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The expedient and efficient copper powder catalyzed direct
ring-opening arylation of benzazoles with aryl iodides in
polyethylene glycol that proceeds in the absence of an added
ligand has been developed. The protocol provides facile ac-
cess to 2-(arylthio)anilines and 2-phenoxyanilines in high
yields with a wide tolerance of functional groups. Trans-
Introduction
2-(Arylthio)aniline and 2-aryloxyaniline are privilegedscaffolds of various compounds that exhibit unique bio-logical and pharmaceutical properties, including antifungal,anticancer, and antiviral activities and caspase-1 inhibitors,and have attracted considerable interest from chemistry andbiology to medicine.[1] Generally, the methods for the syn-
thesis of 2-(arylthio)anilines and 2-(aryloxy)anilines mainlyrely on transition-metal-catalyzed C–S[2] and C–O[3] cross-coupling reactions with aryl halides, respectively. In thesereactions involving aromatic –NH2 and aromatic –SH (and/or aromatic –OH) active groups, site-specific selectivity isdifficult, particularly in the preparation of 2-(aryloxy)-anilines.[4] Moreover, those methods also suffer from theneed to prepare easily oxidized 2-aminothiophenols and 2-aminophenols as starting materials.
To address these problems, a two-step procedure was re-ported: a nitroaryl substrate was used for the cross-couplingreaction and then the nitro group was reduced.[5] Althoughthis process can give excellent selectivity, two steps are re-
quired, which leads to the formation of much more waste.Recently, Duan and co-workers reported that the reductionof the nitro group and C–S cross-coupling can occur con-currently in a one-pot reaction.[6] However, only stronglyelectron-deficient coupling partners could undergo thistransformation smoothly. Alternatively, Li developed a
[a] Jiangsu Key Laboratory of Biofunctional Materials, Key Labo-ratory of Applied Photochemistry, School of Chemistry andMaterials Science, Nanjing Normal University,Nanjing 210023, ChinaFax: +86-25-8589-1455E-mail: [email protected] information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201166.
mission electron microscopy confirmed that the active cata-
lyst results from the in situ generation of copper nanopar-
ticles under standard reaction conditions, which is an alter-
native avenue to develop a highly effective metallic copper
catalyst. Moreover, the catalytic system can be recycled up
to six times.
highly selective and direct coupling reaction for 2-(phen-ylthio)aniline with 2,2Ј-disulfanediyldianiline and phenyltri-methoxysilane in the presence of an air-sensitive and toxicphosphane ligand.[7] To date, it is still a challenge to developan efficient and environmentally benign catalytic system forthe synthesis of 2-(arylthio)anilines and 2-(aryloxy)anilines.
During our research, Xu et al. demonstrated the CuCl-catalyzed cross-coupling reactions of benzothiazole witharyl iodides to form 2-(arylthio)anilines in water,[8] but thisapproach is strictly limited to a single heterocyclic substrate,benzothiazole, and more inexpensive metallic copper is anineffective catalyst in the system.[9]
Metallic copper is cheap, nontoxic, and readily available.However, the ideal catalyst frequently exhibits poor per-formance in cross-coupling reactions.[10] Recently, Ranuand co-workers reported the direct use of metallic copperpowder as a catalyst for C–S bond-formation reactions thatshowed far lower activity than copper nanoparticles pre-prepared by a routine procedure.[11] Therefore, it is highly
desirable to find a way to directly use metallic copper as aneffective catalyst.On the basis of previous studies in cross-coupling reac-
tions in polyethylene glycol (PEG),[12] we report herein anovel ligandless and efficient protocol for preparing 2-(aryl-thio)anilines and 2-phenoxyaniline from the reactions of
Scheme 1. Copper-catalyzed reactions of benzazoles with aryl iod-
Cu Powder Catalyzed Direct Ring-Opening Arylation of Benzazoles
benzazoles with aryl iodides by directly using copper pow-der as the catalyst in PEG-600 (Scheme 1).
Results and Discussion
Initially, the cross-coupling of benzothiazole with iodo-benzene was chosen to optimize the reaction parameters(Table 1). When the reaction was performed in the presenceof copper powder as the catalyst and Cs2CO3 as the basein EG (ethylene glycol),[13] an acceptable yield was obtainedin 6 h (Table 1, entry 1). Gratifyingly, under otherwise iden-tical conditions, reactions that were performed in solventscontaining long alkyl chains, for example, PEG-400 andPEG-600,[14] went to completion to afford desired product3aa in 94 and 96% yield, respectively (Table 1, entries 2 and3), whereas PEG-2000 and PEG-6000 with far longer alkylchains resulted in critically low yields (Table 1, entries 4 and5). Various other copper catalysts were tested: CuCl and
CuI showed good catalytic activity (Table 1, entries 13 and15), whereas CuBr and Cu2O gave product 3aa in moderateyields (Table 1, entries 14 and 16). The base also heavily af-fected the transformation: relative to the reaction per-formed with Cs2CO3 (the best), K2CO3 and K3PO4·3H2Orequired much longer times to reach completion (Table 1,entries 6 and 7), and CH3COOK, KF, Na2CO3, andNaHCO3 were completely ineffective (Table 1, entries 8– 11).
