Cross-coupling of aromatic esters and amides

Ryosuke Takise a, Kei Muto b and Junichiro Yamaguchi *b
aGraduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
bDepartment of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan. E-mail:

Received 10th March 2017

First published on 7th July 2017

Catalytic cross-coupling reactions of aromatic esters and amides have recently gained considerable attention from synthetic chemists as de novo and efficient synthetic methods to form C–C and C–heteroatom bonds. Esters and amides can be used as diversifiable groups in metal-catalyzed cross-coupling: in a decarbonylative manner, they can be utilized as leaving groups, whereas in a non-decarbonylative manner, they can form ketone derivatives. In this review, recent advances of this research topic are discussed.

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Ryosuke Takise

Ryosuke Takise was born in Aichi, Japan, in 1990. He earned his master's degree in chemistry from Nagoya University in 2015. Currently, he is a postgraduate student in the group of Kenichiro Itami at Nagoya University, and working with Prof. Junichiro Yamaguchi as his co-supervisor.

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Kei Muto

Kei Muto was born in Toyota, Japan, in 1988. He received his PhD in 2015 from Nagoya University (Supervisor: Prof. Kenichiro Itami). In 2016, he started his academic career at Waseda University as an Assistant Professor working with Prof. Junichiro Yamaguchi.

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Junichiro Yamaguchi

Junichiro Yamaguchi was born in Tokyo, Japan, in 1979. He received his PhD in 2007 from the Tokyo University of Science under the supervision of Prof. Yujiro Hayashi. From 2007 to 2008, he was a postdoctoral fellow in the group of Prof. Phil S. Baran at The Scripps Research Institute (JSPS postdoctoral fellowship for research abroad). In 2008, he became an Assistant Professor at Nagoya University working with Prof. Kenichiro Itami and was promoted to Associate Professor in 2012. He then moved to Waseda University as an Associate Professor (principal investigator) in 2016. His research interests include the total synthesis of natural products and the innovation of synthetic methods.

1. Introduction

Transition-metal-catalyzed cross-coupling is a reliable method to form C–C and C–heteroatom bonds and is widely applied to the synthesis of natural products, pharmaceuticals, and organic materials.1 Conventionally, cross-coupling uses organic halides as electrophiles. For the past few decades, several advanced methods have been developed to allow for the use of alternative electrophiles such as phenols and anilines.2,3 The use of these electrophiles in cross-coupling is beneficial not only because they do not produce corrosive halogen salts in the reaction, but also because they can provide new synthetic strategies and routes.

Further elaborating on this trend, the development of catalytic cross-coupling of aromatic carboxylic acid derivatives such as esters (ArCO2R) and amides (ArCONR2) has attracted considerable attention.4 Aromatic esters and amides are ubiquitous, readily available, and inexpensive molecular scaffolds. Owing to their ease of handling and facile preparation, they have been used as expedient building blocks in organic synthesis. Although various carboxylic acid derivatives, including acyl chlorides,5 thioesters,6 and aroyl cyanides7 had long been known to participate in metal-catalyzed cross-coupling as electrophiles, the development of cross-coupling of esters and amides was hampered by their comparatively stable C(acyl)–O and C(acyl)–N bonds toward metal catalysts. The cross-coupling of esters and amides can potentially proceed through C(acyl)–O and C(acyl)–N bond activation (oxidative addition) by a transition metal catalyst (M) to give intermediate A (Scheme 1). Thereafter, the reaction can diverge into two reaction pathways: a decarbonylative or a non-decarbonylative addition of nucleophiles.8 In the decarbonylative reaction, the entire functional group works as a leaving group to form a new chemical bond via intermediate B. On the other hand, a non-decarbonylative coupling proceeds without the loss of carbonyl via intermediate C to realize transformations involving mild nucleophiles (nucleophiles that cannot be utilized in traditional substitution reactions).

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Scheme 1 Transition-metal-catalyzed cross-coupling of aromatic esters and amides.

This review highlights the progress in transition-metal-catalyzed decarbonylative and non-decarbonylative cross-coupling of aromatic esters (including acid anhydrides) and aromatic amides (including imides). Although several catalytic cross-couplings of aliphatic esters and amides have been developed,9 this review focuses on the reaction of aromatic derivatives.

2. Overview of cross-coupling of esters and amides

In a pioneering achievement, in 1976, Yamamoto reported that stoichiometric amounts of a nickel complex can insert into the C(acyl)–O bond of an ester (Fig. 1A).10 In this work, Ni(cod)(PPh2)2 was made to react with phenyl propionate to produce complex D. Then, decarbonylation (–CO) of D occurred to give complex E, followed by β-hydride elimination to generate ethylene and complex F. Finally, reductive elimination of phenol and returning a molecule of carbon monoxide (CO) to the nickel atom gave nickel complex G. This was the first observation of oxidative addition of a generally inert C(acyl)–O bond to transition metals, as well as a decarbonylation event from an ester. Nevertheless, a catalytic version of the cross-coupling of aromatic esters and amides did not surface until close to the 21st century.
image file: c7cs00182g-f1.tif
Fig. 1 (A) Early work showing that a Ni complex can inset into the C(acyl)–O bond of an ester. (B) A chronology of catalytic decarbonylative and non-decarbonylative coupling of “esters”. (C) A series of “esters” that can be used in cross-coupling. (D) Effective ligands in the cross-coupling of “ester” substrates. Note: “esters” here include acid anhydrides.

A chronological evolution of catalytic decarbonylative and non-decarbonylative coupling of aromatic “esters” is shown in Fig. 1B (for the purposes of this discussion, “esters” here includes acid anhydrides). The development of decarbonylative coupling of aromatic “esters” began with the use of alkenes as nucleophiles for the Mizoroki–Heck type reaction by the de Vries/Stephan group in 1998 (Section 3.1).11 Thereafter, many types of decarbonylative couplings have been developed, such as alkyne insertion/addition (Section 3.2), Suzuki–Miyaura coupling (Section 3.3), C–H arylation (Section 3.4), Sonogashira coupling (Section 3.5), borylation/silylation (Section 3.6), intramolecular etherification (Section 3.7), amination (Section 3.8), and reduction (Section 3.9). Pd and Ni catalysts are now popular for these kinds of transformations, but Rh and Ru catalysts had also been used in earlier times. Meanwhile, Gooßen reported the first non-decarbonylative coupling of aromatic acid anhydrides using a Pd catalyst in 2001.12 Unlike the decarbonylative methods, non-decarbonylative couplings have only been reported as two reaction types, Suzuki–Miyaura coupling (Section 4.1) and amidation (Section 4.2), and with only two transition metal catalysts, Pd and Ni.

Although the type of nucleophile is important for functional group diversification, the most critical issues are: (i) the type of aromatic “ester” (including acid anhydrides), (ii) the mode of activation of the C(acyl)–O bond by transition metals, and (iii) the type of ligand for the applicable transformation. The classification of aromatic “esters” was performed with mechanistic considerations in mind, as shown in Fig. 1C. Acid anhydrides are not esters, but we include them herein as “activated ester derivatives” that can be cleaved at its C(acyl)–O bond, driven by its electron deficiency (e.g., the de Vries/Stephan group [1998] for decarbonylative coupling11 and Gooßen [2001] for non-decarbonylative coupling12). p-Nitrophenyl esters are also electron-deficient at their C(acyl)–O bond and were used for the first time in a decarboxylative coupling with a carbon nucleophile by Gooßen and coworkers in 2002.13 In 2004, the Gooßen group used enol ethers, whose C(acyl)–O bond cleavage was driven by a stabilized leaving group, since enol ethers produce acetone after oxidative addition of the transition metal.14 Meanwhile, the Murai group reported the first Ru-catalyzed decarbonylative coupling with pyridylmethyl esters,15 which has an activated C(acyl)–O bond, driven by chelation assistance. The Ru atom might be chelated to the pyridyl group, and renders the C(acyl)–O bond active toward oxidative addition. In a similar mode of activation, 2-pyridyl esters were employed by Chatani in 2004 for non-decarbonylative coupling,16 and ethyl esters with a nitrogen-containing directing group were used by Wang in 2012 for decarbonylative coupling.17 Currently, the most used esters are phenyl esters, which were first reported by the Itami/Yamaguchi group in 2012.18 This ester can be classified as a “weakly chelation driven” ester, which might benefit from the interaction between the metal and the π-bonds of the phenyl group as well as the carbonyl group to render the C(acyl)–O bond reactive. Additionally, this report presented the first Ni-catalyzed decarbonylative coupling reaction.

