R2–H Cross-Coupling with Hydrogen Evolution

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Oxidative R1−H/R2−H Cross-Coupling with Hydrogen Evolution Shan Tang,† Li Zeng,† and Aiwen Lei*,†,‡ †

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College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, People’s Republic of China ‡ National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330027, People’s Republic of China Scheme 1. Bond Formation Strategies

ABSTRACT: Oxidative R1−H/R2−H cross-coupling with hydrogen evolution serves as one of the most atom-economical methods for constructing new chemical bonds. This reaction strategy avoids substrate prefunctionalization steps in traditional cross-coupling reactions. Besides, hydrogen gas, which is recognized as a source of green energy, is the only byproduct during the reaction process. The major challenge in this reaction strategy is to achieve selective bond formation and hydrogen evolution at the same time. Over the past few years, novel synthetic techniques especially photochemistry and electrochemistry have provided possibilities for oxidative cross-coupling with H2 liberation. Both C−C and C−X bonds can be constructed without the use of any sacrificial reagents. In this perspective, we will discuss the concept of this reaction strategy and give an overview of its recent development.

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energy source by being stored in suitable hydrogen storage materials.7−11 The major challenge in an oxidative R1−H/R2−H crosscoupling with hydrogen evolution is to achieve C−H/X−H bond activation and hydrogen gas evolution at the same time. Thus, the development of novel catalytic systems is the key to realize this dream reaction pathway. In oxidative R1−H/R2−H cross-coupling reactions, C−H/X−H bonds can be activated through transition metal catalysis12,13 or oxidation-induced bond activation.14,15 The major difference from traditional oxidative cross-coupling is the H2 elimination process. During the past few years, impressive achievements have been made by applying synthetic techniques including thermal transition metal catalysis,16 photochemistry,17 and electrochemistry.18 Thermal H2 liberation of metal hydride, photoredox proton reduction, and cathodic proton reduction have all been used for hydrogen evolution in oxidative R1−H/R2−H crosscoupling reactions. Considering the fast development, it is necessary and significant to provide a conceptual understanding of this emerging area. With continuous interest in oxidative cross-coupling,3,4 we herein provide an overview of the recent developments of oxidative R1−H/R2−H crosscoupling with hydrogen evolution. This perspective focuses on the intermolecular oxidative R1−H/R2−H cross-coupling with hydrogen evolution. Instead of giving a comprehensive review, only representative examples in oxidative C−H/C−H or C− H/X−H cross-coupling will be discussed. The advantages and

INTRODUCTION Transition-metal-catalyzed cross-coupling has been recognized as a powerful tool for the construction of C−C and C−X bonds.1,2 Many name reactions including Kumada reaction, Stille reaction, Negishi reaction, Suzuki reaction, Heck reaction, and Buchwald−Hartwig reaction have been developed through transition metal catalysis.1,2 These reactions successfully deal with the cross-couplings of organic halides with certain nucleophiles. Though these cross-couplings are reliable and efficient, the use of organic halides and organometallic reagents inevitably produces undesirable chemical wastes (Scheme 1a). Moreover, organic halides and organometallic reagents are not readily available substrates and require being prepared through multiple prefunctionalization steps. Over the past decade, developments in C−H activation allow the construction of C−C and C−X bonds from readily available substrates. Oxidative R1−H/R2−H cross-coupling has been considered an ideal way for constructing new chemical bonds.3−6 However, sacrificial chemical oxidants are generally required to remove the hydrogen atoms in the bond formation processes. Stoichiometric amounts of hypervalent metal salts, organic halides, and peroxides are used in most cases (Scheme 1b). It diminishes the atom economy of the overall transformation. A dream reaction pathway is to achieve oxidative R1−H/R2−H cross-coupling with hydrogen evolution (Scheme 1c). This bond formation strategy not only avoids the use of a sacrificial reagent but also releases hydrogen gas as the only byproduct. Importantly, the generated hydrogen gas can be further used as an efficient and green © 2018 American Chemical Society

Received: July 11, 2018 Published: September 27, 2018 13128

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In 2017, Lei and co-workers reported an oxidative C−H/S−H cross-coupling between electron-rich arenes and aromatic thiols under undivided electrolytic conditions (Scheme 3a).29

limitations by applying different synthetic techniques will also be discussed in this paper.

