Chinese Chemical Letters  2018, Vol. 29 Issue (8): 1193-1200   PDF    
Bio-inspired quinone catalysis
Ruipu Zhanga,b, Sanzhong Luoa,b    
a Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China;
b University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Quinoproteins are an important type of redox enzymes for biological oxidation processes. Inspired by the quinone cofactors, particularly from copper amine oxidases, a number of small molecular quinone catalysts have been developed for C-H functionalizations of amines. Bio-inspired quinone catalysts have significantly expanded the substrate scope to include branched primary amines, secondary amines and tertiary amines, far beyond the scope of quinoproteins. This review summarizes the evolution of quinone catalysts, their mechanism and catalytic applications.
Keywords: ortho-Quinone catalysis     Amine oxidation     Organocatalysis     Bio-inspired     C-H functionalization    
1. Introduction

Quinoproteins are recognized as the third redox enzyme besides pyridine nucleotide- and flavin-dependent proteins [1]. The first identified quinone cofactors is pyrroloquinoline quinone (PQQ, Fig. 1) in methanol/glucose dehydrogenases, which is the only quinonecofactor bound to the protein via ionic interaction through its carboxylate groups [2]. The other four quinone cofactors (Fig. 1), such as 2, 4, 5-trihydroxyphenylalanine quinone (TPQ) [3], lysine tyrosylquinone (LTQ) [4], tryptophan tryptophylquinone (TTQ) [5] and cysteine tryptophylquinone (CTQ) [6], are non-dissociable and covalently bound for amines oxidation. CuAOs, consisting of a quinone cofactor-TPQ and a copper ion are ubiquitous in aerobic organisms [7]. In bacteria and yeast, CuAOs mainly use primary amine to provide carbon or nitrogen source for its growth [8]. In eukaryotes, CuAOs have more ambiguous roles but are important for cellular activities.

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Fig. 1. Structures of quinone cofactors.

The aerobic oxidation catalyzed by CuAOs consists of two distinct half-reactions. In the reductive half reaction, TPQ loses two electrons and oxidizes amines to aldehydes; in the oxidative half reaction, reductive TPQ is oxidized by oxygen with the assistant of copper ion and releases NH4+ and H2O2 [9]. TPQ is the only active species promoting amines oxidation, while copper ion just assists oxygen to recycle the TPQ. The chemical mechanism of TPQ in the reductive half reaction has always been an attractive and important area. Two possible pathways have been proposed for this process (Scheme 1): (A) Transamination pathway consisting of the formation and oxidation of an iminoquinone intermediate and (B) Addition–elimination pathway involving amine oxidation via a hemiaminal intermediate. Mechanistic works reported by Klinman [3, 9, 10], Sayre [11], and others [12] demonstrate that the oxidative reaction proceeds through the transamination pathway.

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Scheme 1. Mechanism of amine oxidation mediated by copper amine oxidases: (A) Transamination, (B) Addition-elimination.

TPQ works only with primary amines, but not secondary and tertiary amines [13]. Inspired by CuAOs as well as other quinoproteins, a number of bio-inspired quinone catalysts have been developed for aerobically oxidative transformations of amines (Scheme 2) [14]. In this review, we would like to summarize recent advances along this line.

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Scheme 2. Quinone-based catalysts.

2. Catalyst development and mechanism insight

Largeron and co-workers first reported an oxidative deamination catalyzed by Q1, (IMQ) in situ generated from electrochemical oxidation [15]. In 2012, Stahl reported the aerobic oxidation of primary benzylic amines catalyzed by Q3 (TBHBQ) [16], initially developed by Klinman for biomimetic studies [10f, g]. Two years later, Stahl and co-workers applied a cooperative catalytic system consisting of Q5 (phd) and transition metal cocatalysts for secondary amine oxidation [17]. Kobayashi also unexpectedly discovered that catechol derivatives Q4 could oxidize secondary amines with the assistance of Pt-nanocluster [18]. Luo has developed a series of simple ortho-qunione-based catalysts such as Q6 [19]. The ortho-quinone catalyst Q6 was able to efficient oxidize primary amines, cyclic secondary amines and cyclic tertiary amines. This is the only quinone catalyst which could oxidize three classes of amines. More recently, Oh and co-workers [20] designed o-naphthoquinone Q7 for aerobic oxidation of primaryamines and cyclic secondaryamines with the assistance of metal cocatalysts.