Representative ligands for decarbonylative and non-decarbonylative coupling of aromatic esters are shown in Fig. 1D. At the beginning, no ligands or PPh3 were used, but electron-rich monophosphines such as trialkylphosphine (Alk = methyl, n-Bu, cyclohexyl) were soon known to be effective for both decarbonylative and non-decarbonylative coupling. On the other hand, bisphosphines, particularly, 1,2-bis(dicyclohexylphosphino)ethane (dcype), were found to be quite effective for the decarbonylative C–H arylation of aromatic esters, as reported by the Itami/Yamaguchi group in 2012.18 Thereafter, the dcype ligand was used for other purposes such as Suzuki–Miyaura coupling, silylation, amination, and reduction. Additionally, the Yamaguchi/Itami group developed new bisphosphine ligands, 3,4-bis(dicyclohexylphosphino)thiophene (dcypt)19 and its cyclopentyl analogue (dcppt), and applied them to a Sonogashira coupling and etherification. The Gade group also reported that bis(diisopropylphosphinomethyl)amine can be used for decarbonylative C–H coupling.20 Recently, N-heterocyclic carbene (NHC) ligands such as ICy, SIPr, and IPr are known to be effective not only for decarbonylative borylation, but also for non-decarbonylative coupling such as Suzuki–Miyaura coupling and amidation. Putting it all together, regardless of the hapticity, structural features of these ligands involve electron-rich and bulky substituents, which could activate inert C–O and C–N bonds.

A chronological evolution of the cross-coupling of “amide” substrates is depicted in Fig. 2A (for the purposes of this discussion, “amides” here includes imides). Many types of decarbonylative coupling of “amides” exist, including alkyne insertion/addition (Section 5.1), Negishi coupling (Section 5.2), C–H arylation (Section 5.3), Mizoroki–Heck reaction (Section 5.4), Suzuki–Miyaura coupling (Section 5.5), reduction (Section 5.6), borylation (Section 5.7), and amination (Section 5.8). For non-decarbonylative coupling, reaction types include Suzuki–Miyaura coupling (Section 6.1), Negishi coupling (Section 6.2), esterification (Section 6.3), and transamidation (Section 6.4).

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Fig. 2 (A) A chronological evolution of catalytic decarbonylative and non-decarbonylative coupling of “amides”. (B) A series of “amides” that can be used in cross-coupling. (C) Effective ligands in the cross-coupling of “amide” substrates. Note: “amides” here include imides.

Structures of aromatic “amides” (including imides) used for these decarbonylative and non-decarbonylative couplings are listed in Fig. 2B. Much like acid anhydrides were used as “activated esters” in the early cross-couplings of esters, imides such as phthalimides were initially used instead of amides (i.e., driven by electron deficiency). In 2008, Kurahashi and Matsubara reported the first Ni-catalyzed decarbonylative addition of phthalimides onto internal alkynes. Wang reported a decarbonylative coupling of amides for the first time in 2012:21 methoxy amides can be activated with Pd and proceed with decarbonylation. In 2015, the Garg/Houk group reported the first Ni-catalyzed esterification of aromatic amides.22 In this reaction, they used N-alkyl-N-phenylamides as the most suitable electrophile. The amide was slightly twisted from a cis-configuration due to steric hindrance from the alkyl group, as well as a weak π–π interaction between the aryl and phenyl groups. In this activation of the C(acyl)–N bond, energy differences of the starting amide and the product amide were exploited in a thermodynamically driven process. In parallel, Garg and Szostak reported the use of “twisted amides”.23 Various “amides” (including imides) such as glutarimide amides, tosyl amides, Boc amides, saccharin amides, and di-Boc amides, which are readily preparable, were reported in the development of the cross-coupling of amides, driven by disruption of the amide resonance energy. It is of note that this kind of aromatic electrophile can be difficult to strictly define as an “amide”, and therefore formal “imides” and “acylimides” are currently the most utilized groups in the coupling of amides. Recently, Maiti demonstrated a chelation-assisted approach using pyrazole amides.24 These amides are activated not only electronically, but also with the chelation of the transition metal with the nitrogen on the pyrazole. Taken together, all aromatic amides used in decarbonylative and non-decarbonylative cross-couplings have an amide bonds activated electronically or by steric distortion.

Regarding ligands for the cross-coupling of amides, various types are known (Fig. 2C). Initially, bidentate nitrogen ligands such as 1,10-phenanthroline (phen) and 2,2′-bipyridine were used, but this generally shifted toward more electron-rich ligands such as alkyl phosphines or mono- or di-alkyl phosphines. Garg first reported that NHCs (e.g., SIPr) are effective for the activation of the C(acyl)–N bond by nickel catalysis.22 Subsequently, other NHC ligands were utilized for Ni- or Pd-catalyzed decarbonylative and non-decarbonylative coupling of amides, thus NHC ligands are currently the most widely used. Bisphosphines, such as dppf and dcype, are also effective for amination and reduction, as reported by the Rueping group in 2017.25,26

Although an overview of the cross-coupling of esters and amides is described above, details of each reaction and its mechanism, which are classified based on the type of nucleophile, are described in the following sections.

3. Decarbonylative coupling of aromatic esters

3.1. Mizoroki–Heck type reaction

The Mizoroki–Heck reaction between aryl halides and alkenes has become widely used to construct arylated olefins, not only in academic laboratories, but also in various chemical industries.27 Extensive studies have made it possible for chemists to utilize versatile aryl sources over the past few decades. In 1998, Stephan, de Vries and coworkers reported a Pd-catalyzed decarbonylative Mizoroki–Heck type reaction using aromatic acid anhydrides 1A (Scheme 2).11 This reaction proceeded with alkenes 3 in the presence of catalytic PdCl2 and catalytic NaBr but in the absence of base in N-methylpyrrolidone (NMP) at 160 °C to afford various vinyl arenes 4. Unlike the conventional Mizoroki–Heck reaction, which requires a stoichiometric amount of base, the current non-basic system avoids the generation of stoichiometric amounts of salts as waste.
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Scheme 2 Decarbonylative Mizoroki–Heck reaction of benzoic acid anhydrides and alkenes.

In 2002, the Gooßen group disclosed the first example of a Pd-catalyzed decarbonylative Mizoroki–Heck type reaction using aromatic esters (Scheme 3A).13 They found that the addition of alkali metal salts was required to enable the reaction. Particularly, LiCl dramatically improved the product yields. Isoquinoline worked as an exogenous ligand to stabilize the Pd catalyst and realize this high-yielding reaction. Although various functional groups were tolerated under these conditions, an “activated ester” such as p-nitrophenyl benzoic esters 1B were required for the initial oxidative addition to take place. Typically, linear products 5 were obtained as the main products, and branched products 6 were formed as the by-products. Thereafter, they also found that enol esters can be used as substrates for this transformation, with the advantage of generating only volatile by-products such as acetone and CO (Scheme 3B).14 In this case, a tetraalkylammonium salt, [(n-Bu)3NC2H4OH]Br, was more effective than other alkali metal salts including LiCl. The substrate scope and yields were quite similar to those described in Scheme 3A.

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Scheme 3 (A) Decarbonylative Mizoroki–Heck reaction of aromatic esters 1B and alkenes. (B) Decarbonylative Mizoroki–Heck reaction of aromatic enol esters 1C and alkenes.

