, and spirocyclic heterocycles. Mechanistic aspects, including Lewis acid–base mechanism, photocatalysis and formation of radicals, have been highlighted to establish the correlation between catalyst properties and the reaction outcomes. The sustainability of the methodology is assessed through reaction mass efficiency, atom economy, process mass intensity, E-factor, energy efficiency and catalyst reusability. Critically analysed advancements have been made regarding the development of hybrid catalysts based on g-C3N4. Challenges faced during catalyst deactivation, utilization of visible light, scalability and incompleteness in green metric reporting are addressed.