A novel approach to l-tryptophan grafting on mesoporous MCM-41: A recoverable heterogeneous material for organocatalyzed benzo[N,N]-heterocyclic condensation

Abstract

An unprecedented strategy has been developed to introduce l-tryptophan into MCM-41 by binding the indolic NH group of l-tryptophan to modified MCM-41, resulting in the creation of l-Tryp-N-pMCM-41. Initially, 3-Cl-pMCM-41 was prepared by modifying activated MCM-41 with 3-chloropropyltrimethoxysilane. Subsequently, the covalent attachment between the indolic NH group of l-tryptophan and 3-Cl-pMCM-41 was achieved using n‑butyl lithium in THF at -78 °C under an argon atmosphere. l-Tryp-N-pMCM-41 has been characterized by FT-IR, XRD, TGA, EDS, BET and nitrogen physisorption measurements. The organocatalytic performance of this chemically stable material was assessed in the synthesis of both symmetrical and unsymmetrical quinoxalines. The reaction involved the condensation of a series of aromatic 1,2-diamines with 1,2-diketones in methanol at room temperature. Overall, the organocatalytic reaction proceeded smoothly to afford the corresponding quinoxalines in moderate to excellent yields. Furthermore, the crystal structure of an unsymmetrical quinoxaline has been determined by single crystal X-ray diffraction. This metal-free procedure offers several noteworthy advantages, including the simplicity of the reaction, minimal catalyst usage, recyclability of the catalyst and environmentally friendly characteristics. Moreover, this green heterogeneous organocatalyst is a suitable candidate for preparing various valuable pharmaceutical compounds under ambient conditions avoiding the use of strong acids and without any possibility of metal contamination.