# Dehydrogenative Coupling Reaction for the Synthesis of 5- and 6-Heterocyclic Derivatives including Arylquinolin-2(1H)-ones

Nicolas Joly, Clara Mauclaire, Sylvain Gaillard, Jean Luc Renaud, Albert Poater

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Nicolas Joly, Clara Mauclaire, Sylvain Gaillard, Jean Luc Renaud, Albert Poater. Dehydrogenative Coupling Reaction for the Synthesis of 5- and 6-Heterocyclic Derivatives including Arylquinolin-2(1H)-ones. Asian Journal of Organic Chemistry, 2025, 10.1002/ajoc.202400728. hal-04955690

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# Dehydrogenative Coupling Reaction for the Synthesis of 5- and 6-Heterocyclic Derivatives Catalyzed by Earth-Abundant Metal-Based Complexes

Nicolas Joly,[a, b] Clara Mauclaire,[a, c] Sylvain Gaillard,[b] Albert Poater,[*a] and Jean-Luc Renaud*[b, c]

Heterocycle synthesis is an intense research area due to their significant importance in pharmaceutical, material chemistry and fine synthesis. Key innovations include transition metal-based-catalyzed heterocyclic derivatives synthesis, which facilitates diverse structural modifications, enhances selectivities and efficiency. The necessity to develop more greener technologies for the construction of new C–C and C–N bonds has strived chemists to re-design their synthetic strategy plan and to introduce abundant starting materials. Among these new transformations, acceptorless dehydrogenative coupling (ADC) reactions have received increased attention thanks to their atom-economy, the use of alcohols as pro-electrophiles and the formation of water and hydrogen gas as only by-products.

These methods exemplify the interplay between traditional and cutting-edge techniques, providing a robust synthetic toolkit to address the growing demand for biologically relevant compounds with tailored functionalities. This review will focus on the acceptorless dehydrogenative coupling methodologies for the synthesis of nitrogen-containing 5- and 6-membered heterocyclic derivatives (such as pyrroles, pyrimidines, quinazolines, and quinoxalines) in the presence of 3d-metal complexes. The state of the art of the ADC process underscores the continuous evolution of synthetic strategies and emphasizes its importance in creating valuable and structurally diverse compounds, ensuring their centrality in both academic and pharmaceutical research.

## 1. Introduction

N-heterocycle structures are often present not only in natural products but also in pharmaceutical relevant derivatives,[1] or material science and their synthesis has gained significant attention in the field of organic chemistry. As examples, bioactivities, such as antibiotics,[2] antitumor[3] or antiplatelet[4] activities, have been reported in the literature. This interest has prompted chemists to develop more efficient synthetic methodologies. Among these methodologies, dehydrogenative coupling reactions have emerged as a powerful strategy, providing a direct and efficient pathway to construct diverse heterocyclic frameworks.

Catalytic acceptorless dehydrogenative coupling (ADC) strategy is a growing research area,[5,6] allowing the creation of

carbon-carbon or carbon-nitrogen bonds through the removal of hydrogen gas and liberation of water, as side products, under quite mild reaction conditions.[7] In the context of building up more environmentally friendly processes, the ADC approach is a valuable method as it does not require pre-functionalized substrates, consequently it minimizes the number of synthetic steps, avoids typically protection/deprotection steps, diminishes the waste and side products, allows the formation of bonds with high selectivity and functional group compatibility. The one-pot multistep sequence begins with a metal-based catalyzed dehydrogenation of an alcohol, the pro-electrophile, and generates a carbonyl intermediate and a metal-hydride compound. Then, the condensation of a N-nucleophile followed by a cyclization delivers the targeted N-heterocycle and liberate hydrogen and water as side-products. The last decade has seen the development of many precious metal complexes (such as Pd, Ru, Rh, Ir and Os) for the ADC reactions. More recently, Earth-abundant metal-based complexes (Mn, Fe, Co, Ni or Cu) have been introduced in this emerging field.

This review will focus on the 3d-metal complexes-catalyzed acceptorless dehydrogenative coupling methodology for the synthesis of 5- and 6-member heterocyclic derivatives such as pyrroles, pyrimidines, quinazolines, and quinoxalines, with interesting applications as potential drugs (Scheme 1).

<figure><img src="image_3.png" /></figure>

<footnote>[a] Dr. N. Joly, C. Mauclaire, Dr. A. Poater
Institut de Química Computacional i Catàlisi and Departament de Química,
Universitat de Girona
c/ Mª Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
E-mail: albert-poater@udg.edu</footnote>

<footnote>[b] Dr. N. Joly, Dr. S. Gaillard, Prof. Dr. J.-L. Renaud
Normandie Univ, LCMT, ENSICAEN, UNICAEN, CNRS
6 boulevard du Maréchal Juin, 14000 Caen, France
E-mail: jean-luc.renaud@sorbonne-universite.fr</footnote>

<footnote>[c] C. Mauclaire, Prof. Dr. J.-L. Renaud
Sorbonne Université, Institut Parisien de Chimie Moléculaire, UMR 8232,
75005 Paris, France</footnote>

<footnote>© 2025 The Authors. Asian Journal of Organic Chemistry published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_4.png" /><figcaption>Scheme 1. Examples of bioactive compounds containing a quinolin-2(1H)-one moiety.</figcaption></figure>