Mechanistically, there are four possible pathways for the bioinspired oxidation of amines to imines: transamination (), addition–elimination (), noncovalent direct H-abstraction () and electron transfer () (Fig. 2). Based on model compound studies aswellasUV–visspectra, Largeron [15] identified 4red as an keyintermediate in the electrochemical oxiation of primaryamine, in support of a transamination pathway (Scheme 3). In a bimimetic feature, primary amine such as benzylamine first attacked the imine group, releasing an equimolar amount of ammonia to form a Schiff base, which ungerwent α-proton abstraction, followed by condensation with another benzylamine leading to an imine condensation product. Quinone catalyst would be re-oxidized under electrochemical conditions for another cycle. Other studies also shown that quinone-mediated priamry amines oxidation would proceed via transamination pathway [16, 20].

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Fig. 2. Four possible pathways for primary amine to imine.

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Scheme 3. Transamination mechanism for the aerobic oxidation of primary amines.

Recently, Luo and co-workers have developed a simple ortho-quinone catalyst for the oxidation of α-branched primary amines, for which other quinone catalysts are generally inactive [19]. Mechanism studies were carried out to elucidate catalytic origin of ortho-quinone catalysts. Kinetic isotope effect experiment (KIE= 2.9) indicated that C-H bond cleavage was involved in the rate limiting step, hence SET process with normally fast C-H cleavage was unlikely working and can be ruled out. Both ESI-MS and DFT calculations supported the transamination pathway in this ortho-quinone catalyst system (Scheme 4).

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Scheme 4. Aerobic oxidation of α-branched primary amines via a transamination pathway.

As for secondary amines oxidation, there are only a few enabling quinone catalysts. Stahl and co-workers [17] reported a cooperative catalytic system containing a quinone catalyst and ZnI2. ZnI2 plays two roles: zinc ionwould activates quinone toward amine oxidation and iodide ion would promote aerobic catalytic turnover. Based on stoichiometric reaction of 1, 2, 3, 4-tetrahydroi-soquinoline with Q5 and the characterization of the hemiaminal intermediate Int-1, the authors proposed an addition-elimination pathway instead of the transamination pathway (Scheme 5).

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Scheme 5. Addition-elimination pathway for the oxidation of secondary amines.

ortho-Quinone catalysts developed by Luo and co-workers have also been shown to promote effectively the aerobic oxidation of both secondary amines and tertiary amines (Scheme 6) [19, 21]. In the oxidation of the secondary amines and tertiary amines, the electron transfer was excluded on the basis of the kinetic isotope effects which indicated C-H bond cleavage was involved in the rate determining step. DFT calculation revealed that the reaction barrier for hydride transfer was much lower than additionelimination. For tertiary amines, there are only two possible pathways: electron transfer and hydride transfer. As described above, electron transfer could be ruled out. DFT calculation found that the activation free energyof directhydride transfer for tertiary amine was slightly higher than that of secondary amine, but was still feasible under the reaction conditions.

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Scheme 6. Hydride transfer mechanism for secondary and tertiary amine oxidation.

3. Synthetic application 3.1. Primary amines oxidation

Primary amine is the natural substrate for CuAOs. Hence, many bio-inspired quinone catalysts start with primary amine oxidation. Oxidation of benzylamine could serve as a bench mark for quinone catalysis (Fig. 3). It seems that quinone/metal bicatalytic systems generally show better activity. The use of quinone catalyst alone such as Q3 and Q6 could also promote the reaction effectively.

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Fig. 3. Comparison between different catalytic systems.

