Transition-Metal-Mediated Direct CâH Amination of Hydrocarbons with

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Transition Metal-Mediated Direct C-H Amination of Hydrocarbons with Amine Reactants: The Most Desirable but Challenging C-N Bond Formation Approach Hyunwoo Kim, and Sukbok Chang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00293 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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Transition Metal-Mediated Direct C− −H Amination of Hydrocarbons with Amine Reactants: The Most Desirable but Challenging C− −N Bond Formation Approach Hyunwoo Kim1,2 and Sukbok Chang*2,1 1

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST); Daejeon 307701, Republic of Korea

2

Center for Catalytic Hydrocarbon Functionalizations, Institute of Basic Science (IBS), Daejeon 307-701, Republic of Korea

Abstract

Cross-dehydrogenative couplings (CDCs) have become one of the most straightforward protocols in the C−H bond functionalizations, showing step- and atom-efficiency without need of pre-functionalization of substrates and reactants. However, catalytic C−H amination procedures based on the CDC strategy by employing amine reactants are considered to be challenging mainly due to the highly nucleophilic character of parent amines to inhibit the catalytic turnovers and the difficulty in optimizing proper oxidative conditions. In spite of these concerns, recent efforts have led to notable advances in the C−H amination procedures, particularly in the intermolecular reaction system. In this Perspective, we address the recent achievements in the transition metal-mediated CDC amination reactions with two types of hydrocarbon substrates: (i) direct amination of acidic C−H bonds with parent amines, and (ii) chelation-assisted CDC amination/amidation of non-acidic C−H bonds. Mechanistic aspects are also briefly delineated in representative amination reactions to provide insights for the future development towards highly practical and environmentally more benign processes. KEYWORDS: C−H amination, cross-dehydrogenative couplings, amine reactants, transition metal catalysis, oxidative coupling

1. Introduction

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The direct installation of an amino group into organic molecules has been intensive research interests in synthetic, materials, and medicinal chemistry. This can be mainly ascribed to the straightforward access to the aminated products which have versatile utility with significant biological and materials properties in addition to high synthetic value.1 Moreover, the high binding ability of newly introduced amino moieties to metal centers plays an essential role in coordination chemistry.1b-c Inspired by the pioneering work of Ullman on the copper-mediated N-arylation,2 recent advances in the palladium-catalyzed amination of aryl or vinyl (pseudo)halides with amines (amides), mainly contributed by Buchwald3 and Hartwig4 independently, have guided the C(sp2)−N bond formation protocol to be utilized as one of the most valuable synthetic tools (Scheme 1, top left). However, this procedure inevitably provides a stoichiometric amount of halide salts as a side product, thus requiring a base or its equivalents to facilitate the arylation reaction in most cases. In this context, a more direct approach of using non-prefunctionalized arenes (alkenes or alkanes) instead of aryl (vinyl) halides to react with amines has drawn extensive interests as an ideal alternative. This procedure is anticipated to replace the conventional N-arylation of aryl halides if two hydrogen atoms released from this C−H amination method can readily be trapped under mild and practical oxidative conditions (Scheme 1, bottom left).5 Notwithstanding the notable features of this cross-dehydrogenative C−H amination approach, several issues are still remained to be effectively managed in order to make this more reliable and practical especially in the intermolecular reaction systems.5,6 Firstly, the regioselectivity for a desired C−H bond being activated among several reactive sites is the most challengeable in the metal-mediated processes. Although an electronic and/or steric bias of interested substrates (arenes or alkenes) can lead us to predict the most preferable C−H bonds to couple with amine reactants, the situation becomes more complicated in case where the electronic/steric factor is not obvious, thus resulting in a mixture of regioisomeric aminated products. Secondly, since the highly electrophilic nature of metal catalytic species required for the facile C−H bond activation of arene or alkene substrates, electrophilic catalysts are often more easily susceptible to the parent amine reactants, thus eventually decreasing the catalytic performance. Thirdly, oxidative conditions necessitated for the efficient removal of two hydrogen atoms from each hydrocarbon substrate and amine reactant have to be compatible with the employed catalyst systems as well as amine reactants. In addition, control of chemoselectivity to avoid an over-amination pathway is also important. With these aspects taken


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into considerations all together, the development of efficient and selective C−H amination based on the CDC working mode is highly desirable but challenging.