## 2. Quinolin-2(1H)-Ones Catalyzed by Transition Metals

Quinolin-2(1H)-one is a class of nitrogen-containing heterocyclic compounds, classified as cyclic amides. This structure is found in a wide range of biologically and pharmaceutically active molecules,[8] which exhibit significant bioactivities. Due to its unique structure, quinolin-2(1H)-one derivatives are integral to the pharmacology of various therapeutic agents. For instance, Brexpiprazole acts as a serotonin-dopamine activity modulator and is used as an adjunct therapy for major depressive disorder.[9] Indacaterol and Procaterol are bronchodilators employed in the treatment of asthma,[10] while CHIR-124 is a selective Checkpoint kinase 1 (Chk1) inhibitor (see Scheme 1 for structural examples).[11]

The synthesis of quinolin-2(1H)-one motifs is often accomplished using classical methods like the Vilsmeier-Haak[12] or Friedlander reactions.[13] However, recent advancements have expanded the synthetic toolkit to include methods catalyzed by transition metals, or photoinduced,[14] allowing for greater efficiency, selectivity, and access to more diverse quinolinone derivatives. These developments highlight the importance of

<figure><img src="image_5.png" /></figure>

Dr. Nicolas Joly obtained his Master Degree in Molecular Chemistry in 2020 at the ENSCR in Rennes. In 2023, he completed a joint PhD in Chemistry at the University of Caen Normandy and the University of Girona under the supervision of Prof. Jean-Luc Renaud and Dr. Albert Poater. He then joined the group of Dr. Thibault Cantat at CEA Paris-Saclay as a postdoctoral researcher, followed by a post-doc position in 2025 at the University of Rouen with Prof. Thomas Poisson and Prof. Philippe Jubault.

<figure><img src="image_6.png" /></figure>

Ms. Clara Mauclaire completed her Bachelor's degree in Chemistry in 2022 and her Master's degree in 2024, both at Université Paris Cité. She is currently pursuing a joint PhD at Sorbonne University and the University of Girona under the supervision of Prof. Jean-Luc Renaud and Dr. Albert Poater.

<figure><img src="image_7.png" /></figure>

Dr. Sylvain Gaillard received his PhD from the Université Paul Cézanne/Aix-Marseille III. After a first postdoctoral position at the ENSCR in Rennes, he pursued with a second post-doctoral position under the supervision of Prof. Steven P. Nolan at the University of St Andrews. In 2010, he obtained an associate professor position at the University of Caen Normandy. His research interest is focused on molecular design of transition metal complexes tuning photophysical properties for applications in photocatalysis, hybrid materials and biological activity.

<figure><img src="image_8.png" /></figure>

Prof. Jean-Luc Renaud obtained his Ph.D. degree in 1998 under the supervision of Dr. Aubert and Prof. Malacria. He was a Lavoisier Postdoctoral fellow in 1999 with Prof. Lautens at the University of Toronto then moved to the University of Louvain-La-Neuve in the team of Prof. Riant. In 2000, he was appointed as Maître de Conférences at the University of Rennes and accepted a full Professor position at University of Caen Normandy in 2008. He is being invited researcher at the "Institut Parisien de Chimie Moléculaire (IPCM), Sorbonne University, since October 2023. The research interests focus on organometallic catalysis, organocatalysis, photoredox catalysis and their application towards the synthesis of biologically interesting molecules and processes relevant to fine chemical synthesis.

<figure><img src="image_9.png" /></figure>

Dr. Albert Poater finished his PhD in Chemistry in 2006 from the University of Girona. After research stays in Chile, Montpellier, and a postdoctoral position at the University of Salerno, he became a Ramón y Cajal fellow in Girona in 2010. He has been a Visiting Researcher in Saudi Arabia and Toulouse and, in 2019, was appointed Serra Húnter Associate Professor at the University of Girona. A recipient of the ICREA ACADÈMIA award in 2019, he has authored over 350 publications with approximately 15,500 citations (H-index=68). His research focuses on DFT calculations in inorganic and organometallic catalysis, emphasizing green chemistry.
quinolin-2(1H)-one in both synthetic chemistry and medicinal applications, as well as the ongoing innovations in its synthesis to meet the growing demand for complex, bioactive structures.

The first metal-catalyzed synthesis of quinolin-2(1H)-one was introduced by Takahashi in 1995.[15] In an attempt to produce 1,3-dihydroindol-2-ones from 2-alkynylanilines and carbon monoxide using a rhodium catalyst, Takahashi obtained quinoline derivatives as by-products, with yields reaching 11% (Scheme 2).

Following this work, Larock developed a palladium-catalyzed carbonylative annulation approach for 2-quinolone synthesis. Here, 2-iodoaniline derivatives reacted with various alkynes in the presence of Pd(OAc)₂ under a CO atmosphere, yielding the target products in low to good yields (17–82%) (Scheme 3).[16] Although selectivity was limited with unsymmetrical alkynes, increased steric hindrance, such as with

phenylacetylene or trimethyl(prop-1-yn-1-yl)silane, improved product selectivity. This methodology, however, required protected anilines, as unprotected versions proved ineffective. Subsequently, Liu and Jia adapted this protocol to synthesize 4,5-fused tricyclic 2-quinolones from alkyne-tethered ortho-iodoaniline.[17] More recently, Zhou developed a one-step approach for the synthesis of 2-quinolinone through palladium catalyzed cyclization of **ortho-iodoanilines** with acrylates or acrylic acids and obtained moderate to good yields (28–99%).[18]

In 2004, Manley introduced an alternative cascade approach inspired by the Friedländer condensation,[19] using a Buchwald-Hartwig cross-coupling reaction between 2-halogenobenzaldehyde and acetamides in the presence of a palladium/Xantphos catalyst,[20] The *in-situ*-generated *N*-arylacetamides intermediates cyclized through an enol condensation to produce quinolinones in yields ranging from 32 to 94% (Scheme 4). While this method enabled the formation of *N*-methyl quinolinones, steric hindrance from other *N*-substituted arylacetamides hindered the coupling, though the procedure was successful in synthesizing 4-hydroxy and 4-aminoquinolinones.