3.2. Alkyne insertion and addition type

In 2008, the Matsubara/Kurahashi group reported a Ni-catalyzed alkyne insertion/addition of phthalic anhydrides 1H and internal alkynes 7, providing isocoumarins 8 upon decarbonylation (Scheme 4A).28 The combination of Ni(cod)2 and PMe3 was the optimal catalyst. Lewis acids such as zinc salts and alkali metals, or quaternary ammonium salts were found to be necessary for the reaction, presumably increasing the electrophilicity of the carbonyl moiety. Among the investigated Lewis acids, ZnCl2 gave the best yields of decarbonylative products 8. With the optimized conditions, the reaction of phthalic anhydride and 4-octyne was performed to give the corresponding isocoumarin in 96% yield. Not only dialkylalkynes, but also aryl alkynes could undergo the addition reaction. Nevertheless, a mixture of regioisomers was obtained when substituted phthalic anhydrides 1H were used.
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Scheme 4 (A) Ni-Catalyzed decarbonylative isocoumarin synthesis. (B) A plausible mechanism for the Ni-catalyzed isocoumarin synthesis.

This reaction is presumably triggered by oxidative addition of one of the acyl C–O bonds of 1H to the nickel complex, followed by decarbonylation to form a 5-membered oxa-nickelacycle (Scheme 4B). Alkyne 7 would then insert into the aryl–nickel bond to produce a 7-membered nickelacycle. Reductive elimination from this intermediate would then give product 8 and regenerate the Ni0 species. The authors discovered that ZnCl2 has the role of promoting the reductive elimination step through in situ IR analysis experiments.

3.3. Suzuki–Miyaura type coupling

Currently, the coupling of organohalides and organoboron compounds, namely the Suzuki–Miyaura coupling, is widely utilized as one of the most reliable organic reactions.29 In 2004, Gooßen and a coworker developed a Rh-catalyzed Suzuki–Miyaura type biaryl synthesis with benzoic acid anhydrides 1A and aryl boroxins 9 in a decarbonylative fashion (Scheme 5).30 Although diarylketones (via a non-decarbonylative pathway) were also generated as side products, various substituted (Me, CF3, and CN) biaryls could be obtained in moderate yields. NO2 and Cl substituents, which sometimes impede the activity of transition metal catalysts, were tolerated under these reaction conditions. Moreover, heteroaromatic acid anhydrides could also be converted to the corresponding biaryls, albeit in low yields.
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Scheme 5 Rh-Catalyzed decarbonylative coupling between benzoic acid anhydrides and aryl boroxins.

In 2012, Wang and coworkers developed a cross-coupling of ethyl benzo[h]quinoline-10-carboxylates 1F with arylboronic acids 11 in the presence of a Rh catalyst and CuCl via chelation-assisted C–C bond activation (Scheme 6A).17 Various functionalities were compatible with the reaction conditions, resulting in good to excellent yields of the target products 12. In reactions involving decarbonylation, the dissociation of CO from the metal carbonyl complex to regenerate the active species requires significant energy due to the strong metal–CO bond, generally necessitating high reaction temperatures (usually >150 °C). In this system, DFT calculations suggested that CuCl plays a role in reducing the CO–metal dissociation energy by forming a thermodynamically stable copper carbonyl complex (Scheme 6B). Moreover, the initial oxidative addition step was proposed to occur directly with the C–C bond instead of the typical C(acyl)–O bond.

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Scheme 6 (A) Rh-Catalyzed coupling reaction of ethyl carboxylate and arylboronic acids via chelation-assisted C–C bond activation. (B) A plausible mechanism for the Rh-catalyzed coupling reaction of benzoquinolinecarboxylates and arylboronic acids.

In 2015, Itami, Yamaguchi, and Musaev reported a decarbonylative Suzuki–Miyaura cross-coupling of phenyl aromatic esters 1G using nickel catalysis (Scheme 7A).31 The coupling reaction, effected by an inexpensive Ni(OAc)2/Pn-Bu3 catalyst, exhibits a considerably broad substrate scope, offering a diversified biaryl library including natural product derivatives. In addition, this method can be performed on gram-scale. Not only aromatic esters and arylboronic acids, but also aliphatic esters (benzylic esters only) and alkenylboronic acids could be employed in this coupling. A one-pot protocol from carboxylic acids to biaryls was also established (Scheme 7B): treatment of 2-thiophenecarboxylic acid (13) with diphenyliodonium triflate (Ph2IOTf) and K2CO3 in toluene at 130 °C gave the corresponding phenyl ester 1Ga.32 After removing iodobenzene (by-product of the esterification reaction) under reduced pressure, arylboronic acid 11a, the nickel catalyst and Na2CO3 were added to the same reaction vessel to furnish the decarbonylative cross-coupling product 10a in 61% yield (over two steps).

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Scheme 7 (A) Decarbonylative organoboron cross-coupling of esters by nickel catalysis. (B) One-pot transformation of thiophene-2-carboxylic acid (13) to biaryl 10a.

Love and coworkers subsequently reported a similar reaction (Scheme 8).33 Treatment of phenyl benzoic esters 1G (particularly, phenyl azinecarboxylates) with a Ni/PCy3 catalyst and arylboronic acids 11 afforded biaryl products 10 in low to moderate yields, whereas phenyl 2-pyridinecarboxylates were not applicable in this reaction. When non-heteroaromatic esters were subjected to these conditions, coupling products were generated in low yields along with ketones (via a non-decarbonylative pathway) as by-products. It should be noted that the authors could achieve this transformation at lower temperatures (110 °C) by performing the reaction under a continuous flow of N2.

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Scheme 8 Ni-Catalyzed decarbonylative coupling of aromatic esters and arylboronic acids.

Despite significant advances in the decarbonylative Suzuki–Miyaura coupling, phenyl 2-azinecarboxylates were found to be less reactive in a nickel-catalyzed system. In 2016, Yamaguchi, Itami and coworkers solved this limitation by developing a coupling reaction between 2-azinecarboxylates 1G′ and arylboronic acids 11 using a Pd catalyst (Scheme 9A).34 Extensive ligand screening revealed that dcype [1,2-bis(diphenylphosphino)ethane] was exceptionally effective for this transformation,35 delivering 2-arylazines 10′ with good functional group compatibility. As the Pd(OAc)2/dcype catalyst works selectively for 2-azinecarboxylates, a sequential coupling of 2,4-pyridinedicarboxylate was performed (Scheme 9B). First, 2,4-pyridinedicarboxylate 1G′a was subjected to Pd-catalyzed decarbonylative Suzuki–Miyaura coupling, affording the expected 2-arylated pyridine 10′a in 61% yield with no observation of the other regioisomer. The resulting ester was then arylated in a decarbonylative Suzuki–Miyaura coupling with a Ni catalyst, producing 2,4-diarylpyridine 14 in 65% yield.

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Scheme 9 (A) Decarbonylative organoboron cross-coupling of 2-azinecarboxylates by a Pd/dcype catalyst. (B) Diarylpyridine synthesis by sequential decarbonylative coupling.

3.4. C–H arylation type

The C–H arylation of aromatic compounds has been extensively investigated over the past decade wherein coupling with electrophiles takes place without substrate pre-functionalization.36 In 2009, by utilizing a nitrogen-containing directing group strategy, Yu and coworkers achieved Rh-catalyzed direct C–H arylation and alkenylation reactions via decarbonylation of benzoic and cinnamic anhydrides 1A′ (Scheme 10A).37 Under catalytic conditions, benzo[h]quinoline or 2-phenylpyridine 15 were treated with acid anhydride 1A′ in o-xylene to furnish coupling products 16 bearing versatile functional groups such as Br, Cl, NO2 and OMe. The reaction mechanism proposed by Yu is shown below (Scheme 10B). The anhydride undergoes oxidative addition followed by decarbonylation to produce Rh complex 17. A base-promoted C–H rhodation then occurs, and reductive elimination from the resulting complex 18 gives the coupling product 16 and regenerates the Rh(I) catalyst.
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Scheme 10 (A) Rh-Catalyzed decarbonylative C–H arylation with acid anhydrides. (B) A plausible mechanism for the Rh-catalyzed decarbonylative C–H arylation using acid anhydrides.

