The Application of Copper/Iron Cocatalysis in CrossâCoupling Reactions

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The Application of Copper/Iron Cocatalysis in Cross-Coupling Reactions Jincheng Mao,*[a,b] Hong Yan,[b] Guangwei Rong,[b] Yue He,[b] and Guoqi Zhang*[c]

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C 2016 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim V

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ABSTRACT: For conventional cross-couplings in organic chemistry, precious metal (such as Pd or Rh) complexes have been the preferable choices as catalysts. However, their high cost, toxicity, and potential contamination of products limit their massive applications on some occasions, particularly in the pharmaceutical industry, where close monitoring of the metal contamination of products is required. Therefore, the use of metals that are less expensive and less toxic than Pd or Rh can be greatly advantageous and earth abundant metal (such Fe or Cu) catalysts have shown promise for replacing the precious metals. Interestingly, a certain copper catalyst combined with an iron catalyst displays higher catalytic efficiency than itself in various coupling reactions. Notably, ligand-free conditions make such protocols more useful and practical in many cases. In this account, we summarize the recent progress made in this increasingly attractive topic by describing successful examples, including our own work in the literature, regarding effective copper/iron cocatalysis. In addition, a few examples involving a magnetic and readily recyclable CuFe2O4 nanoparticle cocatalyst are also included. Keywords: alkynylation, cross-coupling, decarboxylation, ligand-free, transition metals

1. Introduction Today’s chemical catalysis relies heavily on the employment of transition metals as catalysts, and indeed, transition-metal catalysts have found widespread application in tremendous organic transformations, including some crucial crosscoupling reactions. During the past few decades, enormous effort was directed towards Pd- and Ni0-catalyzed crosscoupling reactions.[1,2] Despite their high catalytic activity, there are still plenty of problems regarding these metal catalysts, such as low abundance, high cost, and toxicological features that largely limit their use on both laboratory and industrial scales. Therefore, to develop new and more efficient approaches possessing commercial availability, good stability, low expense and toxicity, and environmental friendliness is highly desired in current organic synthesis. Fortunately, the

[a]

J. Mao State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Southwest Petroleum University Chengdu 610500 (P. R. China) E-mail: [email protected] E-mail: [email protected] [b] J. Mao, H. Yan, G. Rong, Y. He Key Laboratory of Organic Synthesis of Jiangsu Province College of Chemistry, Chemical Engineering and Materials Science Soochow University Suzhou 215123 (P. R. China) [c] G. Zhang Department of Sciences John Jay College and The Graduate Center The City University of New York New York NY 10019 (USA) E-mail: [email protected]

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past decade has witnessed great progress in the utilization of such transition-metal catalysts, such as those involving iron or copper metals.[3] Iron and copper are both highly abundant in the Earth’s crust, and they are generally considered to be environmentally friendly. In addition, in contrast to precious metals, iron and copper are inexpensive and readily available, which will greatly favor their large-scale application in the chemical industry. In recent years, bimetallic cocatalyst systems have emerged as excellent candidates for some types of reactions, and in many cases, bimetallic cocatalysts have shown unique catalytic abilities compared with the monometallic ones. Chemists have adverted to study the effects brought by bimetallic catalysts, while exploring the optimized reaction conditions for certain reactions, and as a result, significant progress has been made, although the mechanistic aspects and the roles two metals play during the catalytic process remain largely unexplored. To this end, a variety of bimetallic systems, such as palladium/copper,[4] palladium/ nickel,[5] iron/copper,[6] and copper/silver,[7] etc. have been successfully implemented and optimal results have been observed. Among them, the combination of iron and copper has shown great efficacy and versatility in catalyzing a number of reactions, probably owing to cooperative substrate activity that leads to improved reactivity by two metals. Relevant work has been previously summarized as a short review by Jiao and coworkers in 2011.[8] Since then, significant advancement has been achieved in iron/copper cocatalyzed reactions; in particular, that aiming at new crosscoupling reactions through the efforts from both our and other groups. In this account, we will briefly review the iron/copper cocatalyzed carbon-carbon or carbonheteroatom coupling reactions, with a focus on the most recent reports in the literature.