Building on this approach, Ding explored quinolinone synthesis using Buchwald's conditions for copper-catalyzed amide and aryl iodides and bromides coupling at 110 °C.[21,22] This approach provided yields from 20 to 93% (Scheme 5). Notably, the method also produced 4-hydroxy and 4-amino-quinolinones in good yields (46% and 76%, respectively). For less reactive aryl chlorides, reaction temperatures of 150 °C were required, yielding moderate results (35–42%, Scheme 5).

<figure><img src="image_10.png" /><figcaption>**Scheme 2.** Takahashi's initial metal-catalyzed synthesis of quinolin-2(1H)-one.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Scheme 3.** Palladium-catalyzed synthesis of 2-quinolones by Larock and collaborators.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Scheme 4.** Quinolinone synthesis via amide coupling and enol condensation by Manley and collaborators.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Scheme 6.** Silver-catalyzed quinolinone synthesis by Kuhakarn.</figcaption></figure>

<caption>**Scheme 5.** Copper-catalyzed arylation of amides to synthesize quinolinones by Ding and collaborators.</caption>

In 2018, Kuhakarn developed a two-step quinolinone synthesis beginning with N-(2-alkynylaryl)formamides.[23] In the initial step, formamide derivatives were converted into isonitriles, followed by a silver-catalyzed cyclization in a mixture of DMF and water at 80 °C affording quinolin-2(1H)-ones in yields of 32–80% (Scheme 6). This method accommodated a range of electron-donating and electron-withdrawing substituents, with notable success in obtaining an azido-substituted 3-arylquinolin-2(1H)-one at 32% yield (Scheme 6).

More recently, Qi and Wu reported a quinolinone synthesis leveraging anthranil derivatives and benzyl chlorides.[24] Known for their coordination efficiency in transition-metal-catalyzed C–N bond formations,[25] anthranil compounds were converted in situ to 2-aminobenzaldehydes, which have then been engaged in a carbonylative Buchwald-type coupling with benzyl chlorides. Following an intramolecular condensation, functionalized 3-arylquinolin-2(1H)-ones were obtained with yields from 49 to 99% (Scheme 7). The reaction demonstrated high versatility, tolerating a variety of electron-donating and electron-withdrawing groups.

Regarding the state of the art on the synthesis of quinolinones and dehydrogenative coupling reactions, catalyzed by non-noble metals and leading to heterocyclic products (see section 4, *vide infra*), it was proposed to evaluate the feasibility of the synthesis of quinolinone-type products via dehydrogenative coupling reaction.

### 3. Iron-Catalyzed Dehydrogenative Coupling of Amido-Alcohols: Synthesis of 3-Arylquinolin-2(1H)-Ones

3-Arylquinolin-2(1H)-ones feature three distinct rings (A, B, and C, see Scheme 8), which can be modified either before cyclization or through post-functionalization of the quinolinone product.[26] To introduce structural diversity into the reaction, various substituted N-(2-(hydroxymethyl)phenyl)-2-arylaceta- mides were prepared by Renaud and coworkers and used for the synthesis of mono- and di-substituted quinolin-2(1H)-ones *via* the intramolecular acceptorless dehydrogenative cyclization.[27]
<figure><img src="image_14.png" /><figcaption>**Scheme 9.** Ruthenium-catalyzed synthesis of 3-phenylquinolinone.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Scheme 8.** Representation of the A, B, and C rings discussed.</figcaption></figure>

With a diverse set of amido-alcohols prepared, the quinolinone synthesis via dehydrogenative coupling was tested. Using the ruthenium complex [Ru1] at 100 °C with 2 equivalents of base proved effective (Scheme 9). Best selectivity and conversions were obtained with carbonates. In DMF and with Na₂CO₃ at 100 °C), the reaction led to 96% conversion of 3-phenylquinolin-2(1H)-one.

While these first results were encouraging for the development of substituted quinolinones using an ADC methodology, a better competitive and attractive procedure was developed. The iron congeners [Fe1]–[Fe4] were evaluated in this reaction and the higher reactivity and selectivity was observed in the presence of [Fe3].[28] With the optimized conditions in hands, the scope and limitations were then delineated. Initial experiments were conducted with the first library amido-alcohols

evaluating ring A in Scheme 10. The corresponding 3-arylquinolin-2(1H)-ones were isolated in 48–83% yields (Scheme 10). The ortho substitution did not hamper the reactivity and electron-donating as well as electron-withdrawing groups were also well tolerated in this reaction with products isolated in good yields (54–83% and 62–69% yields, respectively). The introduction of a heterocycle such as thiophene was possible but with only in 48% yield. Next, phenylacetanilides were engaged in this catalytic process (Scheme 10 middle). The electron-donating and electron-withdrawing substituted quinolinones were isolated in good yields (68 and 71% respectively). Neither the steric hindrance generated by the biphenyl motif nor the extension of the aromatic system hampered the reactivity. Finally, the methodology was applied to secondary amido-alcohols (Scheme 10, down). Alkyl-substituted quinolinones were obtained from lower yield (58%) while N-substituted anilide yielded the corresponding compound in very good yield (89%). Among the reported methods allowing the formation of substituted quinolines derivatives, few reported the possibility to bring substitution on any position of this type of
## 4. Dehydrogenative Coupling Approach to Heterocycles Synthesis