In 2012, the Itami/Yamaguchi group discovered the first Ni-catalyzed decarbonylative C–H coupling of azoles with non-activated aromatic esters 1G (Scheme 11A).18 The nickel and dcype catalyst, which had enabled aryl C–O cleavage of phenol derivatives in their related work,35 was found to also mediate catalytic decarbonylative couplings between 1,3-azoles 19 and aromatic esters 1G. Various aromatic esters, particularly heteroaromatics such as thiophenes, furans, pyridines and thiazoles, can be employed in this reaction to afford the corresponding biaryl products 20. It should be mentioned that the decarbonylative alkenylation using α,β-unsaturated esters can also take place under similar conditions, which was successfully applied to a formal synthesis of a naturally occurring molecule.38

image file: c7cs00182g-s11.tif
Scheme 11 (A) Ni-Catalyzed C–H arylation of azoles with aromatic esters in a decarbonylative fashion. (B) A plausible mechanism for the decarbonylative Ni-catalyzed C–H arylation of azoles.

As depicted in Scheme 11B, the proposed catalytic cycle consists of (i) oxidative addition of the ester C–O bond of 1G to Ni0, (ii) CO migration onto the nickel center to produce an Ar–NiII(CO)n+1–OPh species (22, n = 0 or 1), (iii) C–H nickelation of azoles 19 (Het–H) with Ar–NiII(CO)n+1–OPh (22) to generate Ar–NiII(CO)n+1–Het (23), and (iv) reductive elimination to release the coupling product Het–Ar (20) and to generate a Ni0(CO)n+1 species (24). Finally, the active Ni0 catalyst could be regenerated by thermal extrusion of CO from 24.

Ni(dcype)(CO)2 (25) is indeed a competent pre-catalyst: 20a was obtained in 96% yield from the reaction between benzoxazole (19a) and phenyl thiophenecarboxylate (1Ga) in the presence of a catalytic amount of 25, indicating that 25 is a putative intermediate (Scheme 12A). The use of the stable and less expensive NiCl2 in combination with zinc powder also promotes the decarbonylative C–H coupling. Using this novel decarbonylative heterobiaryl synthetic method, a formal synthesis of muscoride A (27),39 a natural product with antibacterial activity, was achieved (Scheme 12B). To this end, a Ni-catalyzed decarbonylative coupling of 19b and 1Gb produced 20b in 39% yield. Subsequent hydrolysis and condensation with prenyl alcohol generated the key intermediate 26, from which the route to muscoride A (27) had been previously described.39b

image file: c7cs00182g-s12.tif
Scheme 12 (A) Alternative protocols for the decarbonylative C–H coupling, including the use of Ni(dcype)(CO)2 (25) (along with its X-ray crystal structure). (B) Formal synthesis of muscoride A (27) through Ni-catalyzed decarbonylative C–H coupling.

In 2013, the Gade group explored other nickel catalysts for the decarbonylative C–H coupling of 1,3-azoles (Scheme 13).20

image file: c7cs00182g-s13.tif
Scheme 13 Ni-Catalyzed decarbonylative C–H arylation of azoles (the unique Ni catalysts were synthesized by Gade and coworkers).

According to their previous synthetic methods for phosphine ligands,40 they synthesized Ni/bidentate phosphine complexes and applied them to the decarbonylative C–H coupling of benzoxazole (19a) with phenyl ester 1Ga, providing the desired product 20a in good yields. NMR- and mass spectrometry-enabled mechanistic studies supported the generation of an Ar–Ni–OPh intermediate, which would be formed by oxidative addition of the C(acyl)–O bond and subsequent decarbonylation. However, acyl–Ni–OPh intermediates could not be observed in any experiment, indicating a rapid decarbonylation step.

In 2016, the Yamaguchi and Itami groups applied the aforementioned Ni-catalyzed decarbonylative coupling (see Scheme 9) for the coupling of 1Gc and 9, and decarbonylative C–H coupling (see Scheme 11) for the coupling of the resulting 28 and 1G, followed by a decarboxylative [4+2] cycloaddition of 29 with α,β-unsaturated carboxylic acids 30 to the synthesis of triarylpyridines 31 (Scheme 14A).41

image file: c7cs00182g-s14.tif
Scheme 14 (A) Synthesis of triarylpyridines via decarbonylative couplings. (B) Formal synthesis of GE2270s.

By using this coupling/cycloaddition strategy, a formal synthesis of thiopeptide antibiotics GE2270s (34) was achieved (Scheme 14B). Two azole esters 1Gd and 28a were coupled under modified conditions to afford the corresponding diaryloxazole 29a in 49% yield. Next, 29a was reacted with thiazolyl acrylic acid 30avia a decarboxylative [4+2] cycloaddition to afford trithiazolylpyridine 31a. Subsequently, removal of the acetal group by trifluoroacetic acid (TFA) afforded 31a in 38% yield over two steps. 31a was then converted to 33 in four steps (acid-promoted transesterification, silylation/bromination, and thiazole formation with 32). Since the conversion of 33 to 34 had been described previously by Nicolaou and coworkers, the formal syntheses of GE2270s (34) were accomplished.42

3.5. Sonogashira type coupling

Coupling of alkynes with aryl halides (Sonogashira coupling) is widely employed in aryl alkyne synthesis to generate frameworks of organic materials.43 Recently, Yamaguchi/Itami developed a decarbonylative coupling between aromatic esters and terminal alkynes using a palladium/copper co-catalytic system, which represents the first example of C(sp2)–C(sp) bond formation in a decarbonylative fashion (Scheme 15A).44 In the presence of Pd(acac)2, CuI, and 3,4-bis(dicyclohexylphosphino)thiophene (dcypt),19 aromatic esters 1G were coupled with TIPS acetylene (35) to afford various alkynylated arenes 36 in moderate to good yields. Since conventional Suzuki–Miyaura coupling employing aryl halides under Pd catalysis is generally compatible with ester groups, orthogonal coupling reactions through Suzuki–Miyaura coupling and the developed decarbonylative Sonogashira type coupling was possible (Scheme 15B). First, pyridine derivative 37 was treated with arylboronic acid 11b to provide biaryl product 1Ge. Subsequent decarbonylative Sonogashira type coupling with TIPS acetylene (35) by Pd/Cu co-catalysis afforded alkynylated compound 36a in 46% yield.
image file: c7cs00182g-s15.tif
Scheme 15 (A) Decarbonylative Sonogashira type coupling of aromatic esters by a Pd/Cu co-catalyst. (B) Orthogonal coupling for the synthesis of multi-functionalized pyridine.

3.6. Silylation and borylation

With respect to the ease of functionalization and chemical diversification, silicon-containing compounds have been employed as useful intermediates in synthetic chemistry. While huge efforts have been made toward the development of novel and efficient methods for C–Si bond formation, only a few examples of decarbonylative silylation of aroyl compounds are known.45 In 2016, Rueping and coworkers established the decarbonylative silylation of aromatic phenyl esters 1G with silylboranes 37 by Ni/Cu co-catalysis (Scheme 16A).46 Under the reaction conditions, boryl, methoxycarbonyl and alkene moieties were well tolerated to give silylarenes 38. Furthermore, indolecarboxylates with an unprotected N–H bond were reacted to give the corresponding product in moderate yields. A wide substrate scope of silylboranes 37 was also demonstrated.
image file: c7cs00182g-s16.tif
Scheme 16 (A) Decarbonylative silylation of aromatic esters by a Ni/Cu co-catalyst. (B) A plausible mechanism for the decarbonylative silylation of aromatic esters.

The proposed catalytic cycle is depicted in Scheme 16B. First, oxidative addition of the phenyl ester 1G to nickel forms an acylnickel(II) phenolate 39. This complex then undergoes transmetallation with the copper silane species, generated by reaction of the copper salt with silylborane 37. After decarbonylation, reductive elimination of 41 releases the desired arylsilanes 38 and regenerates Ni(0).