Scheme 1. Metal-Mediated C− −N Bond-Forming Reactions In this Perspective, we focus on the recent advances in C−H amination reactions using transition metal mediators to deal with the above critical issues. This article covers the recent progresses in the intermolecular CDC amination of more reactive acidic C−H bonds in addition to notable examples of chelation-assisted C−H amination of non-activated hydrocarbons. We are also chosen to highlight a range of metal mediators including Cu, Ag, Co, Mn, Pd, or Ir with brief mechanistic aspects.

2. Direct Aminations of Acidic C− −H bonds with Amines 2.1. Copper Catalysis Copper-mediated carbon-heteroatom coupling reactions have been known for more than a century.7 Considering the fact that copper species in those reactions display a high ability of C−H bond activation8 as well as facile reductive elimination leading to C−N bonds,9 copper catalysts are predicted to play a leading role also in CDC amination reactions. Moreover, its higher abundance, cheaper price, and lower toxicity compared to precious transition metals make a copper-based catalyst system highly attractive. The early examples of aerobic CDC amination reaction in the context of copper catalysis were reported independently by Mori10 and Schreiber,11 thereafter triggering intensive research efforts to couple acidic C−H bonds of


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heterocycles or fluoroarenes with amines or amides (Scheme 2). The reaction developed by Mori’s group covers a variety of azole substrates to react with secondary amines giving rise to C2-aminated azole products by using molecular oxygen as the terminal oxidant (Scheme 2a). Schreiber et al. expanded the scope of (hetero)arenes to include perfluoroarenes in coupling with a range of secondary amines and primary amides as reactants (Scheme 2b). Both of these processes are believed to be initiated by the deprotonation of a C−H bond of azoles to form organocopper(II) species B (Scheme 3). The subsequent pathway was assumed to be a ligand exchange of halide (X) by amine reactant to lead to C that then undergoes a reductive elimination to release the desired aminated product with the concomitant reoxidation of Cu(0) to catalytically active Cu(II) species A by molecular oxygen. Further, Hirano and Miura showed an elegant example of a tandem oxidative CDC amination and annulation by using orthoalkynylanilines to react with oxadiazoles (Scheme 4).12 The authors proposed that Nazolylaniline was formed in situ as an intermediate through a CDC path that is consistent with the previously reported mechanistic pathway.

Scheme 2. Copper-Catalyzed CDC Amination Reactions of Azoles and Perfluoroarenes


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Scheme 3. Proposed Mechanistic Cycle

Scheme 4. Tandem Oxidative CDC Amination-Annulation Reaction Su et al. reported a Cu-catalyzed aerobic intermolecular C−H amination of electron-deficient perfluoroarenes with anilines in the presence of TEMPO (0.5 equiv) and t-BuOK (>2.0 equiv). Although the reaction offers moderate product yields in most cases, it represents a unique example of a direct coupling by using primary arylamines. The reaction scope was rather limited in that only anilines bearing electron-deficient substituents were operative (Scheme 5).13 They proposed a slightly modified mechanism compared to the previous one,10,11 especially in the oxidation state of copper catalyst in that TEMPO works as a co-oxidant to O2 to facilitate the oxidation of organocopper(II) species (C) to Cu(III) intermediate (D), thus eventually leading to a smooth reductive elimination (Scheme 6).


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Scheme 5. Copper-Catalyzed Intermolecular CDC amination with Primary Anilines

Scheme 6. Proposed Mechanism 2.2. Silver, Cobalt and Manganese Catalysis As a mechanistically distinct approach to the Cu-catalyzed CDC amination reactions, our group developed the Ag-mediated, Co- and Mn-catalyzed oxidative C−H amination of oxazoles and thiazoles (Scheme 7).14 Compared to the above copper-catalyzed procedures,10-13 this amination of azoles required an acid additive; this Brønsted acid was proposed to facilitate the formation of


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more electrophilic oxazolinium intermediates (Scheme 8, B) to react with amines, and the employed oxidants participate in the rearomatization process of 2-aminobenzoxazolidine species (C). It was shown that the Co- or Mn-catalyzed C−H amination protocol was more effective than the Ag-mediated procedure in terms of milder reaction conditions and broader substrate scope. It should be emphasized that in the case of Mn catalysis, aqueous ammonia was participated in the amination albeit with moderate efficiency. Mechanistic studies including a preliminary KIE experiment (kH/D = 1.0) indicated that the C2−H bond cleavage of azole substrates was not involved in the rate-determining step, suggesting that the amination occurs through an initial nucleophilic attack of amines to the oxazolinium intermediates.