The transition-metal-catalyzed synthesis of heterocycles has been extensively explored in recent decades.[34] Recently, borrowing hydrogen and dehydrogenative coupling methods have emerged as appealing strategies due to the low toxicity of the by-products they generate (e.g., H₂O, H₂).[5,35] Both noble and first-row metal catalysts have been reported for these reactions. This chapter will focus on heterocycle synthesis via dehydrogenative coupling catalyzed specifically by 3d transition metals. The general mechanism of this reaction is illustrated in Scheme 11. In the initial step, the metal catalyst dehydrogenates an alcohol, forming the corresponding carbonyl compound and a metal-hydride species. Unlike in alkylation reactions,[36] the latter releases molecular hydrogen to regenerate the catalytic species, with or without the assistance of an oxidant or hydrogen trap. The carbonyl compound subsequently undergoes intramolecular condensation, yielding a cyclic unsaturated product.

### 4.1. Copper-Catalyzed Dehydrogenative Synthesis of Heterocycles

In 2006, Cho and colleagues introduced the first copper-catalyzed synthesis of quinolines.[37] A reaction of 2-amino-benzyl alcohol and ketones with KOH (3 equiv.) and CuCl₂ (1 mol%) in dioxane at 100 °C yielded 2-substituted quinolines (Scheme 12). Aromatic and aliphatic ketones afforded 2-alkyl or 2-arylquinolines with good yields (42–82%). Adjustments to the conditions allowed the use of aldehydes, leading to 3-alkyl or 3-arylquinolines (41–67% yield). Later, the same group developed a recyclable version of this protocol, where the catalytic system retained activity for up to 10 cycles.[38]

Subsequent developments by Chen and Wu enabled the synthesis of quinazolines,[39] using CuCl (20 mol%) along with Ce(NO₃)₃ and TEMPO (10 mol% each), allowing for a broad substrate scope, with yields between 55–93% (Scheme 13).

Zhang later reported the synthesis of 1,3,5-triazines via copper-catalyzed reactions between benzamidine hydrochlorides and alcohols, achieving high yields (46–98%).

<figure><img src="image_16.png" /><figcaption>**Scheme 11.** General mechanism of dehydrogenative coupling.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Scheme 10.** Synthesis of 3-arylquinolin-2(1H)-ones with diversity on the A (top), B (middle), C (down) ring.</figcaption></figure>

heterocycle.[29,30,31,32] Among them, Kirchner[30] and Srimani[31] were limited to aryl substituents on the position 4 of the quinoline ring, while Wu and Liu reported only 4-methyl of 4-trifluoromethyl quinolines.[33] The manganese-catalyzed synthesis of quinolines reported by Chai and Zhang is the only method allowing alkyl and aryl substitution on the position 4 of the quinoline product.[29] However, all these methodologies allowed the obtention of the desired heterocycles in similar yields than the present method.
<figure><img src="image_18.png" /><figcaption>**Scheme 12.** Copper-catalyzed synthesis of quinolines.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Scheme 13.** Synthesis of quinazolines.</figcaption></figure>

(Scheme 14).[40] Wang, in 2017, synthesized benzimidazoles from 1,2-diamines and benzyl alcohols using a copper triazole-phosphine complex under basic conditions,[41] yielding function alized benzimidazoles in yields of 62–91% (Scheme 15).

Other notable copper-catalyzed methods include Zhang's three-component pyrimidine synthesis (Scheme 16, top)[42] and Li's synthesis of quinazolinones from 2-aminobenzonitriles and primary alcohols (Scheme 16, down),[43] each offering broad

<figure><img src="image_20.png" /><figcaption>**Scheme 14.** Copper-catalyzed synthesis of triazines.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Scheme 15.** Copper-catalyzed synthesis of benzimidazoles.</figcaption></figure>

functional group compatibility and yields ranging from moderate to excellent (from 42 to 86%).

## 4.2. Manganese-Catalyzed Dehydrogenative Synthesis of Heterocycles

Kirchner, in 2016, introduced the first manganese-catalyzed synthesis of nitrogen-containing heterocycles via dehydrogenative coupling.[30] Using PNP–Mn complex [Mn1] (5 mol%) at 140 °C, 2-aminobenzyl alcohols reacted with secondary alcohols to yield quinolines (61–91%, Scheme 17). This method allowed substitution at the quinoline's 4-position, a notable advancement compared to the method proposed by Cho.[37,38]

Further work enabled pyrimidine synthesis from benzamine and various alcohols, with yields ranging from 58–90%
<figure><img src="image_22.png" /></figure>

(Scheme 18, top). Kempe also reported a three-component synthesis of pyrimidines with moderate to good yields (44–79%, Scheme 18, middle),[44] and a sequential four-component approach yielding tetrasubstituted pyrimidines (Scheme 18, down), in the presence of a modified manganese-pincer type complex [Mn2].

