Chinese Chemical Letters  2018, Vol. 29 Issue (1): 47-53   PDF    
Recent progress in Ru(Ⅱ)-catalyzed C-H activations with oxidizing directing groups
Zhenrong Wang, Peipei Xie, Yuanzhi Xia    
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, China
Abstract: In this review, we highlight the recent development in Ru(Ⅱ)-catalyzed C-H activations under redox neutral conditions. After a brief introduction of the C-H activations with oxidizing direct group by different transition metal catalysts, the examples with Ru(Ⅱ) catalyst were classified and introduced according to different internal oxidants used in the system. The features of each methodology will be highlighted and the plausible reaction mechanism will be presented if available.
Key words: Ru (Ⅱ) catalysis     C-H activation     Redox neutral condition     Oxidizing directing group     Internal oxidant    
1. Introduction

Transition metal-catalyzed C-H activation has become one of the most widely pursued topics in organic chemistry during the past decades [1]. Such kind of transformation provides a direct pathway for construction of C-C or C-X bond and is an economical and efficient alternative to the traditional methods in which activating groups are required for functionalization of arenes. Thus, various novel processes were developed in recent years under the catalysis of different transition metals. Except for tuning of the catalysts' properties, exploring the reactivity of different coupling partners has also been extensively studied [1].

To realize effective and selective C-H activation, a heteroatom-containing directing group is generally required in the substrate. Besides, stoichiometric or excess oxidant needs to be used in most of the transition metal-catalyzed C-H functionalization to maintain the catalytic cycle. Following these essential elements in C-H activation, a novel concept of oxidizing directing group was developed in recent years [2], in which the directing group could also serve as an oxidant for regeneration of the active catalyst. This concept was firstly reported in Pd(0)-catalyzed annulations of acyl oximes with arynes by Neuville and Zhu in 2008 [3], and shortly later by the groups of Cui and Wu and Hartwig [4]. The oxidizing directing group strategy was found much more prosperous applications under Rh(Ⅲ) catalysis after seminal research from groups of Fagnou and Glorius [5]. In this respect, different directing groups containing N-O, N-N, and other moieties were employed as the internal oxidant (Scheme 1) [6], and novel reactivity and synthetic methods were reported. Following the experimental observations, theoretical mechanistic studies of the Rh(Ⅲ)-catalyzed C-H activations with internal oxidants were conducted by our group [7] and others [8]. At the mean time, C-H activations under redox neutral conditions were reported with other metals, such as Co(Ⅲ) and Ru(Ⅱ) complexes, which attract more and more attention due to their relatively low cost and unique activity [9, 10].

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Scheme 1. Typical substrates containing oxidizing directing groups under Rh(Ⅲ) catalysis.

The emerging of Ru(Ⅱ) catalysis has attracted considerable attention as a result of high performance at low cost, and its combination with the internal oxidant strategy has been found wide applications for annulation and olefination reactions in the last few year. In this review, the Ru(Ⅱ)-catalyzed sp2 C-H activations under external oxidant-free conditions in recent years are presented according to different types of internal oxidant contained in the substrates. Most of the examples are related to transformations of N-OMe and N-OH containing benzaimdes, however, reactions from oximes and other substrates are also included. The [Ru(p-cymene)Cl2]2 is the most prevalent precatalyst in these transformations. The features and plausible mechanisms of the reactions are discussed.

