Transition-Metal-Catalyzed Monoarylation of Ammonia - ACS

Review Cite This: ACS Catal. 2018, 8, 405−418

pubs.acs.org/acscatalysis

Transition-Metal-Catalyzed Monoarylation of Ammonia Johannes Schranck*,† and Anis Tlili*,‡ †

Solvias AG, Römerpark 2, 4303 Kaiseraugst, Switzerland Institut de Chimie et Biochimie Moléculaires et Supramoléculaires, Université Claude Bernard Lyon 1, CNRS UMR 5246, 43 Boulevard du 11 Novembre 1918, 69100 Villeurbanne, France

‡

ABSTRACT: The direct monoarylation of ammonia for the synthesis of aniline derivatives constitutes a significant challenge in modern synthetic chemistry. Over the past decade, major efforts have been made in order to develop highly active and selective catalyst systems for this transformation. More specifically, the application of various transition-metal catalysis has enabled substantial progress in substrate scope and reaction conditions as well as catalyst costs and availability. This review describes these advancements focusing on palladium-, copper-, and nickel-based catalyst systems that have been reported until mid-2017.

KEYWORDS: ammonia, anilines, cross-coupling, transition metal, catalysis

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INTRODUCTION Ammonia ranks among the most produced chemicals. Industrially, it is manufactured from dinitrogen and dihydrogen in the Haber−Bosch process. This process is considered as one of the most important technological advances of the 20th century, which has had a significant impact on the increasing growth of the earth’s population. Ever since, it has been a persistent goal to make use of ammonia as an inexpensive and abundant nitrogen source in chemical synthesis. In this regard, the direct arylation of ammonia to form aryl amines is a very attractive reaction. This approach is particularly useful for functionalized (hetero)aromatics that are incompatible with the traditional two-step nitration−hydrogenation process that is used for the production of simple aniline from benzene. Various reaction sequences make use of the resulting aniline derivatives as relevant intermediates for the manufacture of industrial products such as agrochemicals, dyes, and pharmaceuticals.1 Initiated by the groundbreaking work of Buchwald and Hartwig, the transition-metal-catalyzed amination of aryl halides has emerged as one of the most widely applied tools for the construction of arylamines (Figure 1).2 However, the selective monoarylation of ammonia represents a particular challenge since the strong coordination of ammonia to transition metals can lead to the formation for unreactive complexes. Additionally, ammonia possesses a strong N−H bond (107 kcal/mol)3 which makes its activation challenging. The reductive elimination from the intermediate amido complex is recognized to be complicated. Finally, the formed monoarylated products are prone to act as contending nucleophiles to ammonia and in turn lead to the formation of polyarylated products. Consequently, in the past ammonia surrogates needed to be employed followed by a subsequent © XXXX American Chemical Society

deprotection step. For example, the industrial syntheses of the fungicides Sedaxane and Solatenol make use of palladiumcatalyzed benzylamination followed by heterogeneous catalytic cleavage of toluene due to the lack of efficient protocols for direct ammonia monoarylation at the time these processes were developed.4 In an effort to overcome these challenges, the direct and selective arylation of ammonia has been investigated intensively over the past decade. As a result, several methodologies have been disclosed using palladium-, copper-, or nickel-based catalysts which are summarized herein.2f,5 1. Palladium-Catalyzed Monoarylation of Ammonia. The first catalytic reactions for the coupling of ammonia with aryl halides were disclosed by Shen and Hartwig in 2006. The authors demonstrated that aryl iodides, bromides, chlorides, and aryl triflates can be coupled with ammonia gas (5.5 bar) in very good to excellent yield with excellent selectivity toward the corresponding monoaryl amine.6 The key to success is the use of a preformed Josiphos (L1) palladium(II) complex that enables low catalyst loadings (1 mol %). The use of the strong base NaOtBu is essential, and the reactions were performed in DME at 90 °C (Scheme 1). Notably, solid LiNH2 could also be employed as nucleophile to yield the corresponding aniline derivatives under avoidance of gaseous ammonia. Shortly after, the same group7 demonstrated that the association of [Pd(P-otol3)2] and CyPF-tBu Josiphos ligand as catalyst allows for the formations of anilines with only 5 equiv of ammonia gas, and the reaction occurs with catalyst loadings as low as 0.1 mol %. Received: September 24, 2017 Revised: November 23, 2017 Published: November 26, 2017 405

DOI: 10.1021/acscatal.7b03215 ACS Catal. 2018, 8, 405−418

Review

ACS Catalysis

Figure 1. Synthesis of anilines from (hetero)aryl halides.