Contemporaneously, the Shi group also reported a similar decarbonylative silylation of aromatic phenyl esters 1G by combining Ni and Cu catalysts (Scheme 17A).47 Dcype, a bidentate phosphine, was adopted as the optimal ligand in this reaction. The use of silylborane 37a as the silylating agent under co-catalytic conditions provided the desired products, including heteroaromatic compounds, with good functional group tolerance. Moreover, they achieved a Ni-catalyzed decarbonylative borylation of aromatic esters by using B2nep2 (42A) as well. Ni catalysts have advantages in terms of cost, and expensive catalysts like Pd or Ir and Rh require the borylation of aromatic compounds by halogen–boron exchange or C–H borylation.48 Additionally, from the standpoint of synthetic chemistry, diverse methods for the installation of readily convertible borons are desirable. In the present system, the utilization of ICy, an N-heterocyclic carbene ligand, enabled such a transformation with wide substrate scope. The use of NaCl was found to promote the reaction.

image file: c7cs00182g-s17.tif
Scheme 17 (A) Decarbonylative silylation by Ni/Cu catalyst and decarbonylative borylation by Ni catalyst. (B) Catalytic reaction with 44 and stoichiometric reaction with 45. (C) X-ray crystal structures of 44 and 45.

To probe the reaction mechanism, plausible intermediates 44 and 45 were synthesized and their structures were assigned by X-ray crystal analysis (Schemes 17B and C). Treating phenyl 1-naphthoate (1Gf) with a stoichiometric amount of Ni/ICy and NaOt-Bu as a base at room temperature produced 44, most probably generated upon oxidative addition followed by ligand exchange with a chloride anion. By heating complex 44 at 60 °C in toluene, decarbonylation proceeded smoothly to furnish the aryl nickel complex 45 in excellent yield. Conversely, when heating 45 in the same conditions but under CO atmosphere, 45 was re-converted into 44 in high yield. These experiments suggest that the decarbonylation step is a fast reversible process even at low temperatures. In addition, 44 was found to catalyze the reaction under standard conditions, whereas the stoichiometric reaction of 45 with borylating agent in 1,4-dioxane at 80 °C (lower temperature than the standard conditions) produced borylated product 43Af in good yields. Both results support the roles of 44 and 45 as intermediates in the borylation reaction.

Subsequently, the Rueping group reported the borylation reaction of aromatic phenyl esters 1G with B2pin242B using a Ni/Pn-Bu3 catalytic system (Scheme 18A).49 A variety of substituents were well tolerated such as t-Bu, methoxycarbonyl, OMe, and substituted amines. This Ni-catalyzed reaction was also applicable to the conversion of alkenyl and alkyl esters to the corresponding borylated compounds, albeit in slightly lower yields.

image file: c7cs00182g-s18.tif
Scheme 18 (A) Ni-Catalyzed decarbonylative borylation of aromatic esters. (B) Sequential coupling to generate borylated biaryl 43Ba.

In order to demonstrate the synthetic utility of this reaction, sequential couplings were performed, making use of the Pd-catalyzed ortho-selective C–H arylation of aromatic carboxylic acid 46 with phenyl iodide (47), followed by esterification (Scheme 18B). As such, borylated compound 43Ba was obtained in 47% yield over three steps through Ni-catalyzed decarbonylative borylation of the resulting phenyl ester with B2pin2.

3.7. Etherification

The Yamaguchi/Itami groups hypothesized that aromatic esters 1G could be converted to diaryl ethers 49via decarbonylation in the absence of transmetallating agent. After extensive screening, they found that nickel or palladium with dcypt or its analogue, 3,4-bis(dicyclopentylphosphino)thiophene (dcppt), allowed for the synthesis of a variety of 2- or 4-arenoxyazine derivatives in good to excellent yields (Scheme 19).50 Thus, a novel diaryl ether synthesis was accomplished, where the conventional approach is to perform an intermolecular cross-coupling reaction of an aryl halide and a phenol with a copper or palladium catalyst. A caveat for this reaction is that it only proceeds generally on aryl 2-azinecarboxylates.51
image file: c7cs00182g-s19.tif
Scheme 19 Decarbonylative diaryl ether synthesis.

3.8. Amination

Benzoic acid derivatives can usually be converted to aniline derivatives by a Schmidt type rearrangement. Very recently, the Rueping group developed a new aryl amine synthesis from benzoate esters 1G with imine 50 in a decarbonylative fashion using Ni catalysis (Scheme 20).36 To this end, esters 1G were coupled with imines 50 in the presence of a Ni catalyst to afford the corresponding coupling products, followed by acidic hydrolysis to give aryl amines in moderate to good yields and with functional group tolerance. The dcype ligand is also dramatically effective; otherwise, the reaction did not work at all. When secondary amines such as morpholine were used, amide formation occurred.
image file: c7cs00182g-s20.tif
Scheme 20 Ni-Catalyzed amination of aromatic esters.

3.9. Reduction

In 2001, the Murai group developed the first example of a catalytic reaction of esters involving C(acyl)–O bond cleavage followed by decarbonylation (Scheme 21).15 By using ammonium formate as the reducing agent, reductive decarbonylation of esters proceeded using a Ru catalyst. They utilized pyridylmethyl ester 1D, in which the pyridine moiety would function as a directing group, to accelerate the site-selective C(acyl)–O bond cleavage. Aromatic esters 1D with both electron-donating and withdrawing groups were reacted with HCO2NH4 to produce the reduced arene products 52 in good to excellent yields.
image file: c7cs00182g-s21.tif
Scheme 21 Ru-Catalyzed decarbonylative reduction of aromatic esters 1D.

In 2017, a Ni-catalyzed decarbonylative reduction of esters was accomplished by the Rueping group (Scheme 22).25 The hydrogen source was inexpensive and air-stable polymethyl-hydrosiloxane (PMHS), with which aromatic phenyl ester 1G can be reduced to aromatic compounds 52 in the presence of a Ni(OAc)2/dcype catalyst. Although both monophosphine and diphosphine ligands worked in this reaction, dcype was the optimal ligand for this transformation. Additionally, for a user-friendly, stable Ni(II) catalyst, Ni(OAc)2 can be used in this reduction. These reaction conditions can be applied for the removal of the amide moiety as well (see Scheme 37).

image file: c7cs00182g-s22.tif
Scheme 22 Reductive removal of esters by Ni catalysis.

4. Non-decarbonylative cross-coupling of aromatic esters

4.1. Suzuki–Miyaura type coupling

In parallel to decarbonylative coupling techniques, non-decarbonylative couplings of aromatic esters have been studied in the past decades. Generally, ketone synthesis from carboxylic acids goes through a Weinreb amide intermediate, and requires organometallic reagents. However, the use of organometallic reagents results in poor functional group tolerance. Because of this limitation, a new method with mild conditions for ketone synthesis from carboxylic acid derivatives using a transition metal catalyst had long been desirable.

In 2001, Gooßen and a coworker developed an acid anhydride-activated decarbonylative coupling of carboxylic acids with boronic acids using a Pd catalyst (Scheme 23).12 First, they observed that the reaction of aromatic acid anhydrides 1I with arylboronic acids 11 gives diaryl ketones 54. Subsequently, they attempted to avoid the stepwise preparation of anhydrides 1I and found that the combination of the carboxylic acid 53 and pivalic anhydride was effective for the generation of the active mixed anhydride intermediate. The reaction was compatible with various functional groups and gave the desired compounds in good yields.

image file: c7cs00182g-s23.tif
Scheme 23 Pd-Catalyzed synthesis of aryl ketones from carboxylic acids and boronic acids via acid anhydride formation. a Ligands are noted in parentheses.

Another approach for ketone synthesis via Pd-catalyzed cross-coupling of esters with organoboron compounds was reported by Chatani and coworkers in 2004 (Scheme 24).16 Owing to chelation assistance, 2-pyridyl esters 1E were smoothly converted to diaryl ketones in excellent yields under mild conditions. Various functionalities such as OMe, NO2, F, methoxylcarbonyl and Cl were well tolerated, and even o-substituted esters could be reacted to afford the corresponding products 54 in satisfactory yields. This catalytic system enabled them to employ aliphatic esters as starting materials. Moreover, aryl alkyl ketones can be synthesized by using alkylboron compounds under slightly modified conditions.

image file: c7cs00182g-s24.tif
Scheme 24 Pd-Catalyzed coupling reactions of esters and organoboron compounds for ketone synthesis. a The reaction was conducted at 100 °C with PCy3 instead of PPh3. 9-Benzyl-(9-BBN) was used as the alkylboron reagent. Phth = phthalimide.

