Heterocycles are an important class of compounds that are widely used in pharmaceuticals, agrochemicals, dyes, and materials. Multicomponent reactions (MCRs) offer efficient synthetic routes for producing these complex structures. The search for effective and sustainable catalytic processes in organic synthesis has led to the exploration of various nanomaterials as potential catalysts. To this end, carbon nanotubes (CNTs) have recently emerged as promising heterogeneous catalysts for the MCR synthesis of heterocycles due to their unique properties, which include high surface area and reactivity, tunable surface chemistry, excellent electrical conductivity, recyclability, and exceptional thermal and chemical stability. This review provides a comprehensive analysis and overview of the use of CNTs as catalysts for synthesizing heterocycles via MCRs and their advantages.

Carbon nanotubes as heterogeneous catalysts for the multicomponent reaction synthesis of heterocycles
    Abstract
        Graphical abstract
    1 Introduction
    2 Carbon nanotubes
        2.1 Single-walled carbon nanotubes
        2.2 Double-walled carbon nanotubes
        2.3 Multi-walled carbon nanotubes
        2.4 Synthesis of CNTs
        2.5 The mechanism of action of CNTs in MCRs
            2.5.1 Catalytic surface and active sites
            2.5.2 Improved electron transfer
            2.5.3 Thermal conductivity
            2.5.4 Stabilization of intermediates and transition states
            2.5.5 Functionalization for specific reactions
            2.5.6 Mechanistic pathways
        2.6 The perspective of CNTs in green chemistry
        2.7 Future prospects of CNTs and challenges
            2.7.1 Interfacial interaction
            2.7.2 Control of structural parameters
            2.7.3 Agglomeration prevention
            2.7.4 Optimization of efficiency and stability
            2.7.5 Cost and scalability
            2.7.6 Controllable synthesis
            2.7.7 Scale-up production
            2.7.8 Macroscopic assembly
            2.7.9 CNT-based biosensors
            2.7.10 Desalination
            2.7.11 Impurities
            2.7.12 Separation and sorting
            2.7.13 Control over properties
            2.7.14 Environmental and health impact
    3 The synthesis of heterocycles via CNT-catalyzed MCRs
        3.1 N-Heterocycles
            3.1.1 Piperidine scaffolds
            3.1.2 1,4-Dihydropyridines (1,4-DHPs)
            3.1.3 Pyridine scaffolds
            3.1.4 Azole scaffolds
        3.2 O-Heterocycles
        3.3 Spiro compounds
        3.4 Miscellaneous compounds
    4 Conclusions
    References

• Ramin Javahershenas(Department of Organic Chemistry, Faculty of Chemistry, Urmia University, Urmia, Iran) Corresponding author
• Vadim A. Soloshonok(Department of Organic Chemistry I, Faculty of Chemistry, University of the Basque Country (UPV/EHU), Paseo Manuel Lardizábal 3, 20018 San Sebastián, Spain, Basque Foundation for Science, IKERBASQUE, Alameda Urquijo 36‑5, Plaza Bizkaia, 48011 Bilbao, Spain)
• Karel D. Klika(Molecular Structure Analysis, German Cancer Research Center (DKFZ), 69120 Heidelberg, Germany)
• Peter J. Jervis(Center of Chemistry, University of Minho, Campus de Gualtar, 4710‑057 Braga, Portugal)