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  • 4586

    M. C. Henry et al. Short ReviewSyn thesis

    SYNTHESIS0 0 3 9 - 7 8 8 1 1 4 3 7 - 2 1 0 XGeorg Thieme Verlag Stuttgart New York2017, 49, 45864598short reviewen

    Recent Advances in Transition-Metal-Catalyzed, Directed Aryl CH/NH Cross-Coupling ReactionsMartyn C. Henry Mohamed A. B. Mostafa Andrew Sutherland* 0000-0001-7907-5766

    WestCHEM, School of Chemistry, The Joseph Black Building, University of Glasgow, Glasgow, G12 8QQ, United KingdomAndrew.Sutherland@glasgow.ac.uk

    DG

    DG = directing group

    Het NH

    H+

    R1

    R2DG Het

    NR1R2

    Mn, Fe, Co, Ni, CuRu, Rh, Pd, Ag

    Ir or Au

    N

    O

    N

    NO2N

    ON NH

    O

    HN

    N

    F3C

    Received: 06.07.2017Accepted: 14.07.2017Published online: 28.08.2017DOI: 10.1055/s-0036-1588536; Art ID: ss-2017-z0443-sr

    License terms:

    Abstract Amination and amidation of aryl compounds using a transi-tion-metal-catalyzed cross-coupling reaction typically involves prefunc-tionalization or preoxidation of either partner. In recent years, a newclass of transition-metal-catalyzed cross-dehydrogenative coupling re-action has been developed for the direct formation of aryl CN bonds.This short review highlights the substantial progress made for ortho-CNbond formation via transition-metal-catalyzed chelation-directed arylCH activation and gives an overview of the challenges that remain fordirected meta- and para-selective reactions.1 Introduction2 Intramolecular CN Cross-Dehydrogenative Coupling2.1 Nitrogen Functionality as Both Coupling Partner and Directing

    Group2.2 Chelating-Group-Directed Intramolecular CN Bond Formation3 Intermolecular CN Cross-Dehydrogenative Coupling3.1 ortho-CN Bond Formation3.1.1 Copper-Catalyzed Reactions3.1.2 Other Transition-Metal-Catalyzed Reactions3.2 meta- and para-CN Bond Formation4 CN Cross-Dehydrogenative Coupling of Acidic CH Bonds5 Conclusions

    Key words amination, amines, amides, cross-coupling, dehydrogena-tion, transition metals

    1 Introduction

    The ubiquitous presence of aryl CN bonds in naturalproducts, pharmaceuticals, agrochemicals and organic ma-terials has resulted in a wide range of methods for the effi-cient and selective synthesis of this motif.1 Traditionally,aryl CN bonds were formed using a combination of elec-trophilic aromatic nitration, followed by reduction of theresulting nitrobenzene.2 However, with the development of

    catalytic bond-forming reactions (Scheme 1), the use ofcopper- or palladium-catalyzed amination of aryl (pseudo)-halides using UllmannGoldberg, ChanEvansLam or

    Martyn C. Henry (left) was born in 1993 in Livingston, Scotland. He graduated with a 1st class M.Sc. degree in chemistry from Heriot-Watt University in 2016, while carrying out a placement year with Dr. Chris-topher Levy at Kansas State University and a final-year project under the supervision of Dr. Stephen Mansell on novel nickel complexes for olefin polymerization catalysis. Since 2016, he has been part of the Suther-land group at the University of Glasgow working toward his Ph.D., which is focused on the development of new transition-metal-catalyzed cross-coupling reactions.Mohamed A. B. Mostafa (middle) graduated with a B.Sc. degree (with high distinction) in chemistry from the University of Omar Al-Mukhtar, Libya, in 2003. In 2012, he was awarded an M.Sc. degree from the Uni-versity of Wollongong, Australia, while carrying out a research project under the supervision of Professor Paul Keller on the stereoselective synthesis of chiral biaryl natural products. Since 2014, he has been part of the Sutherland group at the University of Glasgow, working toward his Ph.D. on the development of metal-catalyzed, one-pot multi-reac-tion processes for the preparation of polycyclic scaffolds.Andrew Sutherland (right) was born in 1972 in Wick, Scotland. After graduating with a 1st class B.Sc. Honours degree in chemistry at the University of Edinburgh in 1994, he undertook a Ph.D. at the University of Bristol under the supervision of Professor Christine Willis. This was followed by postdoctoral studies with Professor John Vederas at the Uni-versity of Alberta and Professor Timothy Gallagher at the University of Bristol. In January 2003, he was appointed to a lectureship in the School of Chemistry at the University of Glasgow and currently holds the posi-tion of Reader. His research groups interests are on the discovery of new transition-metal-catalyzed methodology and the development of radionuclide- and fluorescent-based molecular imaging agents.