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2. Iron/Copper Cocatalyzed Alkynylation Coupling Reactions 2.1. Iron/Copper Cocatalyzed Sonogashira Couplings It has been well known that aryl acetylenes can be easily accessed from aryl halides and alkynes since the classic Sonogashira reaction was reported in 1975.[9] In the later decades, both biological and organic synthesis has made full use of Sonogashira coupling to obtain important aryl acetylenes. The classic catalyst system for this reaction is a palladium-based phosphine complex catalyst with a copper(I) salt as cocatalyst, which, however, has limited use in industry due to the high cost of the palladium present in the catalyst.[4a] Therefore, a catalyst of equal efficiency, but lower cost, is more desirable for application on a large scale. With a great deal of effort, chem-

Jincheng Mao received his B.Sc. and M.S. degrees from Southwest Petroleum University (SWPU) in 1999 and 2002, respectively. Then he obtained his Ph.D. degree in organic chemistry from Dalian Institute of Chemical Physics, Chinese Academy of Science in 2005 under the supervision of Prof. Boshun Wan and Shiwei Lu. After postdoctoral research in the group of Prof. Can Li in the State Key Lab of Catalysis, he began his independent career in Soochow University in 2006. During 2008 to 2009, he worked in the group of Prof. Thomas R. Ward to study artificial metalloenzymes. In 2015, he came back to SWPU, and now is a professor in the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation. His research interests relate to organic synthesis, functional materials, and green chemistry. Hong Yan received his bachelor’s degree in applied chemistry from Soochow University in 2011. He then pursued his Ph.D. study in the Prof. Jincheng Mao’s group at Soochow University. Currently, Hong is studying at the University of M€ unster as a joint Ph.D. student under the supervision of Prof. Armido Studer. His current research interests focus on cross-couplings and radical chemistry.

ists have been devoted to exploring new catalyst systems in recent years, in which iron/copper cocatalysts have played a crucial role. In 2008, Mao and coworkers reported an effective ligand-free iron/copper cocatalyzed Sonogashira reaction (Scheme 1).[6a] In this work, they found that 20 mol% Fe(acac)3-CuI (1 : 1), partnered with 2 equivalents of K3PO4 as a base in DMSO, displayed the highest catalytic activity. It was noticed that in the control experiments, the use of either copper or iron catalyst alone gave much poorer results or even no reaction, strongly indicating a synergistic effect between two metals. Both aryl iodides and aryl bromides were suitable substrates in this protocol. However, aryl bromides were less favored than aryl iodides, which gave the corresponding aryl acetylenes in good to excellent yields. Additionally, alkyl alkyne could also react with aryl iodides to furnish the desired

unsaturated carboxylic acids and the functionalization of alkynes. Yue He received his B.Sc. in chemistry from China West Normal University in 2014. Then he joined Prof. Jincheng Mao’s group at Soochow University as a master’s candidate. His research focuses on iron- and copper-catalyzed C–N, C–O formation reactions. Guoqi Zhang earned his doctorate in organic chemistry from the Institute of Chemistry of the Chinese Academy of Sciences in 2006. He then moved to the University of Basel in Switzerland and later to the Los Alamos National Laboratory (LANL) in the US to conduct postdoctoral research under the supervision of Professors Wolf-D Woggon, Edwin Constable, and Dr. Susan Hanson. He was a recipient of Novartis Foundation – formerly the Ciba-Geigy Jubilee Foundation – and LANL Director’s Postdoctoral Fellowships. He became an assistant professor at John Jay College of the City University of New York (CUNY) in 2013. He also holds a joint professorship at the CUNY Graduate Center. His research interests include earth-abundant metal catalysis and metallosupramolecular chemistry.

Guangwei Rong obtained his bachelor’s degree in chemistry from Huainan Normal University. After graduation, Guangwei started his Ph.D. study under the supervision of Prof. Jincheng Mao. His research interests include the decarboxylative coupling reactions of a, b-

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Scheme 2. CuI/Fe(acac)3 cocatalyzed decarboxylative Sonogashira reactions with low catalyst loading reported by Mao.[14]

