دانلود رایگان مقاله روش اقتصادی اتم پالادیوم کاتالیز شده برای بنزوتیازول و بنزوکسازول

1. Introduction

Transition-metal-catalyzed direct functionalization of C\\H bonds represents a powerful approach for the development of more sustainable chemical processes [1]. Direct arylation of arenes is an attractive alternative to traditional cross-coupling reactions [2]. These arylation reactions of benzothiazoles and benzoxazoles achieve regioselectivity using transition metal catalysts [3–6]. These moieties are part of compounds having several biological properties such as antibacterial, antitumor and anti-inflammatory [7–9]. The synthesis of such heterocycles and their derivatives has been a topic of research as they have important applications in a variety of fields. Transition metal catalyzed C\\H functionalization reactions of arenes and hetero arenes have gained more attention in the preparation of various organic and biologically important scaffolds [10]. A variety of aryl sources, such as aryl halides [11], arylboronic acids [12], iodobenzenediacetate [13], arenediazonium salts [14], aryl sulfamates [15], aryltrimethylammonium triflates [16], arene benzenesulfonyl chlorides [17] and substituted benzoic acids [18] have been used for the arylation of benzothiazoles and benzoxazoles. Some examples are given in Fig. 1.

2. Results and discussion

Initially, we have optimized various reaction parameters to develop this protocol. PdCl2-Cu(OAc)2 acts as a catalytic system over the model reaction of benzothiazole (1a) and only 0.33 equivalent of triphenylbismuth (2a) to get the arylated product (3a). The experimental results are summarized in Table 1. We first carried out the reaction with PdCl2 as a catalyst and obtained product 3a with 6% GC yield (entry 1, Table 1). We also used Cu(OAc)2 as a catalyst and obtained only 12% GC yield of 3a (entry 2, Table 1). When we employed Pd/Cu (5:10 mol%) catalytic system, it gave 54% yield of 3a (entry 3, Table 1). After getting these encouraging results, we tried to get better product yield of 3a by employing various ligands. We screened various nitrogen containing ligands such as 2,2′-bipyridine, 1,10-phenanthroline and TMEDA. They showed 16%, 66% and 34% yield respectively (entries 4–6, Table 1). Phosphorus containing ligands such as DPPB, DPPP, and DPPF provided 36%, 46% and 49% yields of 3a (entries 7–9, Table 1). We got very surprising results when we used 10 mol% PPh3 as ligand. It offered 90% GC yield and 87% isolated yield of 3a (entry 10, Table 1). We also tried some palladium species such as Pd(PPh3)4, Pd(dba)2, Pd(dppf)Cl2, Pd(PPh3)2Cl2 along with Cu(OAc)2 as a co-catalyst and PPh3 as a ligand but obtained only 74%, 48%, 59% and 72% GC yield respectively (entries 11–14, Table 1). We also tried other bases such as Cs2CO3 and K2CO3 but got only 34% and 21% yield of 3a (entries 15–16, Table 1). The model reaction was carried out using various solvents such as DMF, water, dioxane and DMA. The obtained results revealed that DMSO is the best solvent for this reaction (entries 17–20, Table 1). Reaction using RuCl2(PPh3)3 and NiCl2 afforded only 15% and 8% GC yield respectively (entries 21–22, Table 1). Optimized reaction parameters for the reaction between benzothiazole (0.5 mmol) and triphenylbismuth (0.167 mmol) include the catalytic system PdCl2 (5 mol%), Cu(OAc)2 (10 mol%), PPh3 (10 mol%), K3PO4 base (1.5 mmol), DMSO (3 mL) as a solvent, at 100 °C for 12 h.