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OXIDATION-INDUCED OXIDATIVE C−H FUNCTIONALIZATION WITH HYDROGEN EVOLUTION Among the developed external chemical oxidant-free R1−H/ R2−H cross-coupling reactions, transition metal catalysts are not required in most cases. Instead of metal-catalyzed C−H activation to afford C−M bonds, C−H bonds are usually activated by photochemical oxidation or electrochemical oxidation.14 Radicals, radical cations, and cations can be generated through oxidation-induced C−H/X−H bond activation (eq 1). This activation strategy enables the selective oxidative C−H functionalization of redox active substrates.

Scheme 3. Electrochemical Oxidative C−H Thiolation and Amination

Both of the substrates were found to be redox active under the standard conditions. The C−S bond was proposed to be formed through the radical/radical cross-coupling between an arene radical cation and a sulfur radical. More recently, Lei and co-workers reported an electrochemical oxidative C−H amination of unprotected phenols with phenothiazine derivatives (Scheme 3b).30 Since phenothiazine demonstrated lower oxidation potential than phenols, the amine substrates were likely to be first oxidized to generate radical cations during electrolysis. This transformation was established before by using chemical oxidants while either a high reaction temperature or strong chemical oxidants were used.31,32 In comparison, the electrochemical method could be conducted at room temperature without the use of any sacrificial reagents. Moreover, the reaction was scalable at ambient conditions. Arene radical cations could also be generated under photochemical conditions. In 2015, Nicewicz and co-workers reported the generation of an arene radical cation in a visiblelight-mediated oxidative C(sp2)−H amination of arenes with azoles.33 A dual catalytic system of an acridinium photoredox catalyst and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was used to facilitate the C−N bond formation. Oxygen was used as the terminal oxidant. To further avoid the issue of possible overoxidation by oxygen, Lei and co-workers described a similar oxidative C(sp2)−H amination of arenes with azoles without the use of terminal oxidants (Scheme 4a).34 They utilized a synergistic photoredox and protonreduction catalytic system. 9-Mesityl-10-methylacridinium

Oxidative C−H Functionalization of Arenes. Oxidative arene−arene cross-coupling provides a straightforward approach for the construction of nonsymmetrical biaryls.19,20 In 2008, Kita and co-workers reported a single-electron-transfer oxidation-induced oxidative cross-coupling between naphthalenes and mesitylenes by using a hypervalent iodine(III) reagent as the oxidant.21 Anodic single-electron-transfer oxidation of arenes provides an environmentally friendly method for activating aromatic compounds. Since 2010, Waldvogel and co-workers have investigated the electrochemical cross-coupling between phenols and electron-rich arenes (Scheme 2a).22 Different electron-rich arenes including Scheme 2. Electrochemical Oxidative Arene−Arene CrossCoupling

phenyl ethers,23 phenols,24,25 and thiophenes26 were subsequently applied to couple with phenols under undivided electrolytic conditions. Boron-doped diamond (BDD) anodes and hexafluoroisopropanol (HFIP) solvent were both important for achieving good reaction efficiency with phenols. The key step in these transformations was the anodic oxidation of phenols to generate oxyl radicals. Moreover, the same group also reported the electrochemical oxidative cross-coupling among different aniline derivatives.27 By using a radical cation pool strategy, the Yoshida group was able to achieve the oxidative cross-coupling of two unactivated aromatic compounds under divided electrolytic conditions (Scheme 2b).28 One aromatic compound was oxidized by the anode to generate a radical cation while another aromatic compound was added under nonoxidative conditions. Despite C−C bond formation, electrochemical anodic oxidation was also applied to C−X bond formation reactions.