The strategy using ortho-quinone type catalyst for oxidative coupling of primary amines was firstly explored by Largeron in 2000 [15]. Later on, the same authors extended their Q1 catalyst to the oxidation of nonactivated primary aliphatic amines under the same conditions [22]. A variety of aliphatic amines were tolerated albeit with moderate yields (Scheme 7). Secondary amines did not react at all. The authors also identified a catalyst deactivation pathway. The electrogenerated Q1 catalyst would react with an enamine intermediate arisen from the tautomerization of alkylimines in an inverse-electron-demand Diels-Alder (IEDDA) reaction. This side pathway was further advanced to a synthetic protocol for 1, 4-benzoxazines derivatives [23].

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Scheme 7. Oxidation of nonactivated primary amines under electrochemical conditions.

In 2008, Largeron and co-workers [24] investigated the structural-activity relationship of their quinone catalysts (Scheme 8). Electron-withdrawing group (8c vs. 8d) at the 1- position favors attack of the amines at the meta position and also contribute to proton abstraction during transamination process. The 2—OH group would form hydrogen bond with the imine moiety of the Schiff base which facilitates attack of another primary amine by preventing other side Michael addition reaction.

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Scheme 8. Substituents effects of Q1 in the oxidation of primary amines.

Inspired by the role of copper ion in CuAOs, Largeron and coworkers used copper ion as metal cocatalyst and ambient air as the terminal oxidant to modify the former biomimetic catalytic systems (Scheme 9) [23]. Methanol was the best solvent, because strong solvation of the Q1 would enhance the electrophilicity of its quinonoid moiety, thereby favoring the nucleophilic attack of the amine. Only 0.2 mol% of copper (Ⅰ) 3-methylsalicylate was needed for full conversion of benzylamine in 10 h. Without any copper salt, the reaction proceeded very slowly. Benzylic amines (9a and 9b) generally exhibited high catalytic activity. Heterocyclic amines (9c) afforded imines in good yields, while aliphatic amines (9d) did not react well in this catalytic system. Secondary amines (9f) were not reactive at all and a-branched amines (9e) were found to be inferior substrates. Besides, they also challenged the oxidative cross-coupling of different primary amines. Branched alkylamines (9g and 9h) worked well in the reactions.

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Scheme 9. Cooperative catalytic system reported by Largeron.

In 2012, Stahl and coworkers [16] reported quinone catalyzed amine oxidation without any metal cocatalyst. A TPQ analog, Q3 (TBHBQ) was introduced to the aerobic oxidation of benzylic amines (Scheme 10). Electron-rich amines (10b) showed better activities than electron-deficient amines (10c). Aliphatic primary amines, secondary amines and tertiary amines (10e, f) were not oxidized. Under much more forcing conditions using 10 mol% Q3 and stoichiometric Brønsted base, α-branched benzylamine (10d) could be oxidized to imine with moderate yield after 48 h (Scheme 11). As for the cross-coupling reaction of primary amines, Q3 also showed good catalytic activities albeit with 5 mol% catalyst loading. The sterically hindered tritylamine (10i) and a-branched benzylamine (10g) could serve as effective coupling partners.

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Scheme 10. Organocatalytic aerobic oxidation of primary amines.

In previous reports [16, 25], α-branched primary amines was inferior substrates due to its steric effect. Luo and co-workers [19] developed a simple ortho-quinone catalyst Q6 to achieve the oxidation of α-branched primary amines (Scheme 11). 1-Phenylethanamines substituted with electron-donating and electronwithdrawing groups (11b, c) were well-tolerated to produce corresponding imines with high to excellent yields and good E/Z ratio. However, (1-naphthyl)ethylamine (11e) and 1-phenylpropan-1-amine (11f) underwent deamination instead of oxidative homo-coupling. Investigations about the origins of different performance of Q3 and Q6 have been conducted (Scheme 12). Stoichiometric experiments revealed that 1-phenylethanamine was consumed quickly in reacting with Q3 and the corresponding imine (12a) was formed. DFT calculation indicated a strong intramolecular O-H···N hydrogen bond in imine (12a), which sets the α-C-H in an unfavored geometry for H-transfers. On the other hand, iminoquinone (12b) from Q6 did not have the internal O-H···N hydrogen bond. Instead, a weak C-H···O hydrogen bond, which would facilitate H-transfer, was noted.