Very recently, a collaborative work by Newman and Houk was reported, describing a Pd-catalyzed Suzuki–Miyaura coupling of aryl esters (Scheme 25).52 Esters 1G were coupled with arylboronic acids 11via C(acyl)–O bond cleavage using a Pd/IPr catalyst, furnishing diaryl ketones 54 without generation of decarbonylative products. A wide variety of diaryl ketones 54 can be synthesized with high functional group compatibility under these Pd catalytic conditions. Not only aromatic esters, but also aliphatic esters were applicable in this reaction. For example, when phenyl acetate was used as a starting material, acetophenone was obtained in excellent yield.

image file: c7cs00182g-s25.tif
Scheme 25 Ketone synthesis from esters by Pd/IPr catalysis.

Recently, it has been demonstrated that not only the C(acyl)–O bond but also the C(aryl)–O bond in esters are cleavable depending on the catalyst and the structure of the ester.53 In this catalytic system, DFT calculations showed that the C(acyl)–O bond cleavage is an energetically preferred pathway over the C(aryl)–O bond cleavage.53b,c

4.2. Amidation

In 2016, Garg and Houk explored the first example of methyl ester C(acyl)–O bond oxidative addition in nickel-catalyzed amidation reactions (Scheme 26A).54 Under the reaction conditions, methyl naphthalene-1-carboxylate derivatives (1J) were found to be more reactive with anilines 55 compared to methyl benzoate. The addition of Al(Ot-Bu)3 was found to have a crucial effect in enabling this transformation.
image file: c7cs00182g-s26.tif
Scheme 26 (A) Ni-Catalyzed amidation of methyl esters. (B) Orthogonal coupling via Buchwald–Hartwig amination, condensation and Ni-catalyzed amidation.

Mechanistic studies based on DFT calculations suggested that the coordination of Al(Ot-Bu)3 to the carbonyl group of 1J would decrease the kinetic barrier of the oxidative addition and of the ligand exchange process (OMe to NR1R2). Furthermore, the coordination of Al(Ot-Bu)3 specifically destabilizes naphthalene-1-carboxylate derivatives 1J because of steric repulsion between the aromatic ring and the bulky Lewis acid in the complex, explaining the exceptional reactivity of 1J.

As an application, the possibility to carry out orthogonal cross-couplings was demonstrated (Scheme 26B). Buchwald–Hartwig amination reaction between proline derivative 57 and aryl bromide 58 bearing a methyl ester moiety was performed, followed by acid-promoted selective hydrolysis of the tert-butyl ester to produce the desired product 59 in 49% yield over two steps. After condensation with different amino acids 60, the Ni-catalyzed amidation reaction with aniline 55a was conducted to give dipeptides 1Ja, with no epimerization taking place at any of the stereocenters.

Very recently, the Newman group reported an efficient amide formation of esters using Pd catalysis (Scheme 27).55 Typically, amide formation from activated esters such as pentafluorophenyl esters seems to work without a catalyst. However, the amidation did not work (only in trace amounts) without a catalyst when using phenyl ester 1G, since the phenyl ester is only a “weakly” activated ester. In the presence of a Pd/IPr catalyst, phenyl esters 1G were smoothly converted to the corresponding amides 62. Although only aryl amines 61 were applicable, aliphatic esters instead of aromatic esters 1G also work in this catalytic reaction.

image file: c7cs00182g-s27.tif
Scheme 27 Pd-Catalyzed amidation of aromatic esters.

5. Decarbonylative coupling of aromatic amides

5.1. Alkyne insertion and addition type

Amides, as with esters, are ubiquitous functional groups in organic chemistry. Despite extensive studies regarding the utility of amides as a directing group for regioselective functionalization,56 it has been difficult for chemists to use amides as intermediates in a synthetic sequence due to the robustness of the C(acyl)–N bonds.57

With the aim of rendering a C(acyl)–N bond reactive, Kurahashi and Matsubara revealed that a Ni/PMe3 catalyst could trigger the decarbonylative addition of phthalimides 2A onto internal alkynes 7 (Scheme 28A).58 Diverse isoquinolones 63 could be synthesized in good yields under these reaction conditions, although electron-deficient arenes were required as substituents on the nitrogen atom of 2A. The use of diarylalkynes in 7 was also tolerated, yielding multiply arylated isoquinolones.

image file: c7cs00182g-s28.tif
Scheme 28 (A) Ni-Catalyzed decarbonylative addition of phthalimides with alkynes. (B) A plausible mechanism for the decarbonylative addition reaction of phthalimides.

The first step of this decarbonylative addition reaction is thought to be the oxidative addition of the C–N bond of 2A to the nickel complex to give 64 (Scheme 28B). Upon decarbonylation, a 5-membered aza-nickelacycle 65 is formed, which undergoes alkyne insertion into the aryl C–Ni bond. Finally, reductive elimination proceeds from the resulting 7-membered ring complex 66 to produce isoquinolones 63 and Ni(0).

Matsubara and Kurahashi also reported the decarbonylative cycloaddition of phthalimides 2A′ with 1,3-dienes 67 catalyzed by nickel (Scheme 29).59 Investigation of substituent effects on the nitrogen atom revealed that phthalimides 2A′ requires an electron-withdrawing group, similarly to the above isoquinolone synthesis (see Scheme 28). Among them, an N-pyrrolyl group was found to be the most favorable substituent. This reaction proceeded in a regioselective fashion when the aryl group is substituted: the selectivity was thought to arise from the ease with which one of the two C(acyl)–N bonds is cleaved due to the effect of the substituent on the aromatic ring. For example, CF3 and OMe groups promoted nickel insertion into the closest C(acyl)–N bond due to an electron-withdrawing effect or a chelating effect, respectively. Meanwhile, a Me group interfered with nickel insertion into the closest C(acyl)–N bond, resulting in oxidative addition of the furthest C(acyl)–N bond.

image file: c7cs00182g-s29.tif
Scheme 29 Decarbonylative cycloaddition of phthalimides with 1,3-dienes catalyzed by nickel.

5.2. Negishi type coupling

In 2011, Johnson and coworkers reported the development of a Ni-mediated decarbonylative cross-coupling of phthalimides 2A with diorganozinc reagents 69 (Scheme 30A).60 A broad range of phthalimides were converted to ortho-substituted amides 70 in good to excellent yields in the presence of Ni and 2,2′-bipyridine (bipy) in 1,4-dioxane/THF at 95 °C. Although good functional group tolerance was observed, the present reaction system requires a stoichiometric amount of nickel and bipy. The absence of turnover under catalytic conditions suggested a high kinetic barrier to dissociate CO from the Ni–CO complex, making the regeneration of the active species challenging.61,62
image file: c7cs00182g-s30.tif
Scheme 30 (A) Ni-Mediated decarbonylative cross-coupling of phthalimides with diorganozinc reagents. (B) Ni-Catalyzed decarbonylative cross-coupling of phthalimides with diorganozinc reagents.

In order to achieve the catalytic coupling of phthalimides and organozinc without the need of raising the reaction temperature, further efforts have been made by Johnson and coworkers in 2016 (Scheme 30B).63 They eventually discovered that Ni/bipy could catalyze the decarbonylative coupling reaction at 95–100 °C just by introducing an electron-withdrawing group onto the nitrogen atom of the phthalimide, such as Ts, electron-deficient aromatics, and even OH. These results indicate that CO dissociation from the Ni complex is probably taking place from a nitrogen-coordinated nickel center.

5.3. C–H arylation type

In 2012, Wang and coworkers developed a decarbamoylative C–H arylation of 1,3-azoles 19a with aromatic amides 2J using a Pd catalyst in the presence of oxidants (Scheme 31).21a Phenanthroline (phen) was the effective ligand and K2S2O8 was the optimal oxidant in this transformation. Substituent effects on the amide moiety were examined, uncovering the importance of N–H and N–X (X = OMe, NH2 and alkyl) bonds. Especially, N-methoxybenzamide 2B served as the most suitable arylating agent.
image file: c7cs00182g-s31.tif
Scheme 31 Ni-Catalyzed decarbonylative cross-coupling of phthalimides with diorganozinc reagents.