Kempe also described pyrrole synthesis from secondary alcohols and amino-alcohols,[45] yielding non-symmetrically substituted pyrroles (Scheme 19).

Rueping and El-Sepelgy expanded this field by synthesizing N-substituted pyrroles from 1,4-diols and primary amines,[46] demonstrating good yields compared to complex [Mn2] (ranging from 42 to 92%) and compatibility with electron-withdrawing groups (Scheme 20).

In 2018, Srimani reported the synthesis of quinolines, quinoxalines, pyrazines, and benzothiazoles under neat conditions.[31] The catalytic process utilized a tridentate NNS–Mn(I) complex, Mn4, to facilitate these dehydrogenative
<figure><img src="image_23.png" /><figcaption>Scheme 19. Manganese-catalyzed synthesis of pyrroles.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>Scheme 20. Manganese-catalyzed synthesis of pyrroles (alternative method).</figcaption></figure>

coupling reactions. Quinolines were synthesized from 2-amino-benzyl alcohols and secondary alcohols using a methodology similar to that of Cho[37,38] and Kirchner.[30] Various alkyl- and aryl-substituted quinolines were obtained in yields ranging from 58% to 86%, although this reaction was limited to non-functionalized starting materials, with only two examples of substituted quinolines (F, Cl) reported (Scheme 21).

The same catalyst was subsequently employed to synthesize quinoxalines and pyrazines from 1,2-diamines and 1,2-diols. These reactions tolerated alkyl, halogen, and nitro substituents, yielding products in good yields (52% to 86%, Scheme 22). The conditions were also effective for synthesizing benzothiazoles from 2-aminothiophenol and benzyl alcohols, with good yields and tolerance for electron-donating, electron-withdrawing, and heteroaryl groups (69% to 87%, Scheme 22).

The same research group also reported the synthesis of 2-substituted and 1,2-disubstituted benzimidazoles.[47] In contrast to Chen and Wu's approach,[39] which started from benzaldehydes, Srimani used benzyl alcohols as pro-electrophiles. The reactions utilized various diamines and alcohols under basic conditions (BuOK or KOH) with the NNS-Mn catalyst [Mn5] (5 mol%), yielding the desired mono- or di-substituted products in 35% to 91% (Scheme 23).

<figure><img src="image_25.png" /><figcaption>Scheme 21. Synthesis of quinolines reported by Srimani.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Scheme 22. Synthesis of quinoxalines, pyrazines, and benzothiazoles.</figcaption></figure>

The synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazoles was also explored. Starting from N-benzyl diamines, disubstituted benzimidazoles with various aryl groups were obtained in good yields (61% to 79%, Scheme 24).[47]

This NNS-Mn [Mn5] complex was further applied to synthesize 2,3-dihydro-1H-pyrimidine derivatives using 1,8-diaminonaphthalene and various primary alcohols (Scheme 25).[48]

By adjusting the base (BuOK or KOH) and solvent, the NNS-Mn complex [Mn5] enabled the synthesis of quinazolines or 2-aminoquinolines from 2-aminobenzyl alcohols and nitriles.[49] This methodology afforded various nitrogen-containing heterocycles with both electron-donating and electron-withdrawing groups (Scheme 26).

The authors also explored a two-step, one-pot synthesis of 2-alkylaminoquinolines. After forming the 2-aminoquinoline intermediate, a primary alcohol was introduced to further functionalize the amine.[49] Electron-donating and electron-with-
<figure><img src="image_27.png" /><figcaption>**Scheme 23.** Manganese-catalyzed synthesis of mono- and di-substituted benzimidazoles.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Scheme 24.** Synthesis of disubstituted benzimidazoles bearing various aryl substituents.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Scheme 25.** Synthesis of dihydropyrimidines.</figcaption></figure>

drawing groups were well tolerated, yielding 2-aminoalkylquinolines in 59% to 68% (Scheme 27).

In 2018, Milstein reported the synthesis of pyrazines and quinoxalines catalyzed by a PNP–Mn complex.[50] The homocoupling of amino alcohols in the presence of potassium hydride

<figure><img src="image_30.png" /><figcaption>**Scheme 26.** Synthesis of quinazolines and 2-aminoquinolines from nitriles.</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>**Scheme 27.** Manganese-catalyzed synthesis of 2-alkylaminoquinolines.</figcaption></figure>

(3 mol%) and the manganese complex (2 mol%) in toluene at 150 °C produced symmetric pyrazines in yields ranging from 30% to 95% (Scheme 28). Under similar conditions, reactions between 1,2-diamines and 1,2-diols yielded quinoxalines in 35% to 95%. This approach was effective primarily for alkyl- and aryl-substituted heterocycles.

More recently, Balaraman described the reaction of quinoxalines and quinazolines catalyzed by an in situ generated NNN–Mn complex.[51] Following Milstein’s approach,[49] the reaction of diamines and diols in the presence of tBuOK (40 mol%), ligand (3 mol%), and Mn(CO)₅Br (2 mol%) in toluene at 130 °C afforded various alkyl- and aryl-substituted quinoxalines in good yields (40% to 83%, Scheme 29). Using similar conditions with 2-aminobenzyl alcohol and substituted amides, quinazoline derivatives were also obtained in good yields (58% to 81%). This catalyst demonstrated a high tolerance for functional groups.