Scheme 1. Palladium-Catalyzed Arylation of Ammonia Developed by the Group of Hartwig

Scheme 2. Mechanistic Investigation for the Palladium-Catalyzed Amination of Aryl Halides by the Group of Hartwig

groups in ortho or para position), the subsequent reductive elimination is very slow and occurs over 8 h up to 3 days. In 2007, the group of Buchwald10 introduced the use of biarylphosphane L2 (t-BuDavePhos) in the arylation of ammonia (5 equiv). The catalytic system is based on the use of the commercially available precursor [Pd2(dba)3] (1 mol %) and was applied to phenyl chloride and four different aryl bromides (Scheme 3). The reactions were performed in dioxane at 90 °C. Interestingly, the authors demonstrated that the in situ formation of symmetrical as well as

The same catalyst system is also effective for the coupling of ammonium salts, including ammonium sulfate as an interesting surrogate to ammonia.8 In order to shed some light on the reaction mechanism of the CyPF-tBu Josiphos/palladium catalyzed amination of aryl halides, the group of Hartwig9 studied stoichiometric reactions (Scheme 2). Whereas the formation of the amido complexes occurs very fast within 30 min for differently substituted aryl halides (substituted with electron donating methyl or methoxy 406


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ACS Catalysis Scheme 3. Palladium-Catalyzed Arylation of Ammonia Developed by the Group of Buchwald

Scheme 4. Palladium-Catalyzed Arylation of Ammonia Developed by the Group of Beller

Scheme 5. Palladium-Catalyzed Arylation of Ammonia Developed by the Group of Stradiotto

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ACS Catalysis Scheme 6. Palladium/Mor-Dalphos-Catalyzed Monoarylation of Ammonia Developed by the Group of Stradiotto

temperature between 65 and 100 °C with high yield and excellent selectivity toward the formation of anilines (Scheme 6). Interestingly, this catalyst system promotes the transformation of aryl tosylates at room temperature. Moreover, the Pd/MorDalphos system enables remarkable chemoselectivities as aryl chlorides containing N−H functionalities (primary or secondary amines) can be converted into the desired anilines with excellent yields. Finally, in order to gain more detail on the active catalytic species, the authors isolated a Pd(II) complex by treating [Pd(cinnamyl)Cl]2 with 2 equiv of ligand in the presence of chlorobenzene and NaOtBu. The resulting complex C1 could be isolated in quantitative yield and X-ray crystallographic studies confirmed coordination of L5 to palladium in a κ2-P,N fashion. No reaction was observed upon treatment of C1 with ammonia. However, in the presence of silver triflate and ammonia, the cationic adduct C2 could be obtained at room temperature. The latter complex underwent reductive elimination in the presence of NaN(TMS)2 to yield the corresponding aniline. In a subsequent report, the group of Stradiotto described the ability of complex C1 to catalyze the monoarylation of ammonia with aryl chloride at room temperature.14 In 2011, the group of Beller15 disclosed an imidazoliumphosphane ligand that, in association with Pd(OAc)2, is an effective catalyst for the arylation of ammonia with aryl bromides and chlorides. Interestingly, the formed aniline could be precipitated under acidic conditions and the remaining solution was recharged with the starting aryl halide, ammonia as

unsymmetrical di- and triarylamines could be easily accessed through optimization of the reaction conditions and sequential additions of different aryl halides. After the seminal work of the groups of Hartwig and Buchwald, other groups investigated the synthesis of anilines under palladium catalysis through rational ligand design. In this regard, the group of Beller11 developed N-phenylpyrrole/ indole-substituted and the more sterically demanding imidazole-based phosphines for the arylation of ammonia. Interestingly, an ammonia solution in dioxane (0.5 M, 5 equiv) was used. Among the tested ligands, dialkyl-2-(N-arylimidazolyl)phosphines (i.e., L3) turned out to be the most efficient ones for the transformation of aryl bromides and chlorides into their corresponding anilines (Scheme 4). In 2010, the group of Stradiotto12 reported new P,Nsubstituted phenylene ligands for the arylation of ammonia. Aryl chlorides underwent amination in very good to excellent yield when L4 was applied in association with [Pd(allyl)Cl]2 (1 mol %). Both of the latter methodologies are based on the use of the strong base NaOtBu, and the reactions were performed in dioxane at 110−120 °C (Scheme 5). In order to discover a more efficient system for the arylation of ammonia, the group of Stradiotto13 investigated a series of air-stable phenylene-bridged P,N-ligands featuring the bulky di(1-adamantyl)phosphino (P(1-Ad)2) fragment. Among the tested ligands, L5 bearing a morpholino group (MorDalphos) gave the best results. The authors demonstrated that a large variety of aryl chlorides can be cross-coupled to ammonia at 408


