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

Abstract

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. Highlights

• An efficient and reusable sulfonated multi-walled carbon nanotubes were prepared.
• MWCNTs-SO₃H was used in the synthesis of tetrahydrobenzo[a]xanthene and tetrahydrobenzo[a]acridine.
• All of the reactions were performed under solvent-free conditions.
• The MWCNTs-SO3H was characterized using FE-SEM, TEM, FT-IR, and Raman spectroscopy.
• The catalyst could be reused for seven catalytic cycles.