Scheme 4. Photochemical Oxidative C−H Functionalization of Arenes

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electron-rich arenes with styrenes (Scheme 5c).40 Electron-rich arenes were proposed to act as nucleophiles in this transformation. Instead of oxidative activation of alkenes, the other coupling partner could also be activated in the oxidative C−H functionalization of alkenes. In 2016, Lei and co-workers also reported a sulfonylation of α-methyl-styrene derivatives with sulfinic acids (Scheme 5d).41 Eosin Y was used as the photocatalyst instead of acridines. As a consequence, sulfinic acids were found to be oxidized instead of alkenes. A series of allylic sulfones could be synthesized in good to high yields. Oxidative C−H Functionalization of Aldehydes. Scheidt42 and Studer43 studied the N-heterocyclic carbenecatalyzed oxidative esterification between aldehydes and alcohols in the presence of chemical oxidants including MnO2 and quinone. In 2012, Boydston and co-workers reported a N-heterocyclic carbene-catalyzed oxidative esterification between aldehydes and alcohols under undivided electrolytic conditions (Scheme 6a).44 Instead of oxidation by

perchlorate was used as the photoredox catalyst for the oxidative activation of arene while a cobaloxime complex was used as the proton-reduction catalyst for hydrogen evolution. Similar with the work of Nicewicz, an arene radical cation was proposed to be the key intermediate for C−N bond formation. Unfortunately, only azoles could be utilized as the amination source in C(sp2)−N bond formation. At the same time, Wu and co-workers also reported a photochemical oxidative C(sp2)−H amination and hydroxylation of benzene with water and ammonia (Scheme 4b).35 Compared with the work of Lei, stronger photocatalysts (quinolinum ions) were used, which allowed the direct oxidative functionalization of benzene and electron-deficient arenes. Using a similar catalytic system, they were also able to achieve a photochemical oxidative etherification of benzene with alcohols.36 Oxidative C−H Functionalization of Alkenes. Nicewicz and co-workers have shown that alkene radical cations can also be generated by using acridinium photocatalysts.37 Since there was a lack of oxidation protocols, they were only able to achieve the hydrofunctionalization and difunctionalization of alkenes. In 2016, Lei and co-workers reported an antiMarkovnikov oxidation of alkenes with H2O as the oxygenation source under external chemical oxidant-free conditions (Scheme 5a).38 Similar with their work on photochemical

Scheme 6. Electrochemical N-Heterocyclic CarbenePromoted Oxidative Esterification and Amidation with Aldehydes

Scheme 5. Photochemical Oxidative C−H Functionalization of Alkenes

chemical oxidants, anodic oxidation of the in situ generated Breslow intermediate was the key for this electrochemical transformation. Electrophilic 2-acylazolium species were proposed to be formed. By using thiols instead of alcohols, they were also able to synthesize a series of thioesters.45 As for the synthesis of amides, they needed to prepare the Breslow intermediate at first since amines could directly react with aldehydes in the presence of N-heterocyclic carbenes. In a flow reaction system, they were able to prepare amides in high reaction yields (Scheme 6b).46 Obviously, a stoichiometric amount of N-heterocyclic carbene was required in this transformation. Oxidative C(sp 3 )−H Functionalization. In 2007, Jørgensen and co-workers reported an organocatalytic enantioselective α-arylation of aldehydes with quinones.47 Since quinones could be synthesized from corresponding phenols by electrochemical oxidation, the same group reported the combination of electrochemical oxidation and chiral amine catalysis in the asymmetric oxidative cross-coupling between aliphatic aldehydes and protected 4-aminophenols (Scheme 7a).48 Anodic oxidation of 4-aminophenols and enamine activation of aliphatic aldehydes by chiral amine catalyst worked synergistically for the asymmetric C(sp3)−C(sp2) bond formation. It has been reported that tertiary amines can be oxidized by chemical oxidants to generate iminium cation intermediates.49 By using a similar reaction strategy with

oxidative C−H functionalization of arenes, a synergistic photoredox and proton-reduction catalytic system was applied. C−O bonds were formed through the nucleophilic addition of water to the in situ generated alkene radical cation. Using alcohols or azoles instead of water, they were also able to synthesize multisubstituted enol ether derivatives and Nvinylazoles from simple alkenes under similar reaction conditions (Scheme 5b).39 Importantly, this reaction strategy was also applicable to the oxidative Heck-type reaction of 13130