Review

ACS Catalysis Scheme 7. Recyclable Palladium-Based Catalyst for the Arylation of Ammonia Developed by the Group of Beller

Scheme 8. Discrete Pd-NHC Complexes for the Arylation of Ammonia Developed by the Group of Organ

Scheme 9. Synthesis of Indoles through Monoarylation of Ammonia Developed by the Group of Stradiotto

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ACS Catalysis Scheme 10. Copper-Catalyzed Arylation of Ammonia Developed by the Group of Lang

Scheme 11. Copper-Catalyzed Arylation of Ammonia Developed by the Groups of Taillefer, Chang, and Ma

equipment. Because it is a gas at ambient temperature and pressure, ammonia often makes high-pressure reactors mandatory. In many cases, a commercially available solution of 0.5 M ammonia in dioxane is used. However, this solution is expensive and its ammonia content decreases over time. In this respect, a number of inexpensive, abundant and solid ammonia surrogates have been evaluated for the amination of aryl chlorides by Green and Hartwig.8 In particular, NH4Cl, NH4OAc and (NH4)2SO4 were tested in combination with various reported palladium catalysts. It was found that especially NH4Cl lacks compatibility with many reported palladium catalysts systems. However, a catalyst system composed of Pd[P(o-tol)3]2 and CyPF-tBu Josiphos proved highly versatile and allowed for remarkably low catalysts loadings (0.3 mol %) in combination with ammonia and all three tested ammonium salts. 2. Copper-Catalyzed Monoarylation of Ammonia. The copper-catalyzed synthesis of anilines through the arylation of ammonia or ammonia surrogates has been known since the 1980s. However, early reports relied on the employment of (over)stoichiometric amounts of copper salt, the use of activated starting materials as well as the requirement of harsh reactions conditions, all of which led to a limited

well as the base to observe similarly excellent reactivities even after three consecutive cycles (Scheme 7). Consequently, the employment of ligand L6 facilitates a catalyst recycling feature. The group of Organ16 introduced the first phosphine-free palladium catalyst for the monoarylation of ammonia. By applying discrete Pd-NHC (NHC = N-heterocyclic carbene) complexes, a variety of aryl amines were prepared with very good to excellent yields starting from aryl bromides and chlorides (Scheme 8). The reactions were performed in dioxane at 100 °C. Notably, both complexes, C3 and C4, are air- and moisture-stable. Whereas ortho-substituted aryl halides resulted in low yields in the presence of C3, the less hindered complex C4 enabled an excellent reaction outcome for those challenging substrates. In an effort to amplify the scope of ammonia monoarylation, the group of Stradiotto17 succeeded to incorporate this methodology into a sequential synthesis of indoles (Scheme 9). Starting from ortho-alkynyl aryl bromides, initial amination is followed by an intramolecular hydroamination to form the desired indoles. Among the tested ligands, Josiphos L1 was found suitable leading to moderate to excellent yields. Albeit anhydrous ammonia is inexpensive and abundant, its physical properties make a high demand on the process 410


Review

ACS Catalysis Scheme 12. Developed Ligands for Copper-Catalyzed the Arylation of Ammonia