Scheme 1. CuI/Fe(acac)3 cocatalyzed Sonogashira reactions reported by Mao.[6a]

products in moderate yields. Notably, to expand the usability of the Fe(acac)3/CuI cocatalyst, they successfully applied the same catalyst system to the preparation of indoles and diphenyl ethers, as well as sulfides. Interestingly, Vogel’s group reported a similar Fe(acac)3/CuI cocatalyst system for Sonogashira couplings in the same year.[10] They used 10 mol% Fe(acac)3/CuI (1 : 1) in combination with 2 equivalents of Cs2CO3 and NMP as the solvent for the catalytic reactions. Although Vogel’s method also showed good to excellent catalytic activity towards the reactions of both aryl iodides and alkyl alkynes, aryl bromides were not suited in this case. In addition to those mentioned in Mao’s and Vogel’s work, an Fe2O3/Cu(acac)2 system accompanied by a TMEDA ligand,[11] an Fe powder-CuI with a triphenylphosphine ligand,[12] and CuFe2O4 nanoparticles[13] have also emerged as successful cocatalyst combinations for Sonogashira couplings. 2.2. Iron/Copper Cocatalyzed Decarboxylative Sonogashira Couplings With continuing efforts, Mao’s group disclosed a more effective Fe(acac)3/CuI catalyst system to synthesize aryl acetylenes by the decarboxylation of phenylpropiolic acid with aryl halides in 2011 (Scheme 2).[14] Notably, in this protocol, the loading amount of Fe(acac)3 was as low as 1 mol% (0.5 mol% for CuI). Compared with previous work in relevant syntheses, the decarboxylative couplings of phenylpropiolic acid with aryl iodides were even more efficient, giving the desired aryl acetylenes in higher yields. Moreover, the reaction proceeded well

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Scheme 3. Fe(acac)3/Cu(acac)2 cocatalyzed homo- and cross-couplings of terminal alkynes reported by Chen.[15]

while using aryl bromides and alkyl alkynes as substrates, and the corresponding coupling products were generated in moderate to good yields. Notably, an aryl acetylene was afforded in 95% yield in a scaled-up reaction. Thus, not only does this protocol feature low expense and environmental friendliness, but also it is highly practical. 2.3. Iron/Copper Cocatalyzed Coupling of Terminal Alkynes In 2010, Chen and coworkers[15] demonstrated an iron/copper cocatalyzed coupling of terminal alkynes, which is synthetically important for the generation of 1,3-diynes (Scheme 3). Remarkably, in this work, neither copper salt nor iron salt was able to independently catalyze the reaction; it was rather a synergistic cocatalytic effect made by both copper and iron salts that enabled the required substrate activation. As a result, when only 0.1 mol% Cu(acac)2 was employed in combination with 10 mol% Fe(acac)3, the reaction was brought about with good catalytic efficacy. O2 was proven to play a very important role in the whole process. Preliminary mechanistic studies revealed that 1,3-diyne was probably afforded by the reductive elimination of copper(II) acetylide, with the generation of



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Scheme 5. Iron/copper cocatalyzed N-arylation between N-heterocycles and aryl halides reported by Taillefer.[6b]

Scheme 4. FeCl3/CuTC cocatalyzed coupling reactions between Grignard reagents and halides reported by Hamze.[19]

Cu(I). Subsequently, Cu(I) was readily oxidized to Cu(II) by Fe(III) to generate Fe(II) species which could be oxidized to Fe(III) again by the molecular oxygen in the air, finalizing a catalytic cycle. This method was applied to a broad scope of substrates and various substituted aromatic and aliphatic alkynes were found to react smoothly to afford the desired products in reasonable yields.

3. Iron/Copper Cocatalyzed Coupling of Grignard Reagents Since Hayashi and coworkers reported that iron/copper cocatalysts displayed positive effects on the reaction of Grignard reagents,[16–18] iron/copper cocatalysts have been found to be efficient in the cross-couplings of Grignard reagents. In 2012, the Hamze group revealed that the coupling between a-styryl halide and Grignard reagents occurred smoothly in the presence of both FeCl3 and copper(I) thiophene-2-carboxylate (CuTC) (Scheme 4).[19] Compared with the ever reported Fecatalyzed Grignard procedures, this cocatalyst system is clearly more efficient, and even competes with the Pd- and Nicatalyzed procedures. Consequently, this approach was successfully applied to the reactions involving 1-akenyl iodide, bromide, chloride and triflate derivatives, and enol phosphate. In addition, both alkyl and aryl Grignard reagents were suitable precursors, leading to the isolation of the corresponding 1,1diaryl ethylene products in good to excellent yields.