    Georg Thieme Verlag Stuttgart New York Synthesis 2017, 49, 45864598

  • 4587

    M. C. Henry et al. Short ReviewSyn thesis

    BuchwaldHartwig protocols have superseded the harshconditions of the nitration/reduction approach.3,4 More re-cently, the surge in methods for transition-metal-catalyzedC(sp2)-H activation has resulted in novel approaches for arylCN bond formation.5 Initially, these novel transformationsinvolved coupling of nucleophilic-like metalated intermedi-ates with electrophilic or activated aminating reagents.However, methods have now been established that allowdirect cross-dehydrogenative coupling (CDC) between arylCH bonds and non-activated amines and amides (Scheme1).5 This short review highlights the various methods thathave been reported for both intramolecular and intermo-lecular aryl CN bond formation using the cross-dehydro-genative coupling approach. In particular, highly regioselec-tive ortho-amination and amidation through transition-metal-catalyzed chelation-directed activation of aryl CHbonds will be described, as well as CN bond formationfrom more reactive acidic aryl CH bonds. The various one-pot strategies that have been reported for para-aminationand amidation using activating groups are also discussed.

    Scheme 1 Transition-metal-mediated CN bond-forming processes

    2 Intramolecular CN Cross-Dehydrogena-tive Coupling

    2.1 Nitrogen Functionality as Both Coupling Part-ner and Directing Group

    The first example of an oxidative cross-dehydrogenativecoupling process was reported by Buchwald and co-workersfor the preparation of unsymmetrical N-acylcarbazoles(Scheme 2).6 The palladium-catalyzed process used oxygenand copper acetate as oxidants and allowed the efficientpreparation of a range of N-acylcarbazoles bearing variousfunctional groups and substituent patterns. Importantly,the N-acetamide coupling partner also acted as the direct-ing group, which was necessary to overcome the energybarrier associated with the final C(sp2)N reductive elimi-nation step.

    Scheme 2 Synthesis of N-acylcarbazoles using a palladium-catalyzed CDC process

    Other palladium(II)-catalyzed syntheses of carbazolesusing intramolecular CN bond-forming reactions havebeen reported. Work by the Gaunt7 and Youn8 researchgroups showed that the use of oxidants such as PhI(OAc)2 orOxone permit CN bond formation at ambient temperature.These studies also extended the substrate scope, demon-strating that N-alkyl and N-sulfonamide groups could beused as coupling partners. A similar cross-dehydrogenativecoupling process of 2-aminobiphenyls using iridium(III) ca-talysis has allowed the synthesis of NH carbazoles.9 Againthe nitrogen functionality is used as both the coupling part-ner and the directing group (Scheme 3). Following aminecomplexation of the iridium(III) catalyst and CH activa-tion, reductive elimination then gave the carbazole product.The resulting iridium(I) species was oxidized back to iridi-um(III) using a copper co-catalyst in the presence of air.

    Scheme 3 Iridium(III)-catalyzed synthesis of NH carbazoles

    FG

    X

    FG

    NR1R2

    FG

    H

    FG

    [TM] CN bondformation

    + H N

    R1

    R2

    oxidativeaddition

    CHactivation

    NH

    OPd(OAc)2 (5 mol%)

    Cu(OAc)2, O2

    3 MStoluene, 120 C

    1224 h

    93% 94% 81% 96%

    N

    O

    NAc NAc NAcNAc

    F

    MeO

    CO2Me

    R1R1

    R2 R2

    Cp*IrX2

    NH

    76%

    NH

    77%

    F

    NH

    90%

    NH2

    H2N IrX2

    Cp*

    2 HXHN Ir

    Cp*NH

    Cp*Ir

    2 CuX2

    2 CuX

    air, 2 HX

    H2O

    Georg Thieme Verlag Stuttgart New York Synthesis 2017, 49, 45864598

  • 4588

    M. C. Henry et al. Short ReviewSyn thesis

    This general approach for intramolecular CN bond for-mation has been utilized for the preparation of a range ofN-heterocycle-fused arenes. For example, copper-catalyzedprotocols have been developed for the preparation of ben-zimidazoles10 and pyrido[1,2-a]benzimidazoles,11 whilepalladium(II) catalysis has been used for the synthesis of 2-oxoindoles and 2-quinolinones.12,13 For the palladium(II)-catalyzed synthesis of 2-oxoindoles (and 3,4-dihydroquino-linones), Yu and co-workers used N-methoxamides as theintramolecular N-coupling partner and for activation of theCH bond (Scheme 4).12 Although a CuCl2/AgOAc systemwas used for re-oxidation of the palladium catalyst, result-ing in relatively high temperature reactions, the processwas found to have a broad scope, yielding the products inhigh yields. Furthermore, the N-methoxy groups were read-ily cleaved using either H2/Pd/C or SmI2, allowing efficientaccess to the parent lactam.

    Scheme 4 Palladium(II)-catalyzed synthesis of 2-oxoindoles

    A lower-temperature intramolecular, palladium(II)-catalyzed CN amidation process involving biaryl hydra-zones was reported for the synthesis of 3-arylindazoles.14For unsymmetrical benzophenone tosylhydrazones, the re-gioselectivity of CH activation and subsequent cyclizationwas controlled by electronic factors. For example, aryl ringsbearing electron-donating substituents accelerated CH ac-tivation resulting in cyclization on that particular ring (e.g.,Scheme 5). Electron-withdrawing substituents instead

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