That same year, Chai and Zhang reported the synthesis of quinolines and pyridines using an NN-pyridine-benzimidazole bidentate manganese complex [Mn7].[29] Reactions of secondary alcohols with 2-aminobenzyl alcohols or 1,3-amino-alcohols, in the presence of tBuOK (0.5 equiv.) and [Mn7].complex (3 mol%) in toluene at 110 °C, yielded the desired heterocycles in moderate to good yields (45% to 88%, Scheme 30). This approach also demonstrated a broader functional group
<figure><img src="image_32.png" /><figcaption>**Scheme 28.** Synthesis of pyrazines and quinoxalines reported by Milstein.</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Scheme 29.** Synthesis of quinazolines and quinoxalines reported by Balaraman.</figcaption></figure>

compatibility compared to Srimani's system, which was limited to benzophenone derivatives for substitution at the 4-position of quinolines.[31]

In 2024, a manganese bis-carbene complex [Mn8] was developed for the synthesis of C–C and C–N bonds via dehydrogenative cyclization and hydrogen borrowing reactions.[52] This complex enabled the efficient synthesis of saturated ketones (42–92%), 2-oxindoles (70–83%) and one-pot

<figure><img src="image_34.png" /><figcaption>**Scheme 30.** Synthesis of quinolines and pyridines.</figcaption></figure>

synthesis of quinolines (70–89%), while being environmentally friendly since water is the only by-product formed (Scheme 31).

<figure><img src="image_35.png" /><figcaption>**Scheme 31.** Manganese-catalyzed synthesis of quinolines.</figcaption></figure>
### 4.3. Iron-Catalyzed Dehydrogenative Synthesis of Heterocycles

In 2012, Deng reported the first iron-catalyzed synthesis of heterocycle derivatives through a dehydrogenative coupling strategy.[53] Starting from ortho-nitrophenols and benzyl alcohols in the presence of diphenylphosphino-ferrocene (dppf) (2 mol%) as catalyst in neat conditions at 150 °C, various 2-

<figure><img src="image_36.png" /><figcaption>Scheme 32. Iron-catalyzed synthesis of benzoxazoles.</figcaption></figure>

<figure><img src="image_37.png" /><figcaption>Scheme 33. Iron-catalyzed synthesis of benzimidazoles.</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>Scheme 34. Synthesis of quinolines.</figcaption></figure>

arylbenzoxazoles bearing halogen (F, Cl, Br), methoxy, alkyl or heteroaryl substituents were isolated in good yields (55–88%, Scheme 32).

Following this work, Huang described the synthesis of benzimidazoles.[54] Ortho-nitroanilines were used as precursors along with benzyl alcohols in the presence of dppf (5 mol%) in toluene at 150 °C. Various substituted nitroanilines as well as benzyl alcohols bearing electron-donating or electron-withdrawing groups were engaged in this reaction and the corresponding products were prepared in moderate to good yields (40–88%, Scheme 33).[54]

In 2016, Wu and Liu described the first dppf catalyzed synthesis of quinolines (Scheme 34).[33] Differently substituted quinolines were prepared from 2-nitrobenzylalcohols and secondary alcohols. In this process, formic acid served as a hydride source to reduce the nitro function into the corresponding amine, allowing the subsequent condensation and cyclisation. Nevertheless, this system requires higher temperature and base loading compared to Chai and Zhang's system (110 °C and ¹BuOK 0.5 equiv., Scheme 30).[29]

Singh described an iron(II) phthalocyanine complex-catalyzed synthesis of benzimidazoles, benzoxazoles and benzothiazoles.[55] 2-amino, 2-hydroxy and 2-thioanilines reacted with alkyl and benzyl alcohols in the presence of ¹BuONa (2 equiv.) and Fe(II)Pc (1 mol%) in toluene at 120 °C to yield the 2-substituted heterocycles (41 to 99%, Scheme 35). Lower yields were obtained with aliphatic alcohols due to a greater activation energy of these alcohols over the oxidation step.

Such derivatives can be also synthesized from benzylamines as pro-electrophiles, as described by Nguyen and co-workers, in the presence of FeCl₃ or CoBr₂ at 120–140 °C.[32] Ortho-nitroanilines reacted with various benzyl amines to yield the desired

<figure><img src="image_39.png" /><figcaption>Scheme 35. Synthesis of benzimidazoles, benzoxazoles and benzothiazoles reported by Singh.</figcaption></figure>
five-member rings in good yields (62–92% with cobalt bromide and 30–87% with iron chloride, Scheme 36).

To allow the synthesis of 2-alkylbenzimidazoles, 20 mol% of elemental sulfur has to be added. Actually, sulfur served as an electron-donor to efficiently reduce the nitro function, thus increasing the conversion and yields of the reaction. In these conditions, 2-alkylbenzimidazoles were isolated in low to good yields (10 to 90%, Scheme 37). The authors extended this work to the synthesis of 2-arylquinazolin-4(3H)-ones. 2-nitrobenzamide reacted with benzylamines in the same conditions, and the desired quinazolinones were isolated in good yields (68–75%, Scheme 37). However, this methodology is more limited compared to the process developed by Li in the presence of copper complex.[43] Aiming to improve this catalytic system,[56] a modified version in which elemental sulfur (20 mol%) was replaced by Na₂S·nH₂O (40 mol%) was described then, and the benzimidazoles products were obtained in comparable yields (44 to 90%) than those described by Li.[43]

Barta and Sundararaju reported almost simultaneously the synthesis of N-substituted pyrroles from various primary alkyl and arylamines and 1,4-alkene-diols or 1,4-butyndiols. Barta

reported the use of the Knölker's complex [Fe4] for the synthesis of N-substituted pyrroles.[57] Various primary alkyl and aryl amines reacted with 1,4-diols in the presence of the Knölker's complex (4 mol%) and Me₃NO (8 mol%) in toluene at 130 °C to yield the pyrrole derivatives in 15 to 80% (Scheme 38). A wide range of functions as well as heteroaryl rings were tolerated, making this methodology competitive compared to the one proposed by Rueping[46] or Kempe[45] in the presence of manganese complexes (Schemes 19 and 20).