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Scheme 6. Iron/copper cocatalyzed N-arylation between N-heterocycles and trimethoxysilanes reported by Li.[20]

4. Iron/Copper Cocatalyzed C2N CrossCoupling 4.1. Iron/Copper Cocatalyzed N-Arylation Reactions Pioneering work on the aerobic iron/copper cocatalyzed N-arylation reaction was reported by Taillefer and coworkers (Scheme 5).[6b] Notably, the acetylacetonate ligand which forms a complex with iron(III) proved to be crucial for the cocatalyzed N-arylation. Among various combinations of Fe(acac)3 and copper sources, the Fe(acac)3/CuO system accompanied by PhBr or PhI was found to be the most efficient. A range of nitrogen heterocycles, including pyrazole, imidazole, pyrrole, triazole, and indole were employed to accomplish the N-arylation reactions, and the corresponding products were exclusively isolated in excellent yields (81– 93%). Furthermore, pyrrolidin-2-one, representing one of cyclic amide derivatives, also afforded the desired molecule in 80% yield. A few years earlier than that, Li’s group[20] also disclosed a C–N cross-coupling of aryl silane and N-heterocycles, catalyzed by 3 mol% FeCl3 and 3 mol% Cu (Scheme 6). In this work, solvents were not needed, and both imidazoles and triazoles reacted well with aryltrimethoxysilanes or vinyltrimethoxysilane to afford the coupling products in moderate to excellent yields. Notably, thereafter, in their joint work, Buchwald and Bolm systematically investigated the influence of trace amounts of copper impurities contained in FeCl3 in relevant cross-coupling reactions (Scheme 7).[21] They have confirmed



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Scheme 8. Iron/copper cocatalyzed intermolecular diamination of alkynes reported by Liu.[23]

Scheme 7. The influence of iron and copper in catalytic C–N coupling reactions reported by Buchwald and Bolm.[21]

that the catalytic efficiency of the iron catalyst was significantly promoted by a trace amount of added copper(I) oxide. 4.2. Iron/Copper Cocatalyzed Intermolecular Diamination of Alkynes As we know, the imidazo[1,2-a]pyridine skeleton is a privileged structure widely found in many pharmacologically important compounds.[22] Liu presented such an example of the facile synthesis of imidazo[1,2-a]pyridine via copper(II) and iron(III) cocatalyzed double C–N bond formation. It can be seen that this reaction involves an intermolecular oxidative diamination of alkynes with high chemoselectivity and regioselectivity (Scheme 8).[23]

5. Iron/Copper Cocatalyzed C2O Cross-Coupling Following the pioneering work by Taillefer and coworkers,[6c] Buchwald and Bolm also revealed, in the previously mentioned report,[21] that iron and copper species work cooperatively for the C–O bond- and C–S bond-forming processes. Inspired by these findings, Mao and coworkers then developed a very effective iron/copper cocatalyzed methodology for the formation of dimeric aryl ethers or sulfides without the assistance of ligands (Scheme 9).[24] In this catalyst system, Fe(acac)3 was combined

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Scheme 9. Fe(acac)3/CuI cocatalyzed Ullmann-type C–O couplings reported by Mao.[22]

with one equivalent of CuI to achieve high efficacy for the coupling of a variety of dihalide arenes with phenols or phenthiols, and desired dimeric aryl ethers or sulfides were formed and subsequently isolated in excellent yields. Except for 1,4-diphenol, other diphenols such as 2,20 -biphenol and BINOL were also transformed smoothly to the corresponding diaryl ethers in excellent yields, with preservation of ee values. In 2013, Xu and coworkers improved the Ullmann-type C–O coupling reaction by using heterobimetallic CuFe2O4 nanoparticles as a cocatalyst (Scheme 10).[25] Due to its heterogeneity, magnetism and high stability, the CuFe2O4 nanoparticle catalyst can be easily recovered and recycled by utilizing an external magnet, which is thus far one of the most ideal catalyst precursors for this type of reaction. It was observed in this work that 2,2,6,6-tetramethylheptane-3,5-dione was a key for improved reactivity and almost quantitative diphenyl ether product was obtained under optimized conditions. It was also noticed that sensitive substituents, such as primary amines incorporated in the substrates, were well tolerated without additional pre-protection. On investigating the reusability of the catalyst, it was found that the recycled catalyst from the reaction, beginning with 10 mol% cocatalyst, still afforded the product in 92% yield in the 6th run, and the catalysts themselves remained intact, indicating that the bimetallic cocatalyst was not only highly active, but also very stable and reusable.