This reaction exhibited high functional group compatibility, such as Cl, CN, and ester groups (Scheme 32A). Furthermore, various heteroaryls were well tolerated under these conditions, realizing a diverse biaryl synthesis.

image file: c7cs00182g-s32.tif
Scheme 32 (A) Pd-Catalyzed decarbonylative arylation of azoles with aromatic amides. (B) A plausible mechanism for the Pd-catalyzed decarbonylative arylation of azoles with aromatic amides.

A Pd(II)/Pd(0) catalytic cycle was proposed for this reaction (Scheme 32B). In the first step, ligand exchange gives complex 71, which undergoes decarbonylation to produce Ar–Pd–X intermediate 72. Carbopalladation of the C–N double bond of 1,3-azole 19 then proceeds, followed by β-hydride elimination from 73 to furnish biaryl compound 20. K2S2O8 eventually reoxidizes the generated Pd(0) to Pd(II).

Recently, the Szostak group demonstrated a Rh-catalyzed decarbonylative C–H arylation of 6-membered arenes and aromatic “amides” by using a directing group strategy (Scheme 33).64 In the presence of catalyst, twisted amides bearing a 6-membered ring such as in glutarimide amides 2D, whose amide C–N bond strength is decreased by conjugating the nitrogen lone pair with the leaving group, can be coupled with benzo[h]quinoline or 2-phenylpyridine 15 to give the corresponding coupling product 16via decarbonylation in the absence of bases. The use of twisted amides 2D is critical for this transformation, as other amides such as Boc-amides and sulfonyl amides did not work at all. Although this reaction is similar to Yu's results (see Scheme 10), this is the first example of C–H arylation of twisted amides with 6-membered arenes by double C–H/C–N bond activation.

image file: c7cs00182g-s33.tif
Scheme 33 Rh-Catalyzed C–H arylation of 6-membered arenes with twisted amides.

5.4. Mizoroki–Heck type coupling

As mentioned above, coupling reactions triggered by oxidative addition of amide C–N bonds have been largely unexplored due to the strong nN → πC[double bond, length as m-dash]O* conjugation. In 2015, a decarbonylative Mizoroki–Heck type reaction of aromatic glutarimide amides 2D was reported by the Szostak group (Scheme 34A).65 In order to achieve a successful C(acyl)–N bond activation, they investigated a variety of amides and found that the Mizoroki–Heck type reaction was facilitated by using twisted amides 2D. The Mizoroki–Heck type reaction proceeded without exogenous ligand but in the presence of LiBr in NMP at 160 °C to provide vinyl arenes 4 in excellent yields with high functional group compatibility. A gram-scale synthesis was possible under the optimized conditions.
image file: c7cs00182g-s34.tif
Scheme 34 (A) Decarbonylative Heck reaction of aromatic amides and alkenes. (B) A plausible mechanism for Pd-catalyzed decarbonylative Heck reaction of aromatic amides. (C) Decarbonylative Heck reaction of N-acylsaccharins and alkenes.

A proposed catalytic cycle is shown in Scheme 34B. Sequential oxidative addition of the C(acyl)–N bond/decarbonylation takes place in the first step. Migratory insertion of alkene 3 into Ar–Pd–X 74 then occurs, followed by β-hydride elimination from the resulting 75, affording the corresponding vinyl arene 4 and Pd(II) species. Finally, base-promoted reductive elimination regenerates Pd(0). Szostak and coworkers also demonstrated that N-acylsaccharin amides 2F were also applicable in the decarbonylative Mizoroki–Heck reactions (Scheme 34C).66

5.5. Suzuki–Miyaura type coupling

In 2016, the Szostak group also explored a decarbonylative Suzuki–Miyaura cross-coupling of glutarimide amides 2D catalyzed by nickel (Scheme 35).67 Improvement in product yields was observed when Ni(PCy3)2Cl2 was used as a precatalyst. Various functional groups such as CF3, OMe and ketone moiety were tolerated, as well as heteroaromatics, providing biaryls 10 in good yields.
image file: c7cs00182g-s35.tif
Scheme 35 Pd-Catalyzed decarbonylative Suzuki–Miyaura coupling reaction of aromatic amides and boronic acids.

5.6. Reduction

In 2017, Maiti and coworkers discovered a Ni-catalyzed decarbonylative reduction of amides using tetramethyldisiloxane 76 as the hydride donor (Scheme 36).24
image file: c7cs00182g-s36.tif
Scheme 36 Ni-Catalyzed decarbonylative reduction of amides to aromatic hydrocarbons.

Amides whose nitrogen atom is part of a pyrazole moiety (compounds 2I) were converted to the reduced products 52 in excellent yields under nickel catalysis in toluene at 130 °C. Screening of ligands revealed that a monodentate phosphine ligand facilitates the reduction to produce the corresponding hydrocarbons.

Very recently, the Rueping group also reported a Ni-catalyzed decarbonylative reduction (Scheme 37).25 They utilized PMHS as the hydrogen donor, and the reaction was conducted under the same conditions (Ni(OAc)2/dcype) as with the reduction of esters (see Scheme 22). Glutarimide amides 2D were applicable for this reaction with wide functional group compatibility including heteroaromatics even though high temperatures (170 °C) were required.

image file: c7cs00182g-s37.tif
Scheme 37 Ni-Catalyzed decarbonylative reduction of twisted amides.

5.7. Borylation

In 2016, Shi and coworkers established a new method for borylation with amides and B2nep2 in a decarbonylative fashion under Ni catalysis (Scheme 38A).68 An N-heterocyclic carbene ligand, ICy, was found to enable this transformation in toluene at 150 °C. Unlike in the above-mentioned Pd-catalyzed Mizoroki–Heck and Suzuki–Miyaura type couplings of amides, six-membered amides such as 2D were not suitable substrates. In contrast, the combination of a Boc group and an alkyl or aryl group was found to be necessary for the activation of the inert C–N bond by the Ni/ICy catalyst. Under optimized conditions, this coupling using Boc-amides 2G and B2nep242A exhibited a large substrate scope.
image file: c7cs00182g-s38.tif
Scheme 38 (A) Ni-Catalyzed decarbonylative borylation of amides. (B) Catalytic reaction with 77 and stoichiometric reaction of 78.

In order to shed light on the reaction mechanism, the authors attempted to isolate the key intermediates (Scheme 38B). The reaction of 2Ga with stoichiometric Ni(cod)2, ICy·HCl, NaOt-Bu and K3PO4 in toluene/c-hexane cosolvent system at 35 °C yielded acylnickel(II) chloride complex 77 in 59% yield. Upon mild heating in toluene, 77 could be fully converted to the decarbonylated complex 78, with both structures 77 and 78 confirmed by X-ray crystallography. The observation of decarbonylation at low temperatures suggests that this step is not the rate-determining step. Treatment of a stoichiometric amount of 78 with B2nep2 at 60 °C in toluene/c-hexane in the presence of K3PO4 afforded the borylated product 43Aa in 82% yield, whereas no product was observed without K3PO4, indicating the involvement of the base in the transmetallation and/or reductive elimination. Moreover, catalytic reactions employing 77 as the catalyst precursor proceeded well even at 120 °C. These experiments support the existence of both 77 and 78 as intermediates in the decarbonylative borylation.

5.8. Amination

Only one example of decarbonylative amination of amides with imines was reported by the Rueping group (Scheme 39).26 Employing glutarimide amide 2Da, imine 50 was reacted under Ni catalysis, followed by hydrolysis to give 2-naphthylamine 51a in 54% yield. Dppf was the most effective ligand for this transformation, whereas dcype was best for the amination of esters (see Scheme 20).
image file: c7cs00182g-s39.tif
Scheme 39 Ni-Catalyzed amination of aromatic esters with imines.