Increasing the catalytic loading up to 10 mol% and the temperature to 150 °C, Sundararaju extended the scope of this reaction and various substituted pyrroles were isolated (22 to 96% yields, Scheme 39).[58]

Knölker's complex is also able to catalyze the synthesis of benzimidazoles.[59] Following the same strategy than Srimani,[47,48] 1,2-diamines reacted with primary alcohols in the presence of tBuOK (1.5 equiv.) Me₃NO (8 mol%) and Knölker's complex (4 mol%) in xylene at 150 °C to give the desired disubstituted benzimidazoles in low to good yields (20 to 92%, Scheme 40). Compared to Wang[41] or Srimani,[47,48] this method-

<figure><img src="image_40.png" /><figcaption>Scheme 36. Synthesis of 2-arylbenzimidazoles reported by Nguyen.</figcaption></figure>

<figure><img src="image_41.png" /><figcaption>Scheme 37. Synthesis of 2-alkylbenzimidazoles and quinazolinones.</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>Scheme 38. Synthesis of pyrroles reported by Barta.</figcaption></figure>

<figure><img src="image_43.png" /><figcaption>Scheme 39. Synthesis of pyrroles reported by Sundararaju.</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>**Scheme 40.** Knölker complex-catalyzed synthesis of benzimidazoles.</figcaption></figure>

ology appears to be less efficient, nevertheless, aliphatic alcohols could also be engaged in this process.

This methodology was extended to the synthesis of pyrrolo[1,2-α]quinoxalines.[60] Using the Knölker complex [Fe3] (6 mol%) and Me3NO (12 mol%) in cyclopentyl-methyl ether (CPME) at 160 °C under oxygen atmosphere, various 2-pyrrole nitroarenes reacted with primary alcohols to give the tricyclic products in low to very good yields (32–99%, Scheme 41).

Recently, Darcel described the synthesis of benzimidazoles and quinoxalines catalyzed by the same iron complex [Fe3].[61] Starting from either primary alcohols or 1,2-diols and ortho-nitroaniline in the presence of K3PO4·H2O (1 or 3 equiv.), Me3NO (10 mol%) and the Knölker complex (5 mol%) in mesitylene (160 °C) or toluene (150 °C), the desired heterocyclic compounds

<figure><img src="image_45.png" /><figcaption>**Scheme 41.** Synthesis of pyrroloquinoxalines.</figcaption></figure>

were obtained in low to very good yields (28 to 96%, Scheme 42). The addition of DDQ (0.5 equiv.) after 15 h of reaction was necessary for the obtention of benzimidazoles derivatives. This methodology was limited as only three examples of benzimidazoles, and only methyl- or halogen-substituted quinoxalines were reported. Hong developed almost simultaneously the same methodologies in the presence of the same cyclopentadienone iron tricarbonyl complex [Fe3] and almost the same reaction conditions. Benzoxazoles, benzothiazoles, benzimidazoles were isolated in 21–94% yields,[62] and quinoxalines were obtained in up to 94%.[63] Hong extended this approach also to the synthesis of quinolines and quinolinones starting from 2-nitrobenzyl alcohols and secondary/primary alcohols.[64] The quinolines have been synthesized in 22 to 84% yield.

In 2020, Paul *et al.* introduced an NNN–Fe complex [Fe6] for the synthesis of 2-amino- or 2-alkyl-pyrimidines via a three-component reaction.[65] Various alkyl- and aryl-substituted pyrimidines were prepared by this method in yields ranging from 61 to 88% (Scheme 43). This method allowed the introduction of electron-withdrawing substituents, unlike previous methods (Schemes 13 and 16).[39,42]

### 4.4. Cobalt-Catalyzed Dehydrogenative Synthesis of Heterocycles

In 2016, Milstein reported the first cobalt-catalyzed synthesis of pyrroles starting from 1,4-diols and primary amines.[66] A PNN pincer Co complex (5 mol%) was used with tBuOK (5 mol%) and NaBHEt3 (5 mol%) to activate the catalyst. Various alkyl- or aryl-substituted pyrroles were prepared by this methodology (22 to 93% yields, Scheme 44). This methodology tolerated electron-donating (methoxy, methyl) and electron-withdrawing groups (CF3) without impeding the yield of the reaction.