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Scheme 10. CuFe2O4 cocatalyzed Ullmann-type C–O couplings reported by Xu.[24]

Scheme 11. CuFe2O4 cocatalyzed S-arylations between halides and thiols and diphenyl disulfide reported by Nageswar.[26]

6. Iron/Copper Cocatalyzed C–S Coupling Reactions 6.1. Iron/Copper Cocatalyzed S-Arylation Reactions With the interest of utilization of magnetically separable and recyclable CuFe2O4 nanoparticles in cross-coupling reactions, Nageswar and coworkers have successfully applied the CuFe2O4 cocatalyst in catalytic C–S coupling reactions in the absence of any ligands (Scheme 11).[26] Both aryl/aliphatic iodides and aryl/aliphatic thiols exhibited good reactivity in these reactions and the resulting diaryl or aryl alkyl sulfides were obtained in yields ranging from 70% to 98%. In addition, as an alternative sulfide source, diphenyl disulfide was also used to gain similar results to those by phenyl thiols. The elemental analysis of the CuFe2O4 nanoparticles during the recycling experiments by AAS (atomic absorption spectroscopy) technique indicated that only 0.001% of the metal was lost in the filtrate after the second cycle. Thus, no severe decrease of the catalyst activity was observed after four cycles. Thereafter, the work by the same group was focused on the C–Se coupling reactions by the same catalyst system (Scheme 12).[27] In addition, Tber and Berteina-Raboin have disclosed that such inexpensive, nontoxic, air stable, and commercially available CuO and FeCl24H2O enabled a regioselective S-arylation between 6-iodoimidazo[1,2-a]pyridine and mercaptans, including aliphatic, aromatic, and heterocyclic thiols in the absence of ligands (Scheme 13). The desired products could be obtained in moderate to good yields. The same methodology could also be applied for N-arylation and the corresponding products could be acquired with 41–70% yields.[28] 6.2. Iron/Copper Cocatalyzed S-Vinylation Reactions Recently, S-vinylation reactions have received considerable attention in organic synthesis, owing to their important role in biologically and pharmaceutically active compounds,[29] as well as being useful building blocks for organic transformations.[30] In 2013,

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Scheme 12. CuFe2O4 cocatalyzed Se-arylations between halides and diphenyl diselenide reported by Nageswar.[27]

Scheme 13. Fe/Cu cocatalyzed S-arylations between 6-iodoimidazo[1,2a]pyridine and mercaptans reported by Tber and Berteina-Raboin.[28]

Ranu and coworkers developed a CuFe2O4-catalyzed approach to preparing styrenyl sulfides from vinyl halides and thiols (Scheme 14).[31] Interestingly, the reactions were carried out in water in the presence of a phase-transfer catalyst, TBAB. Notably, the configurations of the vinyl halides were well preserved. (E)Styrenyl halides and (Z)-halides afforded the corresponding (E)styrenyl sulfides and (Z)-sulfides stereoselectively. In addition, this catalyst system also favored S-arylation reactions, and even sterically hindered aryl sulfides were generated in good to excellent yields. When the reusability of CuFe2O4 nanoparticles were tested, there was only a trace loss of activity after ten recycling experiments, which gave the chemists great delight. Most recently, an efficient iron/copper cocatalyzed oxidative coupling of sulfonyl hydrazides with alkyne derivatives for the preparation of vinyl sulfones was reported by Mao and coworkers (Scheme 15).[32] After a thorough exploration of the reaction conditions, the combination of copper acrylate and FeCl24H2O in the presence of tert-butyl peroxide was found to be the most effective catalyst. Sulfonyl hydrazides and alkyne derivatives bearing a series of substituents on the phenyl ring were suitable substrates, providing the expected vinyl



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Scheme 16. Iron/copper cocatalytic photochemical C–H borylation reported by Mazzacano and Mankad.[33]

Scheme 14. CuFe2O4 cocatalyzed S-vinylation of (E)-styrenyl halides and (Z)-halides reported by Ranu.[30]

Scheme 17. Proposed mechanism for catalytic C–H borylation consisting of: i) bimetallic oxidative addition; ii) C2H functionalization; and iii) bimetallic reductive elimination.