applicability.18 As the extensive and continuing development of the Buchwald−Hartwig amination has intensified the interest in a more cost-effective process and the replacement of palladium by a cheaper and more abundant metal has been an ongoing challenge in ammonia coupling. Thus, the renaissance of copper chemistry in the past decade has revealed that the presence of particular additives bearing chelating groups enhance the reaction’s outcome.19 In this context, researchers from the Merck laboratories20 (group of Lang) developed the first catalytic version of the amination of aryl halides under fairly mild conditions (80−100 °C). By using ethylene glycol as solvent, which may also serve as ligand to copper, iodobenzene and mainly heterocyclic bromoarenes could be converted into their aniline correspondents with very good to excellent yields (Scheme 10). Nevertheless, selectivity issues were observed because the formation of the desired product was accompanied by the arylation of ethylene glycol in several cases (ratio up to 4/1). More recently, breakthroughs have been reported independently by the groups of Taillefer,21 Chang,22 and Ma23 who revealed two general methodologies for the direct synthesis of anilines from aryl halides. The key to success has been the application of an ancillary ligand in a copper-catalyzed protocol. On the one hand, the group of Taillefer used L7 (2,4pentandione) in conjunction with commercially available copper(II) acetylacetonate (Scheme 11, eq 1). This catalyst

system is efficient for the conversion of aryl iodides as well as more challenging aryl bromides with aqueous ammonia into their corresponding anilines in moderate to excellent yields. The reactions were performed with cesium carbonate as base in DMSO at 60−90 °C. On the other hand, the methodology developed by the group of Chang is based on the use of Lproline L8 as ligand in conjunction with copper(I) iodide (Scheme 11, eq 2). Making use of this catalyst in combination with K2CO3 as base allows for the arylation of ammonium chloride as well as aqueous ammonia at room temperature. The system shows high versatility with respect to the employed aryl iodide tolerating substitutions with electron-withdrawing as well as electron-donating groups. As for aryl bromides, several electron-withdrawing substituents are tolerated, and the authors demonstrated, in a single example, that an electron-rich aryl bromide (4-bromoanisole) could be converted, albeit in very low yield. Similarly, the group of Ma performed the arylation of ammonia with aryl bromides, substitutes with electron-donating or -withdrawing groups, by using L9 4-hydroxy-L-proline (40 mol %) in conjunction with copper(I) iodide (20 mol %) at 50 °C (Scheme 11, eq 3). Since these pioneering discoveries, the direct arylation of ammonia has attracted several research groups, and increasing efforts have been observed in order to discover new reaction conditions. The main attempt focuses on designing new catalytic systems by employing new ligands or well-defined 411


Review

ACS Catalysis copper catalysts (Scheme 12). The group of Ding24 employed copper(I) bromide (5 mol %) in conjunction with 1-(5,6,7,8tetrahy- droquinolin-8-yl)-2-methylpropan-1-one L10 (10 mol %) which allows for the arylation of ammonia with aryl iodides at room temperature. Moreover, with a higher catalyst loading (10 mol % of copper and 20 mol % of ligand) and at higher temperature (110 °C), aryl bromides can also be coupled. The group of Wan25 demonstrated that copper(II) oxide (5 mol %) in combination with L11, N2,N2-diisopropyloxalohydrazide (20 mol %) promotes the arylation of ammonia with aryl iodides as well as aryl bromides. The reactions were performed in water as solvent in the presence of TBAB (tetrabutylammonium bromide) at 120 °C. The authors also demonstrated that this catalytic system is not suitable for aryl chlorides. Another approach has been undertaken by the group of Zhou26 in order to accomplish the arylation of ammonia in water. They synthesized a discrete, water-soluble copper complex, namely, sulfonato−Cu(salen) (C5). This catalyst is effective at a loading of only 5 mol % for transforming aryl iodides at 120 °C, whereas higher catalyst loadings of 10 mol % are necessary to convert their aryl bromides analogues at the same reaction temperature. The group of Fischmeister, Thomas, Renaud et al.27 demonstrated that the association of copper(I) oxide (5 mol %) with N,N-dimethylethylenediamine L12 enables the formation of aminopyridine starting from heterocyclic aryl bromide. The reactions were performed in ethylene glycol at 60 °C. However, the arylation of the solvent was observed in very low amounts as a side reaction. The catalyst system composed of copper(I) iodide (20 mol %) and 2-carboxylic acid-quinoline-N-oxide L13 (40 mol %), developed by Jiang28 requires a higher loading of 20 mol %. It promotes the monoarylation of ammonia with aryl iodides at mild temperatures of 50 °C. The group of Sekar29 introduced the use of carbohydrate as a ligand for copper-catalyzed ammonia arylation. The authors synthesized anilines starting from aryl iodides and aryl bromides in very good to excellent yields by using copper(I) iodide with D-glucosamine hydrochloride L14 (both present in 10 mol %), and the reactions were performed in a mixture of acetone/water (1/1). Additionally, the group of Wan30 introduced the use of sucrose L15 at a high loading (50 mol %) in conjunction with copper(II) sulfate (20 mol %). Aryl iodides and bromides have been converted into their corresponding anilines in generally very good yield, and the reactions were conducted in water and polyethylene glycol (PEG-200) at 90 °C. Interestingly, the authors demonstrated that the catalyst system is recyclable, and it was shown to perform three times without considerable loss of reactivity. In 2013, the association of copper(II) sulfate (20 mol %) and sodium ascorbate L16 (40 mol %) was reported by the group of Wang.31 A mixture of DMSO/glycerol was used as reaction media at temperatures between 60 and 100 °C. Whereas aryl iodides and bromides substituted with electron-withdrawing groups underwent amination in very good yields, their electrondonating analogues showed less reactivity and led to low to moderate yields. The group of Page32 demonstrated that very low catalyst loadings can be achieved when liquid ammonia is employed. Indeed, only 1 mol % of copper(I) iodide and 1 mol % of ascorbic acid were shown to be sufficient for the conversion of electron-poor and electron-rich aryl bromides as well as