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Scheme 7. Silver, Cobalt and Manganese Mediated CDC amination of Azoles with Amines

Scheme 8. Proposed Pathway of Azole Amination

3. Direct CDC Amination of Non-Acidic Arene C− −H Bonds Despite the fact that the deprotonative copper-catalyzed CDC amination ignited extensive research efforts for the development of the step- and atom-economical direct coupling reactions, the substrate scope was rather limited mainly to (hetero)arenes bearing acidic C−H bonds. As an orthogonal approach, chelation-assisted C−H bond activation stood out as a powerful tool for the deprotonation of inert C−H bonds.15 It was natural to envisage that a catalyst system enabling directed C−H bond cleavage might be effectively utilized for the CDC amination with amines.16 However, because of an inherited challenge associated with the direct use of amine reactants that can easily deactivate metal catalysts,17 the early examples of this approach were confined mainly to electron-withdrawing (sulfon)amide coupling partners (Scheme 9).5,18

Scheme 9. Intermolecular CDC Amination of Non-Acidic Arene C− −H bonds 3.1. Palladium Catalysis In 2006, Che and Yu reported a Pd-catalyzed intermolecular C−H amidation with the use of sulfonamides as the amino source (Scheme 10).19 A key to success was the choice of oxidant: potassium persulfate in this case was superior to other oxidants examined. The reaction covers not only C(sp2)−H bonds but also C(sp3)−H bonds performing the CDC amidation assisted by an oxime chelating group. Because of the given chelating nature of nitrogen atom of oxime, the


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reaction shows exclusive ortho-selectivity in arene substrates. The authors assumed that the reaction pathway includes the formation of a nitrene species via oxidation of amides, and then a reductive elimination of a proposed Pd(IV) species would likely be occurred.

Scheme 10. Pd-Catalyzed CDC Amidation with (Sulfon)amides In addition to nitrogen-containing strong chelators, less Lewis basic ketones were also found to be effectively employed for this CDC process. Liu reported a Pd-catalyzed directed C−H amidation of aryl ketones with assorted amides in the presence of persulfate salts or [F+] oxidants (Scheme 11).20 Detailed mechanistic investigations with the observed noticeable reactivity of Nmethyl sulfonamides suggested that the amination does not proceed via a nitrene intermediate. As depicted in Scheme 12, the authors proposed that the reaction occurs through a cyclometalated Pd(IV)-imido intermediate (C) via oxidation of a Pd(II)-sulfonamide adduct (B). A large KIE value in the product distribution experiment ([P]H/[P]D = 4.9) and poor reactivity of electron-deficient arenes were suggested to be associated with the competitive coordination to sulfonamide reactants.21 These results support that the rate-determining step involves the C−H bond cleavage step, implying that further applications of highly nucleophilic amine coupling partners in this Pd-catalyzed CDC amination would be plausible in the future upon the proper optimization of oxidative conditions.


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Scheme 11. Pd-Catalyzed C− −H Amidation of Aromatic Ketones

Scheme 12. Proposed Mechanistic Cycle for Pd-Catalyzed Cross-Dehydrogenative C− −H Amidation 3.2. Copper Catalysis As shown in the Ullman-type coupling reactions2,7a,22 and azole-amine CDC aminations,5,23 copper species have proven to be effective to facilitate the C−N bond-forming process. Indeed, Yu group24 reported a Cu-mediated chelation-assisted C−H amidation with p-tolylsulfonamide


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and Chatani et al.25 also revealed a stoichiometric CDC with aniline nucleophiles (Scheme 13). Given the fact that the optimized reaction conditions require stoichiometric amounts of copper salts under somewhat harsh conditions (e.g. high temperature), these pioneering works still leave a room for further improvements in the Cu-mediated CDC amination in terms of higher metal turnover numbers, broader substrate scope, milder reaction conditions, and more detailed mechanistic understandings.