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H/C(sp3)−H cross-coupling of tertiary amines with ketones was achieved by using a synergistic multiple catalytic system (Scheme 8b).57 Despite the photoredox catalyst and protonreduction catalyst, a primary chiral amine catalyst was used for the enamine activation of ketones. The mild reaction conditions enabled high chemo- and regioselectivity in the C(sp3)−C(sp3) bond formation. Besides C−C bond formation, considerable efforts were paid to C−N bond formation reactions. In 2016, Zeng, Sun and coworkers reported an electrochemical oxidative cross-coupling of aliphatic ketones with amines (Scheme 9a).58 Actually, the

Scheme 7. Electrochemical Asymmetric Oxidative C−H/C− H Cross-Coupling

Scheme 9. Electrochemical Oxidative C(sp3)−H Amination

Jørgensen, Luo and co-workers achieved the asymmetric oxidative C(sp3)−H/C(sp3)−H cross-coupling between aliphatic ketones and tertiary amines (Scheme 7b).50 A tentative mechanism involved anodic oxidation of tertiary amines and enamine activation by a primary chiral amine catalyst. Great achievements have been made on the photoinduced oxidative C(sp3)−H of tertiary amines in the presence of chemical oxidants.51−55 By using a dual catalytic system, Wu and co-workers reported the first photoinduced oxidative C(sp3)−H/C(sp2)−H cross-coupling of tertiary amines with indoles under external chemical oxidant-free conditions (Scheme 8a).17 Eosin Y was used as a photoredox catalyst, same reaction was achieved by using NH4I as the catalyst and sodium percarbonate as the oxidant.59 In the electrochemical transformation, NH4I was used as a redox mediator. A series of α-amino ketones could be synthesized in moderate to good yields. Mechanistic studies revealed that α-iodination of ketone was the key step for the C−N bond formation. In the same year, Huang and Gong developed an electrochemical oxidative C−H/N−H cross-coupling between γ-lactams and anilines.60 No redox mediator was used in this transformation, and an iminum cation was found to be the key intermediate for C−N bond formation. The following year, Lei and co-workers reported an electrochemical oxidative C(sp3)−H amination with azoles under undivided electrolytic conditions (Scheme 9c).61 C(sp3)−H bonds adjacent to oxygen, nitrogen, sulfur, benzyl, and allyl groups were all suitable in this transformation. Different from the work of Huang, nitrogen radicals were proposed to be generated through direct anodic oxidation.

Scheme 8. Photochemical Oxidative C−H/C−H CrossCoupling with Tertiary Amines

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TRANSITION-METAL-DIRECTED R−H TO R−M AND OXIDATIVE FUNCTIONALIZATION WITH HYDROGEN EVOLUTION Oxidation-induced C−H/X−H bond activation is only applicable to the oxidative cross-coupling with redox active substrates. In order to deal with redox inactive substrates, it is necessary to develop transition-metal-directed R−H to R−M and oxidative functionalization with hydrogen evolution. A big challenge in this transformation is to recycle the transition metal catalysts without the use of external chemical oxidants. To date, thermal H2 liberation of metal hydride (Scheme 10a) and electrochemical recycling of a metal catalyst (Scheme 10b) have been used for achieving this goal. We will discuss the representative reports of these two reaction strategies in this section.

and graphene-supported RuO2 was used as a proton-reduction catalyst. Hydrogen gas was produced as the only byproduct in quantitative yields. Later, they found that a homogeneous proton-reduction catalyst, Co(dmgH)2Cl2, was able to replace graphene-supported RuO2 for the same transformation (Scheme 8a).56 In both cases, a photocatalyst would deliver electrons to a proton-reduction catalyst after reductive quenching. Synergistic cooperation of substrate oxidation and proton reduction was the key for the success of this reaction. An iminium cation intermediate was proposed to be generated during the reaction. Besides tertiary amines, the same group expanded the method to oxidative C(sp3)−H/C(sp2)−H cross-coupling with benzylic ethers.36 In a collaborative study by Wu and Luo, a visible-light-promoted asymmetric C(sp3)− 13131