Table 1. Effect of reaction conditions.[a]
Entry Cat. Base Solvent t [h] Yield[b] [%]
1 Cu Cs2CO3 EG 6 722 Cu Cs2CO3 PEG-400 6 943 Cu Cs2CO3 PEG-600 6 964 Cu Cs2CO3 PEG-2000 6 95 Cu Cs2CO3 PEG-6000 6 66 Cu K2CO3 PEG-600 24 947 Cu K3PO4·3H2O PEG-600 15 968 Cu CH3COOK PEG-600 6 – 9 Cu KF PEG-600 6 010 Cu Na2CO3 PEG-600 6 0
11 Cu NaHCO3 PEG-600 6 012[c] Cu Cs2CO3 PEG-600 6 6513 CuCl Cs2CO3 PEG-600 6 8914 CuBr Cs2CO3 PEG-600 6 7215 CuI Cs2CO3 PEG-600 6 8716 Cu2O Cs2CO3 PEG-600 6 7017[d] Cu Cs2CO3 PEG-600 6 86
[a] Reaction conditions: benzothiazole (0.5 mmol), iodobenzene(1.0 mmol), base (1.0 mmol), solvent (2.0 g), 140 °C, N2 atmo-sphere. [b] Yield of isolated product after column chromatographyon silica gel. [c] Air atmosphere. [d] Loading of copper was de-creased to 5 mol-%.
With the above-optimized reaction conditions (10 mol-%
Cu, 2.0 equiv. Cs2CO3, PEG-600, N2), a series of benza-
zoles and aryl iodides were evaluated (Table 2). The reac-tions with aryl iodides bearing either electron-rich or elec-tron-deficient substituents at the phenyl moiety proceededefficiently to afford the desired products (i.e., 3aa – 3da) inexcellent yields in short times. The reaction times were ob-served to be only loosely correlated with the electronic na-ture of the aryl iodides. This protocol could tolerate halidessuch as F and Br, which can be used for further functionali-zation. The structure of 3ce was confirmed by single-crystalX-ray diffraction (see Supporting Information).[15] In ad-dition, a free amino group was also tolerated under normalconditions. Recently, Liu and co-workers disclosed that thenitro group could be reduced in PEG-600.[6] Interestingly,in our case, the nitro group did not disturb the high selectiv-ity and remained intact in PEG-600. Sterically hinderediodo-2-methylbenzene was a good substrate and providedthe corresponding products in high yields (i.e., 3ab and3cb). Moreover, a doubly functionalized product could be
Table 2. Copper-catalyzed reactions of benzazoles with aryl iod-ides.[a,b]
[a] Reaction conditions: benzothiazole (0.5 mmol), iodobenzene(1.0 mmol), base (1.0 mmol), solvent (2.0 g), 140 °C, N2 atmo-sphere. [b] Yield of isolated product after column chromatography
L. Yao, Q. Zhou, W. Han, S. WeiSHORT COMMUNICATION
obtained in good yield under analogous reaction conditionswhen 1,4-diiodobenzene was used as the aryl iodide[Eq. (1)]. Notably, complicated aryl iodide 2i was quite ef-fective in the reaction with benzothiazole and furnished de-sired product 3ai in 91% yield [Eq. (2)], which is potentiallyuseful for the synthesis of biologically important naturalproducts and pharmaceuticals.[16]
(1)
(2)
Diverse benzazoles were tested in this transformation.Benzothiazoles having electron-rich and electron-deficientgroups worked well under the optimized reaction condi-tions (Table 2). The reactivities of the electron-deficientbenzothiazoles were higher than those of the electron-richbenzothiazoles, which was found to be proportional to theC–H acidity at C2 of the benzothiazole. Notably, benzo-xazole also underwent the transformation successfully togive product 3da in 93% within 3 h. However, the desiredproducts were not obtained when thiazole and N -methyl-benzimidazole were used as substrates.