electron deficient aryl chlorides with very good to excellent yields at 100 °C. Finally, another recyclable catalyst system has been developed by the group of Zhu.33 This catalytic system makes use of copper(II) oxide (10 mol %) and diethylenetriaminepentaacetic acid as ligand L17 (20 mol %). Using water as the sole solvent, the amination of aryl iodides and bromides with ammonia can be facilitated at 100 °C. When the recyclability of the system was investigated, the reaction outcome was uniform in the first two runs, and lower yields were observed for the third and fourth run. Simultaneously to the design of new ligand-enabled copper catalysts able to promote the amination of aryl halides, significant efforts have been made to develop catalytic ligandfree systems (Scheme 13). The key to success has been the Scheme 13. Ligand-Free Copper Systems for the CopperCatalyzed Arylation of Ammonia

employment of an additive or solvent which is capable of stabilizing catalytically active copper species. For example the group of Wolf34 proved that by a judicious choice of solvent (water/NMP: 1/1) aryl iodides and bromides can be transformed into their corresponding anilines with 5 mol % of Cu2O as catalyst at 80 °C. Interestingly, aryl chlorides could also be activated at higher temperature (110 °C) under microwave irradiation. In 2009, the group of Darcel35 reported that catalytic amounts of ferric oxide in conjunction with copper(I) iodide allow for the amination of aryl iodides in very good to excellent yields. An example for use of gaseous ammonia has been reported by Fantasia et al.36 The protocol relies on the application of copper(II) acac as catalyst, DMF as solvent and potassium phosphate as base. Heterocyclic aryl bromide derivatives have been aminated in moderate to very good yields at a reaction temperature of 90 °C. Although this catalyst system does not require the addition of any external ligand, the authors acknowledge the possibility that the acetylacetonate counteranion in the copper salt can act as a ligand. In 2015, the first general methodology for the direct coppercatalyzed formation of aniline derivatives from aryl chlorides and aqueous ammonia or gaseous ammonia was disclosed by the group of Ma.37 This remarkable progress has been enabled by the application of bisaryl oxalic diamide L18 (namely, [N,N′-bis(2-phenyl-4-methylphen-yl)-oxalamide) as ligand in conjunction with copper(I) iodide (both 5 mol %). The catalytic system shows high functional group tolerance, and the desired anilines have been obtained in very good to excellent yields (Scheme 14). Notably, the scalability of the reaction has been demonstrated when the reaction was performed on a 490 mmol scale without any loss of reactivity. In order to shed some light on the reaction mechanism, a radical clock experiment has been carried out (Scheme 14, eq 2). As no cyclization occurred, intermediate radical formation can be excluded. 412


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ACS Catalysis Scheme 14. Copper-Catalyzed Arylation of Ammonia with Aryl Chlorides

Scheme 15. Copper-Catalyzed Arylation of Ammonia with Aryl Iodides and Bromides at Low Catalyst Loadings

process has been reported by the group of Fu39 in 2009 (Scheme 16). Anilines could be obtained in good to excellent isolated yields from a wide range of aryl boronic acids substituted with electron-withdrawing or -donating groups and aqueous ammonia. The catalyst system relies on the application of 10 mol % of copper(I) oxide, methanol as solvent, and air as the oxidant. It should be noted that under these conditions, the arylation of methanol was not observed. Moreover, the scope could be extended to aryl boric acid esters resulting in similar yields. 3. Nickel-Catalyzed Monoarylation of Ammonia. As the interest to employ cheaper and more abundant metals in ammonia mono arylation persists and most of the reported copper-catalyzed protocols have been limited to the application of sterically unhindered aryl iodides or activated bromides,