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Journal of the American Chemical Society Scheme 10. Transition-Metal-Directed Oxidative R1−H/ R2−H Cross-Coupling with Hydrogen Evolution

Scheme 11. Ruthenium-Catalyzed Oxidative Esterification and Amidation

Oxidative C(sp3)−H Functionalization. Esterification and amidation are among the most fundamental transformations in chemistry.62 Traditional methods for synthesizing esters and amides largely depend on nucleophilic substitution to obtain carboxylic acid derivatives. In most cases, the carboxylic acid derivatives used in esterification and amidation are acyl halides and anhydrides which are synthesized by multiple steps. In comparison, alcohols and aldehydes are readily available substrates. In 2005, Milstein and co-workers developed a ruthenium-catalyzed reaction system for the self-esterification of primary alcohols with hydrogen evolution.63 A dearomatized PNP-Ru complex was used as the catalyst. The alcohol was activated by the dearomatized PNPRu complex, and an aromatized complex was formed. A ruthenium hydride complex could be generated through β-H elimination. Hydrogen gas could be released from the generated ruthenium hydride complex upon heating. By using the aromatization−dearomatization strategy, the same group reported a ruthenium-catalyzed oxidative cross-esterification between primary alcohols and secondary alcohols in 2012 (Scheme 11a).64 In order to achieve good reaction selectivity, 2.5 equiv of secondary alcohol were required. Besides oxidative esterification, they also applied this reaction strategy to achieve the oxidative amidation between primary alcohols and primary amines (Scheme 11b).65 Notably, excellent reaction selectivity and efficiency could be achieved with a 1:1 ratio of alcohols and amines. After this pioneering work with ruthenium catalysis, Milstein and co-workers also utilized manganese as the metal center for the oxidative amidation between alcohols and amides.66 Another way for directly converting amines to amides is the oxygenation of aliphatic amines. In 2014, Milstein and co-workers reported the synthesis of lactams from cyclic amines with water as the oxygenation source (Scheme 11c).67 An acridine-based PNPRu complex was used as the catalyst. Mechanistic studies indicated that the actual catalyst was the dearomatized form of the precatalyst.68 A coordinated imine was proposed to be generated through β-H elimination. Water not only acted as the oxygenation source but also played a role in catalyzing the H2 evolution. Oxidative C(sp2)−H Functionalization. Oxidative Heck reaction has been well established by using the combination of transition metal catalysts and oxidants.69 In 2007, Jutand and co-workers reported an electrochemical palladium-catalyzed

oxidative Heck reaction of N-acetylanilines (Scheme 12a).70 Instead of using a stoichiometric amount of benzoquinone to Scheme 12. Electrochemical Oxidative Heck Reaction

recycle the palladium catalyst, they used a catalytic amount of benzoquinone in a divided cell. The hydroquinone generated after each catalytic cycle was oxidized by the anode to regenerate benzoquinone. At the same time, protons were reduced at the nickel foam cathode to release hydrogen gas. More recently, Ackermann and co-workers reported an electrochemical rhodium-catalyzed oxidative Heck reaction of benzoic acids (Scheme 12b).71 Instead of using redox mediators, the rhodium catalyst was directly recycled by anodic oxidation in a simple undivided cell. In these two cases, anodic oxidation played a key role in the recycling of metal catalysts. Besides electrochemical recycling of metal catalysts, acid could also promote the recycling of metal catalysts in oxidative Heck reactions. In 2016, Jeganmohan reported a rutheniumcatalyzed oxidative Heck reaction of aromatic amides, 13132

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Journal of the American Chemical Society ketoximes, and anilides (Scheme 13a).72 Interestingly, hydrogen gas could be detected by GC at room temperature.

at the same time, Lei and co-workers reported an electrochemical cobalt-catalyzed oxidative C−H amination of aromatic amides with secondary amines (Scheme 14c).82 The major difference from the work of the Ackermann group was that an inexpensive carbon cloth anode and nickel plate cathode were used instead of a reticulated vitreous carbon (RVC) anode and platinum plate cathode.