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Scheme 11. Aerobic oxidation of α-branched primary amines.

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Scheme 12. Calculated geometries of iminoquinone.

Recently, Oh and co-workers [20] reported an ortho-quinone catalyst Q7 (Scheme 13) for primary amine oxidation. The catalysis still requires a copper or TFA cocatalyst for effective turnover. The reactions worked only with linear primary amines.

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Scheme 13. o-Naphthoquinone-catalyzed aerobic oxidation of amines reported by Oh.

Oxidative cross-coupling of primary amines remains difficult. For this end, Largeron and coworkers have found that replacing CuMeSal with Cu(OAc)2 would increase the selectivity of crosscoupling imine [26]. Branched (14a) and linear (14b) aliphatic amines could react with 4-methylbenzylamine affording the desired imines in high yields (Scheme 14). Anilines (14d) were less reactive substrates, but with excellent imine selectivity.

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Scheme 14. Chemoselective oxidative cross-coupling of primary amines.

Recently, Largeron and co-workers [27] developed a new ortho-quinone catalyst on the basis of the structure of natural purpurogallin (Scheme 15). Cross-coupling reaction between different substituted benzylamines and aliphatic amines could be achieved to afford the corresponding products (15a–c) under mild conditions without the assistance of any metal cocatalysts. Aniline (15d) could also be applied, but with low reactivity due to its poor nucleophilicity.

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Scheme 15. Cross-coupling of amines catalyzed by a novel ortho-quinone catalyst.

In a sequential electrochemical setting, the anode oxidation could be coupled with cathode reduction to afford secondary amines from primary amineswith Q1 as a mediator [28]. At first, using Pt anode, benzylamine would couple with α-branched primary amine to give a cross imine in 3 h. After exhaustive anodic oxidation, changing Pt anode to Hg cathode rendered the reduction of imine to the corresponding secondary amine (Scheme 16). The switch to a mercury cathode was a disadvantage of the process in consideration of practical operation.

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Scheme 16. One-pot electrocatalytic synthesis of secondary amines.

ortho-Quinone catalyst have also been applied under heterogeneous conditions. Doris and co-workers [29] combined the AuONT nanohybrid with gallacetophenone Q1 for the oxidation of benzylamines (17a–d) (Scheme 17). α-Branched primary amine (17e) and secondary amine (17f) showed low activity or no reaction, respectively. This is the only catalytic system that was conducted in aqueous media.

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Scheme 17. Heterogeneous catalysis combining ortho-quinone catalyst and nanotube.

3.2. Secondary amines oxidation

In nature, CuAOs works only with primary amines, but not secondary amines. The latter are always recognized as inhibitors of copper amine oxidases. In 2012, Kobayashi reported a catechol/PtIr nanoclusters combined catalysis for secondary amine oxidation [18]. Dibenzylamines substituted with electron-donating and electron-withdrawing groups (18a, b) were tolerated to afford the desired imines in good to high yields. Bulky substrates like tertbutyl-substituted benzylamine (18d) required higher catalyst loading. Tetrahydroquinoline (18e) could also undergo oxidation in high yield (Scheme 18). The authors proposed a hydride-metal transfer mechanism. A single electron-hydrogen transfer mechanism could not be excluded.

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Scheme 18. Cooperative catalytic system of metal nanoclusters and catechol derivatives for amine oxidation.