Scheme 13. Copper-Mediated C− −H Amination: Early Works In 2011, Nicholas et al. reported a catalytic version of the arene C−H amination using substoichiometric amounts of copper species under oxygen atmosphere (Scheme 14).26 The reaction was shown to be satisfactory with 20 mol % of copper(II) acetate to work for various amine reactants such as sulfonamides, benzamides and 4-nitroaniline. In their mechanistic description, molecular oxygen was proposed to act as a terminal oxidant and the intermediacy of Cu(III) species was also suggested (Scheme 15), from which a reductive elimination was followed to afford the desired amidation/amination products, consistent with the Stahl’s stoichiometric C−Ν bond formation experiments.9


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Scheme 14. Copper-Catalyzed Cross-Dehydrogenative C− −H Amidation/Amination

Scheme 15. Proposed Mechanistic Cycle It was also revealed that strongly-chelating bidentate directing groups27 can facilitate the copper-catalyzed CDC amination even with aliphatic amines which are considered to be the most challenging reactants. In 2013, Daugulis group developed a C−H amination reaction between non-acidic arenes and alkylamines by the action of a dual Cu-Ag catalytic system (Scheme 16). The reaction proceeds smoothly using co-catalytic amounts of Cu(OAc)2 and Ag2CO3 in the presence of stoichiometric amounts of N-methylmorpholine oxide (NMO) as a terminal oxidant.28 Various types of secondary amines including morpholine derivatives and primary amines efficiently participated in the desired amination process. Even though the authors did not provide an explicit proposal on the mechanistic pathway, they assumed the intermediacy of arylCuIII species via the oxidation of aryl-Cu(II) by NMO.29


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Scheme 16. Cu-Ag Catalyzed C− −H Amination with Alkylamines

Scheme 17. Copper-Mediated C− −H Amination/Amidation with Heterocyclic Amines More recently, Yu et al. reported a copper-mediated C−H amination/amidation of arenes with sulfonamides or heterocycle-containing amines (Scheme 17).30 Interestingly, the reaction conditions displayed a high level of compatibility with heteroarenes and heterocyclic amines. A key to success for this efficient reaction was the choice of the amide-oxazoline bidentate directing group in the presence of stoichiometric amount of copper salts. This coordinating group can easily be removed after the desired amination to afford N-arylaminobenzoates.30 Although the desired reaction proceeds under the stoichiometric conditions, it still offered a notable


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improvement in the CDC amination working with heterocycles that have a range of important functional groups. 3.3. Iridium Catalysis Among several conceivable transition metal-based systems for the more efficient C−H amination reactions, our group payed a special attention to half-sandwich iridium complexes considering the fact that they were known to display superior activity towards the C−H bond cleavage.31 In fact, a number of chelating group-containing arenes can form stable and often isolable iridacycles with half-sandwich iridium complexes at ambient conditions.32 Based on the stoichiometric conversion of an iridacyclic aniline adduct to an aminated benzamide product (Scheme 18), we were fortunate to optimize a room-temperature catalytic C−H amination of benzamides in reaction with diverse anilines (Scheme 19).33 A variety of diarylamine products were obtained efficiently from this procedure with excellent functional group tolerance, thus showcasing the unprecedented compatibility of the C−H functionalization processes with external oxidants based on a half-sandwich iridium catalyst system.

Scheme 18. Stoichiometric Oxidative Conversion of Amine-Bound Iridacycle


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Scheme 19. Ir-Catalyzed Oxidative C− −H Amination with Anilines As a logical extension, the newly developed iridium catalyst system was successfully applied to the C−H amination of arenes with more challenging alkylamine reactants (Scheme 20).34 The reaction proceeded smoothly at 60 oC with linear alkylamines as well as cyclic congeners without giving over-aminated side products. The choice of a solvent system turned out to be crucial for achieving high efficiency in the amination with alkylamines. It was reasoned that the use of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as a co-solvent with 1,2-dichloroethane prevents the conceivable catalyst poisoning presumably by forming a hydrogen bonding between alkylamine reactants and HFIP solvent which is known in the literature.35

