Multicomponent Synthesis of Tetrahydrobenzo[a]xanthene and Tetrahydrobenzo[a]acridine Derivatives using Sulfonated Multi-Walled Carbon Nanotubes as Heterogeneous Nanocatalysts

This study is the first report of the application of sulfonated multi-walled carbon nanotubes (MWCNTs-SO3H) in the synthesis of tetrahydrobenzo[a]xanthene and tetrahydrobenzo[a]acridine derivatives. The catalyst was prepared via a chemical approach and the sulfonated groups were attached to the side-wall of MWCNTs with total density of 2.58 mmol.g-1. In order to prove functionalization of the MWCNTs-SO3H, the catalyst was characterized using FE-SEM, TEM, FT-IR, and Raman spectroscopy techniques. A three-component reaction including 2-naphthol, dimedone, and aromatic aldehydes were applied in the synthesis of tetrahydrobenzo[a]xanthene in the presence of 15.5 mol% of MWCNTs-SO3H under solvent-free conditions. Also, a four-component reaction including 2-naphthol, dimedone, aromatic aldehydes, and ammonium chloride was used in the synthesis of tetrahydrobenzo[a]acridine in the presence of 12.9 mol% of MWCNTs-SO3H under solvent-free conditions. All the derivatives of tetrahydrobenzo[a]xanthene and tetrahydrobenzo[a]acridine were obtained in good to excellent yields. The MWCNTs-SO3H was reused in seven consequent catalytic cycles without loss of their catalytic activity.