Zhiqing Wu

A Simple and Efficient Catalytic System for N-Arylation of Imidazoles in Water

N-Aryl imidazoles are key structural motifs in a wide range of agrochemicals, pharmaceuticals, and biologically active compounds and have been exploited as important precursors of versatile N-heterocyclic carbenes, a powerful class of ligands for transition-metal catalysis or room-temperature ionic liquids. A traditional method of introducing this functionality is nucleophilic aromatic substitution of imidazole with activated aryl halides (SNAr reactions) or classic Ullmann-type coupling reactions. However, these well-known methods suffer from several drawbacks, such as high reaction temperature, stiochiometric amounts of metal reagents, and low functional-group tolerance, which has limited their applications. Recently, Buchwald et al. and Taillefer et al. found several Nand O-based ligands could greatly facilitate the copper-catalyzed path for the N-arylation of imidazoles. After the discovery of these ligands, ACHTUNGTRENNUNGinterest in this field has increased spectacularly in the last few years and resulted in the golden age of copper-catalyzed Ullmann-type coupling reactions. While many important results have been achieved via this methodology, it is worth noting that these protocols are generally operated in volatile organic solvents with transitionmetal ions. From the standpoint of green chemistry, the development of more environmentally benign reaction media, such as water, in place of organic solvents would be desirable. Henceforth, as part of our endeavors toward the development of environmentally friendly protocols, efforts were directed towards performing the N-arylation reaction by using water as the sole reaction medium. To date, salen ligands have been recognized as one of the most efficient auxiliaries, and many metallosalen complexes are used as excellent catalysts in various organic transformations. Herein we report the first example the N-arylation of imidazoles directly catalyzed by sulfonato–CuACHTUNGTRENNUNG(salen) complex 1 under mild conditions in water with satisfactory results. This catalytic system contains several advantages: 1) Water, a green solvent, is used in place of volatile organic solvents; 2) the catalysis reactions could be performed without an inert gas atmosphere and with low catalyst loading; 3) the work-up procedure is simple with excellent yields; 4) the catalyst could be easily recovered and used again. As shown in Scheme 1, catalyst 1 could be conveniently synthesized in 82 % yield by a one-pot reaction of three

Palladium-Catalyzed CH ortho Arylation of Benzoic Acids with Diaryliodonium Salts in Water

Palladium-catalyzed C C bond-formation reactions between aryl halides or triflates and organometallic reagents are among the most useful methods to construct biaryls, which are usually structural motifs found in important nature products, pharmaceuticals, and organic materials. The corresponding organometallic nucleophilic starting materials, however, are often not commercially available and sometimes lead to the formation of undesired side products. Over the past decade, extensive efforts have been made to explore the transition-metal-catalyzed direct arylation of C H bonds as ecologically and economically friendly alternatives. Furthermore, owing to the requirement of site selectivity, directing groups such as pyridyl, acylamino, hydroxy, and phenolic ester groups have been broadly introduced into substrates. The majority of these directing groups cannot be easily removed from the products, and a few of them have to be deprotected by further transformations. In recent years, transition-metal-catalyzed decarboxylative coupling reactions have emerged as a powerful strategy to form carbon–carbon and carbon–heteroatom bonds from cheap, diverse, and readily available aryl carboxylic acids. The carboxylic acid group can efficiently be removed or converted into a variety of useful functional groups, which makes the directed ortho C H functionalization of carboxylic acids exceptionally practical for applications in organic synthesis. In 2007, the groups of Yu and Daugulis pioneered the direct ortho-arylation of benzoic acids with aryl halides by using palladium as a catalyst (Scheme 1 a). Also, Yu and co-workers demonstrated that phenylboronic acid esters and aryltrifluoroborates can be utilized as coupling partners for the ortho functionalization of benzoic acids (Scheme 1 b). 7] Recent work has identified that diaryliodonium salts, which are commercially available or easily synthesized, are important alternatives that can be used as arylation reagents in palladium-catalyzed C H arylation reactions. However, application of diaryliodonium salts in the catalytic ortho-arylation of benzoic acids has not yet been reported. The use of water as an environmentally benign, nonflammable, and nontoxic medium for organic transformations is strongly desired because of green chemistry. 10] As part of our continuous interest in aqueous catalysis, herein we report an efficient and practical protocol for the palladium-catalyzed C H ortho arylation of benzoic acids with diaryliodonium salts by using water as the solvent (Scheme 1 c). In our initial optimization study, m-toluic acid and phenyl(2,4,6-trimethylphenyl)iodonium triflate (2 a) were chosen as model substrates. As illustrated in Table 1, no desired product was found in the absence of catalyst by using water as the solvent (Table 1, entry 1). Gratifyingly, the substrates could be transformed into desired ortho-arylated product 3 a in 85 % yield by using a catalytic amount of Pd(OAc)2 (Table 1, entry 2). Next, a variety of organic solvents were tested; nevertheless, only a trace amount of the product was formed (Table 1, entries 3–10). The results suggest strongly that the choice of water as the solvent is crucial for this reaction. Replacing Pd(OAc)2 by PdCl2 resulted in a slightly lower yield of the product (78 %; Table 1, entry 11). Furthermore, low temperatures decelerated the reaction rate; product 3 a was obtained in 54 % yield if the reaction was performed at 80 8C (Table 1, entry 12). A symmetrical iodonium salt was also compatible with the reaction, and it provided a similar result (Table 1, entry 13). Scheme 1. Palladium-catalyzed C H ortho arylation of benzoic acids. Ac = acetyl, BQ = 1,4-benzoquinone, Mes = 2,4,6-trimethylphenyl, Tf = trifluoromethanesulfonyl.

Quaternary ammonium salt as alkylation agent in three-component reactions for the synthesis of benzothiazoles in water

Substituted benzothiazoles are synthesized by metal-catalyst-free three-component reactions of o-iodoaniline, quaternary ammonium salt, and sulfur powder in water with moderate-to-excellent yields up to 95%.