The same group also demonstrated that the arylation of aqueous ammonia starting with aryl iodides and bromides can be conducted at very low catalyst loadings (down to 0.1 mol % of Cu2O).38 This advancement of the copper catalyst’s efficiency was achieved by the addition of N-(naphthalen-1yl)-N′-alkyl oxalamides (0.1 mol %) as ancillary ligand. (Scheme 15). More specifically, L19 was found to promote the amination of aryl iodides at 60 °C. For aryl bromides, L20 was applied at a slightly higher reaction temperature (80 °C). The scope includes a number of heterocyclic compounds, and the desired products were formed in very good to excellent yields. As an alternative approach to the employment of aryl halides, anilines can also be obtained through the coupling of aryl boronic acids with ammonia. Such a Chan−Lam-type coupling 413


Review

ACS Catalysis Scheme 16. Copper-Catalyzed Arylation of Ammonia with Aromatic Boronic Acids Developed by the Group of Fu

Scheme 17. Nickel-Catalyzed Arylation of Ammonia or Ammonium Acetate Developed by Stradiotto

Scheme 18. Nickel-Catalyzed Arylation of Ammonia or Ammonium Acetate Developed by Hartwig

finding a convenient first-row transition-metal catalyst for the amination of inexpensive and broadly available (hetero)aryl chlorides and pseudohalides has remained an attractive goal. Because nickel complexes are known to undergo oxidative addition of aryl chlorides and to catalyze aryl chloride amination with primary and secondary amines,40 several groups

set out to develop the nickel-catalyzed synthesis of primary arylamines from aryl chlorides. In 2015, the groups of Stradiotto41 and Hartwig42 independently disclosed the first examples for the monoarylation of ammonia under nickel catalysis. The breakthrough has been enabled by the employment of electron-rich Josiphostype ligands. Straditotto and co-workers made use of a catalyst 414


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ACS Catalysis Scheme 19. Arylation of Ammonia with an Air-Stable Ni(II) Dichloride Complex

Scheme 20. Arylation of Ammonia with an Air-Stable Ni(II) Aryl Chloride Complex

Scheme 21. Air-Stable Ni(II) Complex for the Arylation of Ammonia or Ammonium Acetate

A common drawback of these methodologies is the use of an air-sensitive nickel (0) precatalyst or precursor. In order to address this challenge, both research groups developed different strategies that demonstrate the applicability of well-defined and air-stable nickel(II) Josiphos complexes in such a process. The group of Stradiotto synthesized Ni(II) Josiphos chloride by mixing the desired Josiphos ligand with the air-stable nickel(II) chloride ethylene glycol dimethyl ether complex. The formed complex is able to perform the amination of 5-chlorobenzothiophene in very good yield under similar conditions (vide supra). However, the reduction of the nickel(II) precatalyst by adding 35 mol % of phenylboronic acid pinacol ester has been mandatory (Scheme 19). The group of Hartwig undertook another approach and formed a nickel(II) complexes from oxidative additions between C8 and aryl chlorides which could be isolated in excellent yield starting from electron poor aryl chlorides and with moderate yield when the arenes were substituted with electron-donating groups. The newly formed nickel(II)

formed in situ from commercially available Ni(cod)2 in conjunction and L21 (both in 10 mol %). In contrast, Hartwig and co-workers isolated a discrete nickel (0) Josiphos complex with a side-bond benzonitrile (η2-NCPh), which they applied in the monoarylation of ammonia. As the authors demonstrated previously, benzonitrile is able to stabilize the Ni (0) intermediate that undergoes the initial oxidative addition of the catalytic cycle.43 The system developed by Stradiotto allows for the formation of (hetero)anilines starting from aryl bromides, chlorides, and tosylate derivatives (Scheme 17). It promotes the use of ammonia as a dioxane solution as well as ammonia gas in a mixture of 1,4-dioxane/toluene at 110 °C for 16 h. Interestingly, in addition to ammonia solution, the group of Hartwig demonstrated that ammonium acetate could also serve as ammonia surrogate (Scheme 18). Their nickel catalyst C6, has been shown to transform aryl chlorides into the corresponding anilines in moderate to excellent yield within only 3−7h at 100 °C. 415


