Metal-organic-framework-derived copper-based catalyst for multicomponent C–S coupling reaction

Abstract

Copper-based metal-organic frameworks (Cu-MOFs) are a promising multiphase catalyst for catalyzing C–S coupling reactions by virtue of their diverse structures and functions. However, the unpleasant odor and instability of the organosulfur, as well as the mass-transfer resistance that exists in multiphase catalysis, have often limited the catalytic application of Cu-MOFs in C–S coupling reactions. In this paper, a Cu-MOFs catalyst modified by cetyltrimethylammonium bromide (CTAB) was designed to enhance mass transfer by increasing the adsorption of organic substrates using the long alkanes of CTAB. Concurrently, elemental sulfur was used to replace organosulfur to achieve a highly efficient and atom-economical multicomponent C–S coupling reaction.

1. Introduction

Thioethers, as important intermediates in organic and drug synthesis, are extensively used in the manufacture of various chemicals, such as polymeric materials, pharmaceuticals, and agrochemicals [1,2]. Accordingly, the C–S coupling reaction, which is one of the significant ways to construct thioether compounds, has turned into a major research topic in synthetic chemistry recently [3]. In the research of constructing C–S bonds, homogeneous transition-metal catalysts have made significant contributions, especially copper-based catalysts, which are notable for their rich redox chemistry and cost-effectiveness [4]. Over the past few years, numerous efforts have been dedicated to exploring copper-catalyzed C–S coupling reactions. Examples include that Cu(OAc)2-catalyzed C3-sulfenylation of indoles with sulfur powder and aryl iodides have exhibited superior yields [5]. Similarly, Cu(CO2CF3)2 provided high catalytic activity in the Chan–Lam-type C–S coupling reaction with aryl sodium sulfite as the substrate [3]. However, copper-based catalysts also inevitably suffer from the common problems of homogeneous catalysts, including difficulties in separation and recycling [6]. Multiphase catalysts are more stable than homogeneous catalysts and are easier to recycle and reuse, thus the conversion of copper-based homogeneous catalysts to multiphase catalysts is a promising approach for the sustainable development of C–S coupling reactions [7,8].

The distinctive composition and structure of metal-organic frameworks (MOFs) make them stand out among many multiphase catalysts [9–11]. MOFs, consisting of metal-containing nodes connected by organic linkers, are novel nanomaterials that have undergone rapid development in the recent decades [12–17]. The metals located at the nodes of the MOF structure have free coordination sites after the thermal removal of aqueous ligands, without significant structural change. Additionally, the shapes and sizes of MOFs are adjustable according to design, providing size-selective catalysis [18–22]. Leveraging these unique advantages, the Cu-based MOFs have captured widespread attention in the enclosure of catalyzing C–S coupling reactions [23]. For instance, Phan et al. [24] synthesized a copper-based framework, Cu2(OBA)2(BPY), to catalyze the direct C–S coupling reaction of sodium sulfinates and oxime acetates. The results showed that the catalytic activity of the Cu-based MOF surpassed a series of conventional copper-based homogeneous catalysts, yielding β-sulfonylvinylamines at rates as high as 87%. In addition, Wang et al. [25] reported a PCN-60 metal-organic skeleton prepared from a copper compound and 1,3,5-triazine-2,4,6-triyl tribenzoate (H3TATB) ligand, exhibiting excellent catalytic performance in catalyzing the reaction of disulfide with aryl iodide and sulfonyl chloride to form sulfonyl diphenylsulfides. However, the copper-based MOF-catalyzed C–S coupling reactions mentioned earlier primarily rely on organosulfur agents, whose unpleasant odor and structural instability limit the advancement of C–S coupling reactions [26–29]. Furthermore, as a multiphase catalyst, the catalytic activity of Cu-based MOFs is also constrained by the mass transfer of reactants and products [30]. Therefore, selecting a suitable sulfur source and improving the mass transfer of reactants and products are key considerations in multiphase catalytic C–S coupling reactions.