In 2014, Stahl and co-workers [17] achieved aerobic oxidation of secondary amines and nitrogen containing heterocycles using a cooperative catalytic system of Q5 (phd) and ZnI2 with ambient air (oxygen) as the oxidant (Scheme 19). Electron-rich dibenzyl amines gave the desired imines in good yields. A large variety of nitrogen heterocycles such as tetrahydroisoquinolines, tetrahydro-β-carbolines and tetrahydroquinazolines went well under the conditions. Moreover the tertiary amine substrate N-methylindoline could also be applied toproducethe oxidized indole productin moderate yield. Control experiments revealed that both Zn2+ and iodide are important to the catalytic system. Mechanism studies indicated that the coordination between phd and zinc ion would enhance the amine oxidation activity of phd and iodide served as electron-transfer mediator to promote aerobic catalytic turnover.

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Scheme 19. Oxidation of secondary amines and nitrogen heterocycles with Q5.

Stahl further developed an improved catalytic system involving Ru-Q5/Co(salophen) for secondaryamine oxidation. This [Ru(Q5)3] (PF6)2/Co(salophen) catalyst system worked well for a number of tetrahydroquinolines and required much shorted reaction times than the Zn-Q5 system [30]. In this catalysis, Ru2+ ion is more effective than Zn2+ in activating Q5 and Co(salophen) would promotethe oxidation of reductive Q5, enabling the reaction under ambient air (Scheme 20). Oh and co-workers [20] reported an Q6- Ag2CO3 binary catalyst for (Scheme 21) the oxidation of cyclic secondary amines (21a–d). The catalysis did not work well with tetrahydroquinoline and indolines. In 2015, Doris and co-workers [31] presented another example of N-heterocycles dehydrogenation using a carbon nanotube-rhodium nanohybrid (RhCNT)-Q4 catalytic system. Tetrahydroquinolines, tetrahydroisoquinolines, tetrahydro-β-carbolines, tetrahydroquinazolidine and acyclic secondary amines were suitable substrates giving excellent yields at room temperature under air (Scheme 22).

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Scheme 20. Modified quinone catalyst system for dehydrogenation of tetrahydroquinolines.

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Scheme 21. o-Naphthoquinone-catalyzed aerobic oxidation of secondary amines.

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Scheme 22. Cooperative dehydrogenation of N-heterocycles.

Luo and coworkers [19] also challenged the oxidation of cyclic secondary amines by Q6. Although reaction conducted under a slight higher temperature, this catalytic system did not require any additives and metal cocatalysts (Scheme 23). Tertrahydroisoquinolines with electron-donating group (23a-c) furnished dihydroisoquinolines with excellent yields. Meanwhile, 1-phenyl substituted tetrahydro-β-carbolines (23d) exhibited good reactivity under the same condition. However, the reaction with acyclic secondary amine (23e) and 3, 4-dihydroisoquinoline (23f) was very sluggish.

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Scheme 23. Aerobic oxidation of secondary amines by Q6.

3.3. Tertiary amines oxidation

Quinone mediated tertiary amines oxidations are rare. Only a single example was reported in Stahl's work for the oxidation of N-methylindoline [17]. Luo and co-workers recently reported a simple quinone catalyst Q6 could efficiently promote aerobic oxidation of N-phenyltetrahydroisoquinoline, furnishing cross-dehydrogenative coupling product. A range of N-phenyltetrahy-droisoquinolines afforded the corresponding products in good yields. Different nucleophiles, such as malonic esters (24e), indoles (24f) and dialkyl phosphates (24g) were well-tolerated under the reaction conditions [21] (Scheme 24).

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Scheme 24. Tertiary amines oxidation by Q6.

3.4. Heterocycles synthesis

In primary amine oxidation, in-situ generated imine intermediates would be attacked by another amine leading to homo- or heterocoupling imines. To trap the imine intermediate with another nucleophiles is synthetically attractive. This strategy has been applied to N-heterocycles synthesis by employing primary amine and another nucleophile. Largeron and co-workers [32] reported a mild method for benzimidazole synthesis using oaminoanilines as nucelophiles. The reactions were carried out using benzylamines and o-aminoanilines in the presence of Q1 and a copper salt. Benzylamines bearing electron-donating or electronwithdrawing group (25b) could give the products in good yields, even nonactivated aliphatic amines (25c) went wellwith moderate yields (Scheme 25). While using the less reactive o-aminoanilines (25d), the yields would be slightly lower than N-substituted oaminoanilines (25a).