2. Ru(Ⅱ)-catalyzed C-H/N-O functionalizations with N-substituted benzamides

The first Ru(Ⅱ)-catalyzed external oxidant free Csp2-H activation of N-OMe substituted benzaimde (1) with alkynes (2) was reported in 2011 by Li and Wang to afford isoquinolone products (3)(Scheme 2) [11], which was conducted under similar conditions as the Rh(Ⅲ)-catalyzed reactions by Glorius et al. [5a]. 3 mol% of [Ru (p-cymene)Cl2]2 in combination with 20 mol% NaOAc in MeOH solvent were used as an inexpensive catalytic system, and most of the reactions were conducted mildly at room temperature. Thus, the N-OMe moiety in the benzamide substrates is not only an oxidizing directing group, but also an activator for the substrate. This effect could be highlighted by comparison with the [Ru(pcymene)Cl2]2-catalyzed reactions with external oxidant by the Ackermann group [12]. In the latter reaction the N-Me substituted benzamides were substrates and 2.0 equiv. of Cu(OAc)2 were required as the oxidant at 100 ℃. The proposed mechanism for the Ru(Ⅱ)-catalyzed isoquinolone synthesis from 1 and 2 is shown in Scheme 3, which indicates the in situ formed [Ru(p-cymene)OAc2] may be an active species for C-H activation. Following the generation of the 5-membered ruthacycle intermediate A, the migratory insertion will form B and B', from which the final product could be formed by concerted or stepwise pathways.

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Scheme 2. Ru(Ⅱ)-catalyzed annulations of N-OMe substituted benzaimde with alkyne.

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Scheme 3. Plausible mechanism for Ru-catalyzed reactions of 1 with 2.

Almost at the same time, Ackermann reported the Ru(Ⅱ)-catalyzed C-H/N-O bond functionalization in water, providing a green methodology for isoquinolone synthesis from 1 and 2 (Scheme 4a) [13]. In this case the acetate salts were less effective than other carboxylate additives, and sterically hindered KO2CMes was used in the optimal condition. Notably, the reaction with 2.5 mol% [Ru(p-cymene)Cl2]2 and 30 mol% KO2CMes in water was more efficient than the reactions in organic solvents such as MeOH, DMF, and toluene.

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Scheme 4. Ru(Ⅱ)-catalyzed annulations and olefinations in water.

The same reaction could occur when free hydroxamic acid 4 is used instead of 1 (Scheme 4b), which is a more atom-economical pathway for generation of isoquinolone 3 by Ru(Ⅱ) catalysis annulations in water under redox-neutral conditions. A more detailed study on reaction of 4 and 2 was carried out in 2014 [14], which uncovered the subtle effect of carboxylate ligands and found the reaction is the most efficient when the Ru(Ⅱ) complex is derived from the electron-deficient carboxylic acid 3-(F3C) C6H4CO2H. Mechanistic study indicated that the carboxylate assisted C-H metalation is the kinetically relevant step, which is followed by migratory alkyne insertion, reductive elimination, and intramolecular oxidative addition. The dehydrative alkenylation was studied under the same conditions by reaction of 4 with activated alkenes, and styrene derivatives 5 were obtained in moderate to good yields (Scheme 4c).

In 2012, Li and Wang studied the external oxidant-free Ru(Ⅱ)-catalyzed oxidative couplings of N-methoxybenzamide 1 with different olefins (Scheme 5) [15], in which the [Ru(p-cymene)Cl2]2 and NaOAc were used as catalytic precursors. Interesting chemo-selectivity was observed when different olefins were used. The olefinated products 5 were obtained by reactions with acrylate esters in methanol solution (Scheme 5a), whereas 3, 4-dihydroisoquinolinone derivatives 6 and 7 were formed in reactions with styrene and norbornadiene in TFE, respectively (Scheme 5a).

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Scheme 5. Ru(Ⅱ)-catalyzed chemoselective olefinations and annulations.

When gem-disubstituted allenylsilanes 8 were used as the coupling partner, the allenylations of benzamides 1 occur mildly at 22 ℃ with the catalytic system of [Ru(p-cymene)Cl2]2 and NaOAc (Scheme 6) [16]. This reaction is featured with high regioselectivity and functional group tolerance. Mechanistic studies by deuterium labeling indicated that the C-H bond cleavage is the rate-determing step and is irreversible in reactions. Original mechanism proposed by Ackermann et al. suggests the regioselectivity of the migratory insertion should be determined by steric effect, and the N-O bond cleavage occurs after the syn-β-H elimination (Scheme 7). The silyl substituent was found to enhance the reactivity of allenes, however, its detailed effect on this reaction has not been understood.

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Scheme 6. Ru(Ⅱ)-catalyzed allenylation of 1 with allene.