Review

ACS Catalysis Scheme 22. Air-Stable Ni(II) Catalyst for the Arylation of Ammonia or Ammonium Sulfate

Scheme 23. Air-Stable Ni(II) for the Arylation of Ammonia or Ammonium Sulfate with Aryl Carbamates, Sulfonates Derivatives

identification of a catalyst system able of activating the right C−O bond of the (hetero)aryl carbamate and promoting the monoselective arylation of ammonia. A comprehensive highthroughput experimentation revealed a Josiphos type ligand bearing four cyclohexyl substitutents to exhibit the best chemoselectivity. Subsequently, the synthesis of an air-stable Ni(II) catalyst C10 was performed. This catalyst has been employed in the transformation of various aryl carbamate into their corresponding anilines with moderate to excellent yields by using ammonia in solutions or ammonium sulfate (Scheme 22). A very similar Ni(II) catalyst C11 has been employed by the group of Stradiotto, which is effective in the cross-coupling of ammonia or primary alkylamines with (hetero)aryl sulfamates, carbamates, or pivalates. Under the reported conditions, a broad spectrum of heterocyclic functionality within both reaction partners, as well as ether, nitrile, pyrrole, trifluoromethyl, and boronate ester substituents are tolerated (Scheme 23).

complexes could be engaged in the amination process as they readily undergo reductive elimination under coupling conditions. The authors demonstrated that both ammonia and ammonium sulfate can be arylated under these conditions in excellent yield (Scheme 20). The proposed reaction mechanism is based on a Ni(0)/Ni(II) catalytic cycle. Notably, a radical clock experiment was performed which excludes the formation of aryl radical intermediates. In an effort to enhance the operational simplicity, new airstable Ni(II) precatalyst have been developed. The Stardiotto group designed a new ancillary biphosphine ligand, namely, PAd-DalPhos.44 Via an efficient two-step synthesis, this ligand could be incorporated into a new Ni(II) precatalyst. The coordination of the nickel(II) chloride ethylene glycol dimethyl ether complex with the PAd-DalPhos diphosphine ligand occurs in THF at room temperature with 77% yield. Afterward, this intermediate is reacted with (o-tolyl)MgCl to furnish the active complex C9 in 93% yield. The catalyst proved capable of transforming aryl iodides, bromides, and chlorides into the corresponding anilines in generally very good to excellent yield (Scheme 21). Moreover, various aryl sulfonates (i.e., triflates, tosylates, mesylates, and imidazoylSO3) could be used as starting materials, and the desired products could be obtained either by using ammonia in solution, ammonia gas, or ammonium acetate. Very recently in 2017, the arylation of ammonia with the challenging aryl carbmates as halogen- and sulfonate-free starting materials was encompassed. Independently, Schranck, Tlili and co-workers45 as well as the Stradiotto group46 addressed this challenge successfully. In this effort, an initial experiment made by Schranck et al. focused on the

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CONCLUSIONS Since the seminal reports, ammonia monoarylation has experienced profound advancements. State-of-the-art crosscoupling methodologies enable the transformation of structurally diverse (hetero)aryl electrophiles featuring a range of functionalities. The technical challenges of employing gaseous ammonia have also been addressed, and protocols for the employment of inexpensive ammonium salts such as NH4Cl, NH4OAc, and (NH4)2SO4 have been established. A number of palladium-based catalysts and ligands have been developed and are commercially available. Moreover, the catalytic monoarylation of ammonia benefits from the recent efforts toward 416


Review

ACS Catalysis the employment of inexpensive and more abundant first-row transition metals. In particular, copper and nickel have become complementary metals to supersede thus far predominant palladium catalysts. Following this trend, with respect to copper and nickel, many catalysts and ligands are commercially available on a gram scale but only a fraction of them can be purchased in industrially relevant multikilogram amounts. Consequently, it remains an ongoing challenge to make the latest academic findings available to industrial applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Anis Tlili: 0000-0002-3058-2043 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Anis Tlili is grateful to the CNRS and the Institut de Chimie de Lyon for financial support. The authors kindly acknowledge the anonymous reviewers for their constructive comments.

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Transition-Metal-Catalyzed Monoarylation of Ammonia - ACS

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