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Scheme 25. Synthesis of benzimidazoles derivatives.

Luo and co-workers also applied this strategy to heterocycles synthesis using a simple ortho-quinone catalyst [19, 33]. In the oxidation of α-branched primary amines, when the reaction was conducted at a higher temperature, unexpectedly, an oxidative trimerization adduct, imidazolinone was formed (Scheme 26). A number of 1-phenylethanamines bearing electron-donating substitutes could afford products with moderate yields, providing a distinctive pathway for the synthesis of imidazolinones. Next, they employed in-situ generated imine intermediate for other heterocycles synthesis. With the assistance of Brønsted acid, benzimidazoles could be obtained in excellent yield. Electron-rich benzylamines (27a) showed better reactive activity than electron-deficient benzylamines (27b). The more reactive N-substituted o-aminoanilines afforded the desired products (27c) in excellent yields. Thus, nonactivated aliphatic primary amines such as cyclopropylmethanamine (27d) and cyclohexylmethanamine could reactwith N-substituted o-aminoanilines albeit with slightly lower yields. When using 2-phenethylamine as substrate, the unexpected quinoxaline was isolated instead of the desired benzimidazole. Increasing the amount of Brønsted acid led to excellent reaction outcome. 2-Phenethylamines bearing electronwithdrawing or donating group were well tolerated affording products in high to excellent yields. Besides, they found a side product (27h) resulted from the condensation of o-aminoanilines and ortho-quinone catalyst which led the deactivation of quinone catalyst. By increasing the loading of 2-phenethylamine, the formation of (27h) can be largely inhibited. Furthermore, benzoxazoles also could be synthesized in good to high yields with benzylamine and o-aminophenol as substrates. Benzylamines bearing an electron poor substituent (27j) gave better yields than electron rich substituents (27k). Alkyl substituted o-aminophenols (27l) could be generally tolerated to afford products in good yields. Without any metal cocatalysts and harsh conditions, benzimidazoles, quinoxalines and benzoxazoles could be obtained in moderate to excellent yield catalyzed by a sole quinone catalyst, providing a green and mild approach for heterocycles synthesis (Scheme 27).

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Scheme 26. Oxidative trimerization of 1-phenylethanamines.

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Scheme 27. Synthesizing heterocycles catalyzed by a quinone catalyst.

4. Conclusions and prospect

In the past few years, great progress has been made in the development of the aerobic oxidation of amines. Inspired by quinoproteins which mainly control the metabolism of amines, chemists developed a number of quinone catalysts. These quinone catalysts, alone or in combination with metal cocatalysts could effectively catalyze amine oxidation under mild conditions.Several quinone catalysts surpass the activity of CuAOs by expanding the substrates scope from primary amines to secondary amines, even tertiary amines. However, most quinone catalysts work only with activated amines such as benzylamines, the activity as well as the scope remains to be further improved. To elucidate the structureactivity relationship of the quinone catalysts will certainly aid in catalyst evolution and discovery. On the other hand, the exploration quinone-metal co-operative or synergistic catalysis will significantly expand the scope and domain of quinone catalysis. In addition, there are five known quinone cofactors in Nature and among these PQQ could mediate alcohol oxidation. The development of bio-inspired quinone catalysts for alcohol oxidation remains a challenging, yet attracting aim. From the synthetic point of view, the harness of quinone catalysis alone or in concert with metal catalyst is expected to have impact on the formation and transformations of heterocyclic compounds.

Acknowledgments

We thank the National Natural Science Foundation of China (NSFC, Nos. 21390400 and 21521002) and the Ministry of Science and Technology, Chinese Academy of Sciences (No. QYZDJSSWSLH023) for generous financial support. S. Luo is supported by National Program for Support of Top-notch Young Professionals and CAS onehundred talented program (D).

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