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Scheme 7. Plausible mechanism for the Ru(Ⅱ)-catalyzed allenylation.

In 2016, Chegondi et al. discovered an interesting Ru(Ⅱ)-catalyzed cascade annulations of 1 with 2-acetylenic ketones 10 (Scheme 8), in which 5 mol% [Ru(p-cymene)Cl2]2 and 2 equiv. NaOAc were used for generation of the active catalyst in MeOH [17]. This reaction generates tetracyclic product 11 with high efficiency and opens new access to complex heterocyclic structural motifs for pharmaceutical applications. In comparison with the previous Rh(Ⅲ)-catalyzed reaction by Lin et al. [18], a reversed regioselectivity was observed in the cascade annulation in Scheme 8. Proposed mechanism shows that upon the generation of ruthenacycle A by N-H deprotonation and C-H cleavage of 1 with [Ru(p-cymene)(OAc)2] (Scheme 9), the regioselectivity for migratory insertion of 10 could be directed by the weak coordination of the carbonyl group. This was proposed as the key factor for reversal of the regiochemistry. From tetracyclic intermediate C, the oxidative C-N bond formation affords intermediate D, from which isoquinolone intermediate is generated easily by protonation with acetic acid. Finally, the tandem product 11 could be formed by intramolecular cyclization of 11' under basic conditions.

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Scheme 8. Ru(Ⅱ)-catalyzed regioselectivity reversed annulation of 2-acetylenic ketones.

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Scheme 9. Plausible mechanism for carbonyl-assisted reverse regioselective cascade annulations.

More recently, the propargyl alcohols 12 were employed as coupling partners by Liu et al. in Ru(Ⅱ)-catalyzed redox-neutral [4 + 1] annulations of N-ethoxybenzamide 1' (Scheme 10) [19]. While in previous works the internal alkyne were incorporated into the isoquinolone products as a two-carbon synthon, the propargyl alcohols 12 in this reaction act favorably as one-carbon units. The same selectivity could also be obtained by Rh(Ⅲ) catalysis [20]. Thus, a series of N-substituted quaternary isoindolinones 13 were synthesized by mild [4 + 1] annulations. According to the plausible mechanism in Scheme 11, migratory insertion of 12 into Ru-C bond of A forms intermediate B, from which the [4 + 2] and [4 + 1] products could be generated by reductive elimination and β-H elimination, respectively. In reactions only minor amount of [4 + 2] product 3 was obtained. The β-H elimination forms π-allylic species E, and then the C-N bond formation by reductive elimination generates intermediate F. Finally, oxidative addition of the N-OEt moiety to Ru(0) forms complex G, which could release product 13 and regenerate the active species [Ru(p-cymene)(OAc)2] by reaction with acetic acid.

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Scheme 10. Ru(Ⅱ)-catalyzed [4 + 1] annulations.

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Scheme 11. Mechanism for Ru(Ⅱ)-catalyzed [4 + 1] annulations.

3. Ru(Ⅱ)-catalyzed C-H/N-O functionalizations with oximes

Besides N-substituted benzamides, oximes are also known as an important class of substrate containing oxidizing directing group [11]. In this context, the Ru(Ⅱ)-catalyzed external oxidant-free C-H activations have lead to new access to substituted isoquinolines. In 2012, the Ackermann group reported alkyne annulations with oximes 14 catalyzed cationic Ru(Ⅱ) catalyst (Scheme 12) [21]. In this reaction the catalytic active species was formed from 5.0 mol%[Ru(p-cymene)Cl2]2 in presence of 30 mol% KPF6 as an additive, which showed excellent catalytic performance for synthesizing a number of isoquinoline derivatives 15 from reactions of both keto- and aldoximes 14 with different internal alkynes, and only water was generated as the side product.

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Scheme 12. Synthesis of isoquinolines by Ru(Ⅱ)-catalyzed annulations of oxime with alkyne.