Scheme 20. Ir-Catalyzed Oxidative C− −H Amination with Alkylamines It was proposed that the protic reaction media employed in the amination of alkylamines might change the mechanistic pathway especially in the C−H activation stage when compared to that of anilines (Scheme 21). The C−H bond cleavage step in case of alkylamine reactants was determined to be reversible showing no KIE value in parallel rate comparison tests. In contrast, a significant KIE value was measured in an irreversible C−H bond activation step when anilines were examined. The key iridacyclic intermediates (B and C) were isolated and the coordination modes were unambiguously characterized by X-ray crystallographic analysis. Although more comprehensive studies are needed to precisely delineate the amino-group transfer stage, the reaction was proposed to proceed through an Ir(V)-imido species in accordance with stoichiometric oxidative conversion as well as the observed poor reactivity of secondary amines.


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The requisite oxidative conditions are believed to be essential for the formation of the postulated high valent Ir(V) intermediate D,36 eventually leading to an aminated product via a protodemetalation of an imido-inserted iridium amido species (E).

Scheme 21. Proposed Mechanism A remaining issue in making this amination procedure more practical and applicable is the use of super-stoichiometric amount of Ag(I) oxidant under the present conditions. In this regard, environmentally more benign oxidants such as molecular oxygen or hydrogen peroxide would be an ideal alternative. Moreover, photocatalytic37 or electrochemical38 oxidative conditions can lead to be an attractive amination procedure for further applications. In this line, Cho et al. recently reported a successful utilization of the photocatalytic oxidative conditions for the development of an intramolecular C−N dehydrogenative coupling of o-arylanilides leading to carbazole products,37 thus representing a high promise for future guideline (Scheme 22).


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Scheme 22. Photoredox and Pd-Catalyzed Intramolecular CDC Amidation

5. Summary and Outlook Given the crucial utility of nitrogen-containing compounds in diverse research and application areas, the development of more efficient and selective C−N bond formation has been highly desirable, thus leading to numerous amination procedures. Cross-dehydrogenative coupling (CDC) is the most straightforward and step-economical approach towards this goal in that both hydrocarbon substrates and amine reactants are directly employed without pre-functionalization. In this regard, the utilization of this direct C−H amination procedure in organic synthesis has been actively pursued to overcome intrinsic difficulties in the CDC strategy by taming the high nucleophilic character of amine reactants and/or by making the requisite oxidative conditions milder and more convenient. During the past decade, there have been remarkable progresses in the development of transition metal-mediated C−H amination by successfully optimizing reaction conditions. In regard to the substrate scope, while the early examples mainly include (hetero)arenes bearing acidic C−H bonds to react with amines, chelation-assisted C−H amination has recently become a powerful tool for the amination of a broad range of arenes, alkenes, and alkanes largely irrespective of the acidity of the targeted C−H bonds. Despite the great success witnessed recently in the development of intermolecular CDC amination reactions with more detailed mechanistic understandings, there is still a room for further explorations especially in the aspect of (i) reaction conditions (environmentally more friendly oxidants and milder reaction parameters), (ii) catalyst systems (first row transition metals, high turnover numbers, and minimum additives), (iii) substrates (that do not have acidic C−H bonds or strong chelation groups), and (iv) amine reactants (covering primary and secondary aryl/alkyl amines/amides, heterocycles, and ammonia). By taking all these considerations into account, the practical C−H amination reactions are anticipated to be developed in near future, thus finding their high utility in organic synthesis, medicinal chemistry, and materials science. AUTHOR INFORMATION CORRESPONDING AUTHOR *E-mail: [email protected] NOTES The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work supported by the Institute for Basic Science (IBS-R10-D1) in Korea.

ABBREVIATIONS FG, functional group; DMSO, dimethyl sulfoxide; TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl; T-HYDRO, tert-butyl hydroperoxide; EWG, electron-withdrawing group; NMP, N-Methyl-2-pyrrolidone; DMF, dimethylformamide; Cp*, pentamethylcyclopentadienyl; dFppy, 2-(2,4-difluorophenyl)pyridinato; phen, phenanthroline

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