6. Non-decarbonylative coupling of aromatic amides

6.1. Suzuki–Miyaura type coupling

In 2015, Zou's group achieved a Pd-catalyzed Suzuki–Miyaura type ketone synthesis using tosyl benzamides 2E with arylboronic acids 11 using a Pd/PCy3 catalyst (Scheme 40).69 Subsequently, the Szostak group discovered a similar transformation of twisted amides 2D with arylboronic acids 11. The Szostak group also reported that different amides bearing 6-membered imide groups (2D),70 saccharin (2F)71 or di-Boc groups (2H)72 were compatible with this transformation, delivering diverse diarylketones 54 with high functional group tolerance. In all cases, acids such as H3BO3 were found to promote the reaction. In the case of benzamides with di-Boc groups, Lewis bases, especially NEt3, could be used as promoters as well. Very recently, Pd/IPr catalysis was applied to glutarimide amides 2D, tosyl amides 2E, and Boc-amides 2G in the ketone synthesis with high turnover numbers by the Szostak group.73
image file: c7cs00182g-s40.tif
Scheme 40 Summary of Pd-catalyzed ketone synthesis with aromatic amides.

Nickel also enables catalytic cross-coupling involving inactive C–N bond cleavage of amides, much like palladium.74 Garg and coworkers successfully used a Ni catalyst for the Suzuki–Miyaura type coupling of amides bearing a Boc-activating group to form diaryl ketones (Scheme 41A).75,76 The reaction catalyzed by Ni/SIPr in the presence of K3PO4 and water in toluene at 50 °C proceeded to furnish a wide variety of ketones 54 in good to excellent yields. Diarylketones 54 containing a heteroaromatic moiety were also accessible under these reaction conditions.

image file: c7cs00182g-s41.tif
Scheme 41 (A) Ni-Catalyzed ketone synthesis with aromatic amides. (B) Sequential catalytic ketone synthesis and etherification.

Ni/SIPr can catalyze the Suzuki–Miyaura type coupling with selective C(acyl)–N bond cleavage of amides (Scheme 41B). Diamides 2Gb could be converted to the corresponding ketone 54a, showing that only the N-Boc-N-Bn substituted amide group is reactive. After Boc-activation of the remaining amide group, their developed amide esterification procedure (see Scheme 44)22 with (−)-menthol (80) using a Ni catalyst proceeded to afford 81 in 57% yield for the two-step operation.

6.2. Negishi type coupling

In 2016, Garg and coworkers showed that the Ni/SIPr system is an effective catalyst for the formation of alkyl aryl ketones 83via Negishi type coupling of amides (Scheme 42A).77 Not only primary organozinc reagents but also sterically hindered secondary organozinc reagents could be used as alkylating agents.
image file: c7cs00182g-s42.tif
Scheme 42 (A) Negishi type ketone synthesis with amides and alkylzinc. (B) Gram-scale synthesis of 83a in Pfizer's synthesis of 84.

To showcase its synthetic utility, this Negishi type ketone synthesis was applied to a gram-scale synthesis of intermediate 83a in Pfizer's synthesis of glucagon receptor modulator 84 (Scheme 42B). To this end, Boc-amide 2Gc was coupled with cyclohexylzinc chloride 82a in the presence of Ni catalyst to afford 83a.

In the same year, the Szostak group also discovered a Ni-catalyzed Negishi coupling reaction between benzamides 2H with two Boc groups on the nitrogen and arylzinc agents 82 at room temperature (Scheme 43).78 High functional group compatibility was exhibited, resulting in the formation of various ketones 54.

image file: c7cs00182g-s43.tif
Scheme 43 Negishi type ketone synthesis with amides by Ni catalysis.

6.3. Esterification

The classical esterification of amides has often been carried out under harsh conditions with the need for excess amounts of alcohol. Ni-catalyzed amide-to-ester conversion via the activation of amide C(acyl)–N bonds was reported by Garg and Houk as a new esterification methodology (Scheme 44A).22,79 It should be emphasized that this reaction requires neither base nor excess amounts of alcohol. Through DFT experiments, the authors calculated the Gibbs free energy over the entire reaction and the barrier of the rate-determining oxidative addition step. The results indicate that these energies depend on the amide N-substituents. Among the possible amide substrates, N-methyl-N-phenylbenzamide was found to be the most favorable one, computationally and experimentally. The substrate scope is illustrated in Scheme 44A. Alcoholysis of N-alkyl-N-phenylbenzamides 2C catalyzed by Ni/SIPr provided aromatic esters 86 with a variety of functionalized alcohols 85 such as cyclopropyl or N-protected pyrrolidine derivatives.
image file: c7cs00182g-s44.tif
Scheme 44 (A) Ni-Catalyzed esterification of amides. (B) Chemoselective esterification. (C) A plausible mechanism for Ni-catalyzed esterification of amides.

Also, biologically relevant saccharide- and estrogen-based esters could be obtained in excellent yields using this method.

To test the chemoselectivity, a proline-containing diamide was treated with (−)-menthol (80) under the Ni catalytic system, affording the desired product in 83% yield upon selective C–N bond scission (Scheme 44B). It is notable that no epimerization occurred during this process.

A plausible mechanism for the esterification is depicted in Scheme 44C. Oxidative addition of the C(acyl)–N bond of 2C to Ni(0), followed by ligand exchange of 88 with 85 gives an acyl–Ni–OR intermediate 89. Finally, reductive elimination of 89 releases the corresponding ester 86 and regenerates Ni(0).

6.4. Transamidation

Transamidation reactions are a hardly explored transformation due to the difficulty of efficient conversion.80 However, Ni/SIPr could also enable transamidation reactions via the activation of secondary amides by Boc groups (Scheme 45).81 Under mild reaction conditions (base-free and low temperatures), Boc-activated aromatic amides 2G were reacted with secondary amines 90 to give the corresponding products 91 in excellent yields. Primary aniline derivatives could be used as substrates as well. Moreover, heteroaromatics were well tolerated under the reaction conditions.
image file: c7cs00182g-s45.tif
Scheme 45 Transamidation of Boc-activated amides by Ni/SIPr catalysis.

A two-step approach to achieve secondary amide transamidation by nickel catalysis is highlighted in Scheme 46. As the first step, indolylamides 92 were treated with Boc2O to activate the amide moiety, affording the corresponding intermediate in quantitative yields. Subsequently, transamidation reactions with a range of nucleophiles derived from amino acids were performed to form secondary or tertiary amides 91a in excellent yields. It is noteworthy that the resulting products were generated with complete retention of stereochemistry.

image file: c7cs00182g-s46.tif
Scheme 46 Two-step approach for the transamidation.

Szostak's group has also reported the transamidation reaction of Boc-activated amide under Pd/IPr catalysis (Scheme 47).82 The use of Pd/NHC complex enhanced the reaction efficiency. In this reaction, not only aniline derivatives but also alkyl amines were applicable as nucleophiles.

image file: c7cs00182g-s47.tif
Scheme 47 Transamidation of Boc-activated amides by Pd/IPr catalysis.

7. Conclusions

The utilization of aromatic esters and amides as synthetic equivalents of aryl halides in cross-coupling offers advantages because they can be readily prepared, are stable, and generally have low toxicity. In comparison with aryl halides and aryl sulfonates, they are environmentally benign building blocks when considering the resulting waste. In addition, they offer simple solutions for transformations that usually require several steps and/or the use of strongly basic organometallic reagents. As highlighted in this review, significant progress in the catalytic reactions of aromatic esters and amides has been made in recent years. Owing to the advances in this field, esters and amides have been gradually recognized as practical electrophiles. Now, we all synthetic chemists can conduct the various transformations of aromatic esters and amides with wide functional group compatibility and high turnover numbers. Although this coupling reaction is still associated with significant drawbacks such as the requirement of high temperatures and carefully engineered C(acyl)–O or C(acyl)–N bond activation, new catalysts will continually be made to address these issues in the near future. It is our hope that endeavors to refine aromatic ester/amide cross-coupling techniques will accelerate the development of new catalysts and promote the understanding of new reaction mechanisms, providing far-reaching benefits for all synthetic chemists.


This work was supported by JSPS KAKENHI Grant No. JP16H01011, JP16H04148, JP16K13085, and 16H01140 (to J. Y.), JP16H07291 (to K. M.), and a JSPS research fellowship for young scientists (to T. S.). We thank Dr Yoshihiro Ishihara (Vertex Pharmaceuticals) for fruitful discussion and critical comments. This work is conducted as part of the Research Institute for Science and Engineering, Waseda University.

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Dedicated to Professor Teruaki Mukaiyama in celebration of his 90th birthday (Sotsuju).

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