Concurrently, the Geganmohan group reported the same annulation by a neutral Ru(Ⅱ) catalytic system (Scheme 13), and both C-H activations of aromatic and heteroaromatic ketoximes 14 were developed [22]. Notably, the terminal alkynes were also compatible in the neutral system and the corresponding isoquinolines 15 were formed in a regioselective manner. In addition, the O-methyl oximes 14' also underwent smooth cyclizations with alkynes, which is unprecedented in the literature. In 2013, this group extended the above reaction to cyclization of O-methylbenzohydroximoyl halides 16 with alkynes in CF3CH2OH solution [23]. Thus, a regioselective synthesis of 1-haloisoquinolines 17 was achieved with an electron-donating substituent on the phenyl ring (Scheme 14), which could be easily functionalized by further steps. Notably, a one pot formation of 1-O-alkylisoquinolones 18 was realized when the R group is a hydrogen or halogen. The mechanism for the CF3CH2OH-incoperated reaction is shown in Scheme 15, in which the substitution step occurs from inter-mediates after the C-H activation step, as no reaction was observed in reaction of 16a with CF3CH2OH under different conditions.

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Scheme 13. Ru(Ⅱ)-catalyzed cyclization of ketoximes with alkynes.

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Scheme 14. Synthesis of 1-O-alkylisoquinolines by Ru(Ⅱ)-catalyzed cyclization of O-methylbenzohydroximoyl halides with alkynes.

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Scheme 15. Mechanism for introduction of trifluoroethoxy moiety.

In 2014, the Ackmermann group developed the annulations of ferrocenylalkynes 2a by Ru(Ⅱ)-catalyzed isohypsic C-H/N-O bond functionalization of oximes 4 [24], affording 3-ferrocenyl-substituted isoquinolines 19 in high yields and excellent regioselectivity (Scheme 16). This observation suggests the ferrocene fragment only has marginal influence on the regioselectivity of the reaction. Consistently, when N-methoxy benzamides 1 were used instead of the oxime 4, similar annulations were observed to give 3-ferrocenyl-substituted isoquinolones 20 selectively. Interestingly, slightly different catalytic systems were used in the reactions of oximes and benzamides. In the former reaction only catalytic [Ru (p-cymene)Cl2]2 (5 mol%) was used as the catalyst, while in the latter one the combination of [Ru(p-cymene)Cl2]2 and KO2CMes was required.

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Scheme 16. Synthesis of ferrocenyl-substituted isoquinolines and isoquinolones.

4. Reactions of substrates containing other oxidizing directing groups

In presence of the Ru(Ⅱ) catalyst, a number of new annulations of alkynes were devised with different aromatic compounds containing the oxidizing directing group. In 2014, Liu and Lu reported the Ru-catalyzed redox-neutral C-H functionalization of N-phenoxyacetamide 21 for synthesis of benzofurans 22 (Scheme 17) [25], which is a continuation of their previous works with Rh(Ⅲ) catalysis [26]. The reaction occurs under very mild conditions with 2.5 mol% [Ru(p-cymene)Cl2]2 and 25 mol% K2CO3. The N-O bond is cleaved as the internal oxidant, and involvement of Ru(Ⅳ) as a key intermediate was proposed (Scheme 18).

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Scheme 17. Synthesis of benzofurans by Ru(Ⅱ)-catalyzed C-H activation of N-phenoxyacetamide.

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Scheme 18. Plausible mechanism for benzofuran formation by Ru(Ⅱ)-catalyzed C-H activation of N-phenoxyacetamide.

While in all above reactions the N-O bond was involved as the oxidizing directing group, examples with other reactive moieties as the internal oxidants were reported in recent years. In this respect, Huang et al. discovered the ruthenium-catalyzed redox-neutral C-H activation via N-N cleavage of 23 [27], which lead to novel synthesis of N-substituted indoles 24 (Scheme 19). This method was the first example with an N-N internal oxidant, compatible with terminal alkynes with excellent regioselectivity, and an atom-economic transformation by internal cleavage of the directing group. The mechanism of this transformation is shown in Scheme 20, in which a Ru(Ⅱ)-Ru(Ⅳ)-Ru(Ⅱ) catalytic cycle was proposed.

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Scheme 19. N-N bond as an internal oxidant for Ru(Ⅱ) catalyzed C-H activation.