During the reaction between benzothiazole and iodo-benzene under the standard conditions, the copper catalystwas examined by transmission electron microscopy (TEM).The in situ generated copper nanoparticles had an averagesize of 3 nm (Figure 1). This is an alternative to the conven-tional protocol used to prepare metal nanoparticles, whichleads to problems of waste formation and environmentalpollution.[17] The use of copper nanoparticles as a highlyefficient catalyst in C–S bond-formation reactions has beenreported.[11,18] Gratifyingly, the in situ formed catalyst canbe recycled up to six times by using the model reaction toafford the corresponding product in 96, 95, 95, 94, 90, and
To gain preliminary insight into the process of the trans-formation, control experiments were performed.[19] In 2007,Ranu and co-workers reported copper nanoparticle cata-lyzed C–S bond formation from the cross-coupling of thio-phenols with aryl halides and proposed a radical mecha-nism.[11] To check this mechanism in our case, 2,2,6,6-tet-ramethylpiperidine N -oxide (TEMPO, 1.0 equiv. relative tothe benzothiazole), a radical scavenger, was added to themodel reaction under otherwise identical reaction condi-tions. The reaction was not affected at all by the presenceof TEMPO, which suggested a radical pathway was not inoperation. Earlier works have indicated that azoles possessacidic C–H protons and that they can be ring opened upondeprotonation.[20] In our case, the reactivity is proportionalto the C–H acidity at C2 of the benzothiazoles (Table 2),
which supports a deprotonation pathway. Moreover, the re-action between 2-aminothiophenol and iodobenzene pro-ceeded smoothly to afford 3aa in 94% yield within 1 h[Eq. (3)]. These findings imply that a deprotonation-in-duced ring-opening step may be present.
(3)
Although the detailed mechanism for the copper-cata-lyzed ring-opening arylation reaction remains to be eluci-dated, a possible catalytic cycle is proposed. As shown inFigure 2, ring-opened intermediate A is formed by depro-tonation of a benzazole under the assistance of base andsubsequently is hydrolyzed to intermediate B. The catalyticcycle is initiated by oxidative addition of the aryl iodide tothe copper nanoparticles to generate intermediate C, fol-lowed by attack of intermediate B to give key intermediateD. Finally, reductive elimination of intermediate D fur-
nishes the desired product and regenerates the catalyst.
Cu Powder Catalyzed Direct Ring-Opening Arylation of Benzazoles
Figure 2. A possible catalytic loop for the reaction of benzazoleswith aryl iodides.
Conclusions
In conclusion, a simple, practical, recyclable, and envi-ronmentally benign protocol has been developed for the di-rect synthesis of pivotal 2-(arylthio)aniline and 2-phen-oxyaniline intermediates through the in situ generated cop-per nanoparticle catalyzed ring-opening arylation of benza-zoles with aryl iodides in PEG-600. The approach over-comes the disadvantages of classic coupling reactions that
involve the use of substrates that can be readily oxidizedand that suffer from selectivity problems. Various benz-azoles were treated with aryl iodides to afford the desiredproducts in high yields with a remarkable compatibility of functional groups. The starting materials are readily avail-able and stable and can be used to prepare more reactiveproducts with free amino groups that have been widely usedin the synthesis of drug-like and/or pharmaceutically rel-evant molecules. Further work will be dedicated to the ap-plication of the present system for the construction of com-plex molecules and to the investigation of the detailedmechanism.
Experimental Section
General Procedure for the Copper Powder Catalyzed Cross-Coupling
of Benzazole with Aryl Iodides: A 25-mL flask was charged withcopper (10 mol-%, 3.2 mg), Cs2CO3 (1.0 mmol, 330 mg), and PEG-600 (2.0 g) before standard cycles of evacuation and back-fillingwith dry and pure N2. Benzazole (0.5 mmol) and the aryl iodide(2 equiv., 1.0 mmol) were then added successively. The mixture wasstirred at 140 °C for the indicated time. At the end of the reaction,the reaction mixture was poured into a saturated aqueous NaClsolution (20 mL) and extracted with ethyl acetate (3 ϫ 15 mL). Theorganic phases were combined, and the volatile components were
evaporated under reduced pressure. The crude product was purified
by column chromatography on silica gel (petroleum ether/diethylether/triethylamine = 30:1:1).
Supporting Information (see footnote on the first page of this arti-cle): Experimental procedures, characterization data, 1H NMR and13C NMR spectra of the products, and X-ray data for compound3ce.
Acknowledgments
The work was sponsored by the Natural Science Foundation of Jiangsu Provincial Colleges and Universities (12KJB150014), theScientific Research Start-up Foundation of Nanjing Normal Uni-versity (2011103XGQ0250), and the Priority Academic ProgramDevelopment of Jiangsu Higher Education Institutions. We thankProf. Min Fang and Dr. Min Han for assistance with X-ray struc-ture analysis and TEM analysis, respectively.
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Received: August 30, 2012Published Online: November 6, 2012