Scheme 15. Iron/copper cocatalytic synthesis of vinyl sulfones reported by Mao.[32]

sulfones in moderate to good yields. The mechanistic investigation suggested that a radical mechanism was probably involved. The generation of a sulfonyl radical, resulting from the activation of substrates by the bimetallic FeII/CuII catalyst system, was the key step in the whole reaction cycle. The prompt addition of alkyne derivatives to the sulfonyl radical then followed to furnish the reaction with the desired product.

7. Iron/Copper Cocatalyzed Borylation 7.1. Iron/Copper Cocatalyzed Photochemical C–H Borylation Considering that cyclic arylboronic esters are highly useful reagents in the organic chemist’s toolbox, Mazzacano and Mankad reported that a heterobimetallic Cu-Fe complex could catalyze C–H borylation, a transformation that previously

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required noble-metal catalysts. The optimal catalyst, (IPr)CuFeCp(CO)2, exhibited efficient activity at 5 mol% loading under photochemical conditions, showed only minimal decrease in activity upon reuse, and was able to catalyze borylation of a variety of arene substrates (Scheme 16).[33] In this paper, an effective strategy was developed on metal-metal cooperativity between two different base metals. Thus, this heterobimetallic Cu-Fe complex could be used as an efficient, robust, and versatile catalyst for C–H borylation, a sophisticated catalytic transformation of established importance to organic synthesis that previously required the use of noble metals, such as Rh or Ir. A plausible mechanism for heterobimetallic C2H borylation, shown in Scheme 17, consisted of three main stages: i) B2H bimetallic oxidative addition with metalmetal bond cleavage; ii) photochemical C2H borylation by the resulting boryliron intermediate; and iii) H2H bimetallic reductive elimination with metal-metal bond formation. 7.2. Iron/Copper Cocatalyzed Borylation of Aryl Bromides with Pinacolborane Recently, Labre and Chavant found a more direct way, using a cooperative, readily available Fe(acac)3/CuI catalyst for the



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cocatalysts. Furthermore, transforming the existing iron/copper cocatalyst systems to large-scale applications of industrial manufacturing would be an ultimate goal in the future.

Scheme 18. Iron/copper cocatalytic borylation of aryl bromides with pinacolborane reported by Labre and Chavant.[34]

Acknowledgements The research is supported by the Open Fund (PLN1409) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the State Education Ministry, and the Key Laboratory of Organic Synthesis of Jiangsu Province. G.Z. acknowledges the American Chemical Society Petroleum Research Fund for partial support.

REFERENCES [34]

Scheme 19. Hypothetical mechanism proposed by Labre and Chavant.

borylation of aryl bromides with pinacolborane (Scheme 18).[34] Differently, the toxic, precious metal Pd was avoided. Furthermore, they proposed the possible mechanism, which is closely analogous to the copper-assisted Sonogashira coupling: two catalytic cycles cooperate via transmetalation, as shown in Scheme 19.

8. Outlook With the strong appeal of environmentally friendly and atomeconomical catalysis in the 21st century, enormous efforts made by organic chemists are oriented towards the exploration of simple and inexpensive catalyst systems for the construction of carbon-carbon or carbon-heteroatom bonds. An alternative approach is reviewed in this account. The low-cost iron/copper cocatalysts have found more and more uses in various coupling reactions and have displayed even higher catalytic efficacy than those ever reported for noble-metal catalysts. In addition, the magnetic bimetallic CuFe2O4 nanoparticles also show great promise of practical application in industry, owing to their ready preparation and high stability, as well as good recyclability. Built on the current progress in iron/copper cocatalyzed reactions, future development would mainly concern the following three aspects: 1) observation of new coupling reactions based on those effective iron/copper systems previously discovered; 2) design of new, better-performing iron/copper catalysts for known reactions through various combinations of diverse iron and copper salts, the introduction of suitable organic ligands, or the fabrication of different types of bimetallic nanocatalysts; and 3) mechanistic investigation of typical coupling reactions for uncovering the synergistic effect of iron/copper

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Received: October 22, 2015 Published online: March 29, 2016



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The Application of Copper/Iron Cocatalysis in CrossâCoupling Reactions

The Application of Copper/Iron Cocatalysis in CrossâCoupling Reactions

The Application of Copper/Iron Cocatalysis in CrossâCoupling Reactions

The Application of Copper/Iron Cocatalysis in CrossâCoupling Reactions