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Scheme 20. Plausible mechanism for indole synthesis by Ru(Ⅱ)-catalyzed C-H activation of 23.

A more recent work from the Zhu group used the N-amino group in 25 as an oxidizing directing group in Ru(Ⅱ)-catalyzed traceless synthesis of indoles 26 and alkenylanilines 27 by reactions with alkynes and activated olefins, respectively (Scheme 21) [28]. The authors found that both reactions could be scaled up for gram-scale synthesis with a very low catalyst loading of 0.1 mol%. It should be noted that the combination of [Ru (p-cymene)Cl2]2 with 1.2 equivalent of Zn(OTf)2 is a more efficient catalytic system for indole synthesis, thus the in situ generated Ru (p-cymene)OTf2 was regarded as the active catalyst. Amonium is generated from cleavage of the N-N internal oxidant, which may be trapped by complexation with Zn(OTf)2 as proposed by Zhu et al.

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Scheme 21. N-Amino group as an oxidizing directing group in Ru(Ⅱ)-catalyzed traceless synthesis of indoles and alkenylanilines by reactions.

In 2016 the Gogoi group discovered that the amide C-N bond of isatins 28 could be used as the oxidizing directing group in Rucatalyzed annulations via C-H activation (Scheme 22) [29]. This reaction leads to pharmaceutically interesting 8-amido isocoumarins 29 by using catalytic amounts of [Ru(p-cymene)Cl2]2 and CsOAc in DCE/H2O (9:1) solution under air, and 1.0 equiv. of carboxylic acid is required to introduce the acyl group of the amide moiety in product 29. The plausible reaction mechanism gives good explanations for the important roles of water and oxygen in the reaction (Scheme 23). The 28B should be formed first by reaction of 18 with water. Interestingly, oxygen is required for decarboxylation (H to I) but is not an oxidant in the reaction, and Ru(Ⅳ) intermediate J may be formed by incorporation of the acid component.

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Scheme 22. Amide C-N bond of isatins as an oxidizing directing group for Ru(Ⅱ)-catalyzed C-H activation.

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Scheme 23. Plausible mechanism for Ru(Ⅱ)-catalyzed redox neutral C-H activation of isatin.

It is commonly observed that the internal oxidant is cleaved and a small molecule is released in the redox-neutral C-H activation reactions; however, a rare example from the Zhao group indicates that the C=N moiety in aromatic imines 30 could act as an oxidizing directing group in Ru(Ⅱ)/NHC catalyzed [3 + 2] carbocyclization with internal alkynes (Scheme 24) [30]. In this mild reaction the π-allyl complex [(cod)Ru(η3-C4H7)2] is used as the catalyst precursor and the IPr is a more effective NHC ligand in most cases. The imine directing group is reduced into an amine in product 31 [31], thus no external oxidant is required. In a similar way, the authors found the [3 + 2] ketone/alkyne annulation could also be achieved with catalytic amouns of [Ru(p-cymene)Cl2]2, IPr, and NaOAc, and the carbonyl group is a hydrogen acceptor to form a hydroxyl group in the product.

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Scheme 24. Ru(Ⅱ)/NHC catalyzed [3 + 2] carbocyclization of ketimines and ketones.

5. Conclusion and perspective

The using of [Ru(p-cymene)Cl2]2 as a cheap and effective precatalyst for C-H activations has attracted much attention and notable development was achieved by reactions under external oxidant free conditions. New reactions from C-H activations of N-substituted benzamides and oximes were the most studied for remarkable annulations with alkynes and olefinations with alkenes, with the N-O bond in the directing group as the internal oxidant. Examples with other internal oxidants were also paid more and more attention, and novel transformations in the field are still highly desirable. In future research, more prosperous Ru(Ⅱ)-catalyzed C-H activations under redox neutral conditions should be expected based on more detailed mechanistic understanding.

Acknowledgments

We thank the NSFC (Nos. 21372178 and 21572163) for financial support. Z. Wang is supported by the Scientific Innovation Program for College Students of Zhejiang Province (No. 2016R426061).

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