Scheme 13. Acid-Promoted Oxidative Heck Reaction

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CONCLUSION Oxidative R1−H/R2−H cross-coupling with hydrogen evolution provides an environmentally friendly way for the construction of new chemical bonds. Due to the fast development of synthetic techniques especially photochemistry and electrochemistry, this dream reaction pathway has been proven to be feasible for selective bond formation. No sacrificial reagent is required, and hydrogen gas is produced as the only byproduct. Moreover, good functional group tolerance can be achieved in the cross-coupling reaction since no external chemical oxidant is used. This environmentally friendly synthetic strategy is one of the future directions in organic synthesis. Increasing attention has been focused toward this emerging research area. The key in this synthetic strategy is to achieve C−H/X−H bond activation and hydrogen gas evolution in a single reaction system. In the developed methods, C−H/X−H bonds are activated either by oxidation-induced bond activation or by transition metal catalysis. At the same time, hydrogen gas can be liberated by thermal H2 elimination of metal hydride, photoredox proton reduction, or cathodic proton reduction. Based on the types of energy sources applied, three reaction protocols have been utilized in oxidative R1−H/R2−H crosscoupling with hydrogen evolution: (1) Photochemistry (a photoredox catalyst is used for C−H/X−H bond activation while a proton-reduction catalyst is used for hydrogen evolution); (2) Electrochemistry (direct anodic oxidation or anodic oxidation promoted transition metal catalysis is used for C−H/X−H bond activation while cathodic reduction of protons can release hydrogen gas); (3) Thermal transition metal catalysis (the same transition metal is used for both C− H/X−H bond activation and hydrogen evolution). Due to the diversity of catalytic systems, photochemistry and electrochemistry are applicable to many oxidative R1−H/R2−H crosscoupling reactions. Hopefully, further developments in new catalytic systems will allow oxidative R1−H/R2−H crosscoupling with hydrogen evolution to take the place of oxidative R1−H/R2−H cross-coupling reactions using oxidants.

Mechanistic study indicated that acetic acid could react with ruthenium hydride generated after β-hydride elimination to form H2. Later, Dong and co-workers reported a similar strategy for rhodium-catalyzed oxidative Heck reaction of 7azaindoles (Scheme 13b).73 In both cases, a stoichiometric amount of acetic acid was used to recycle the metal catalysts. Electrochemical recycling of cobalt catalysts in oxidative C(sp2)−H functionalization of arenes has also been explored. Actually, a big problem for cobalt-catalyzed oxidative C−H functionalization is that stoichiometric amounts of high-valent metal salts especially silver salts and manganese salts are generally required to recycle the cobalt catalysts.74−79 For example, Song and co-workers have achieved the cobaltcatalyzed oxidative C(sp2)−H etherification and amination by using silver salts as oxidants.78,79 In 2017, Ackermann and coworkers reported an electrochemical cobalt-catalyzed C−H etherification of phenyl amides with aliphatic alcohols (Scheme 14a).80 The reactions could be conducted in both divided and undivided cells. The cobalt catalyst was directly recycled by electrochemical anodic oxidation during electrolysis. Later, they also achieved the electrochemical cobalt-catalyzed C−H amination of aromatic amides with cyclic secondary amines (Scheme 14b).81 Renewable g-valerolactone (GLV) was found to be the optimized solvent for the amination reaction. Almost Scheme 14. Electrochemical Cobalt-Catalyzed Oxidative C− H Etherification and Amination

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Aiwen Lei: 0000-0001-8417-3061 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21390402 and 21520102003) and the Hubei Province Natural Science Foundation of China (Grant 2017CFA010). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also 13133

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appreciated. This paper is dedicated to Professor Xiyan Lu on the occasion of his 90th birthday.

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