<figure><img src="image_46.png" /><figcaption>**Scheme 42.** Synthesis of heterocycles reported by Darcel.</figcaption></figure>
$$ \underset{\text{R1}}{\text{OH}} + \underset{\text{R2}}{\text{OH}} + \underset{\text{H}_2\text{N}}{\text{R}^3\text{NH}} \xrightarrow{\text{[Fe6]} (3 \text{ mol } \%) \quad \text{[Fe6]} (\text{BuOK} (0.5 \text{ equiv.}))} \text{toluene} \\ \text{100 }^{\circ}\text{C, 16 h} \quad \text{R1}^3\text{N} \xrightarrow{\text{41 examples}} \text{R2}^3\text{N} $$

41 examples
61-88 %

$$ \underset{\text{[Fe6]}}{\text{[Fe6] =}} \quad \text{[Fe6]} \xrightarrow{\text{41 examples}} \text{[Fe6]}^3 \quad \underset{\text{83 \%}}{\text{[Fe6]}} \quad \text{[Fe6]}^3 \xrightarrow{\text{62 \%}} \text{[Fe6]}^3 \quad \underset{\text{83 \%}}{\text{[Fe6]}} $$

<caption>Scheme 43. Synthesis of pyrimidines.</caption>

$$ \underset{\text{R1}}{\text{OH}} \xrightarrow{\text{R1}} \underset{\text{H}_2\text{N}^+}{\text{R}^2} \xrightarrow{\text{[Co1]} (5 \text{ mol } \%) \quad \text{[Co1]} (\text{NaBHEt}_3 (5 \text{ mol } \%) \quad \text{[Co1]} (\text{BuOK} (5 \text{ mol } \%) \quad \text{toluene}} \quad \underset{\text{15 examples}}{\text{R1}^3\text{N}} \xrightarrow{\text{15 examples}} \text{R1}^3\text{N} $$

15 examples
22-93 %

$$ \underset{\text{[Co1]}}{\text{[Co1] =}} \quad \underset{\text{89 \%}}{\text{[Co1]}} \xrightarrow{\text{25 \%}} \underset{\text{70 \%}}{\text{[Co1]}} \xrightarrow{\text{90 \%}} \underset{\text{89 \%}}{\text{[Co1]}} $$

<caption>Scheme 44. Cobalt-catalyzed synthesis of pyrroles by Milstein and collaborators.</caption>

Following this work, the same catalyst was used for the synthesis of benzimidazoles.[67] Diamines reacted with various primary alcohol in the presence of NaBHEt₃ (5 mol%) and the PNN–Co catalyst (5 mol%), without any additional base. The desired benzimidazoles were isolated in 28 to 99% yields (Scheme 45). Various functional groups were tolerated allowing the formation of functionalized heterocycles.

Later on, Balaraman reported the synthesis of pyrroles and quinolines using a dimeric-cobalt complex bearing a SNS ligand.[68] 1,2-amino-alcohols reacted with secondary alcohols in the presence of 'BuOK (1 equiv.) and the dimeric complex (2.5 mol%) in refluxing xylene. Various non-symmetric alkyl- and aryl-substituted pyrroles were isolated following this methodology in 45 to 89% yields (Scheme 46). The same conditions were used for the reaction between 1,3-amino- alcohols and secondary alcohols, allowing the synthesis of pyridines and quinolines in 63 to 78% yields (Scheme 46). This

$$ \underset{\text{R1}}{\text{NH}_2} \xrightarrow{\text{R1}} \underset{\text{NaBHEt}_3}{\text{NH}_2} \xrightarrow{\text{[Co1]} (5 \text{ mol } \%) \quad \text{[Co1]} (\text{NaBHEt}_3 (5 \text{ mol } \%) \quad \text{toluene}} \underset{\text{150 }^{\circ}\text{C, 24 h}}{\text{R1}^3\text{N}} \xrightarrow{\text{18 examples}} \underset{\text{4 A MS}}{\text{R1}^3\text{N}^2} $$

18 examples
28-99 %

$$ \underset{\text{70 \%}}{\text{[Co1]}} \xrightarrow{\text{70 \%}} \underset{\text{28 \%}}{\text{[Co1]}} \xrightarrow{\text{28 \%}} \underset{\text{45 \%}}{\text{[Co1]}} \xrightarrow{\text{45 \%}} \underset{\text{99 \%}}{\text{[Co1]}} \xrightarrow{\text{99 \%}} \underset{\text{82 \%}}{\text{[Co1]}} \xrightarrow{\text{82 \%}} \underset{\text{95 \%}}{\text{[Co1]}} $$

<caption>Scheme 45. Cobalt-catalyzed synthesis of benzimidazoles.</caption>

$$ \underset{\text{R1}}{\text{NH}_2} \xrightarrow{\text{R1}} \underset{\text{NaBHEt}_3}{\text{NH}_2} \xrightarrow{\text{[Co2]} (2.5 \text{ mol } \%) \quad \text{[Co2]} (\text{BuOK} (1 \text{ equiv.})) \quad \text{xylene}} \underset{\text{15 examples}}{\text{R1}^3\text{N}} \xrightarrow{\text{15 examples}} \underset{\text{45-89 \%}}{\text{R1}^3\text{N}} \xrightarrow{\text{reflux, 24 h}} \underset{\text{11 examples}}{\text{R1}^3\text{N}} $$

15 examples
45-89 %

$$ \underset{\text{R1}}{\text{NH}_2} \xrightarrow{\text{R1}} \underset{\text{NaBHEt}_3}{\text{NH}_2} \xrightarrow{\text{[Co2]} (2.5 \text{ mol } \%) \quad \text{[Co2]} (\text{BuOK} (1 \text{ equiv.})) \quad \text{xylene}} \underset{\text{15 examples}}{\text{R1}^3\text{N}} \xrightarrow{\text{15 examples}} \underset{\text{45-89 \%}}{\text{R1}^3\text{N}} \xrightarrow{\text{reflux, 24 h}} \underset{\text{11 examples}}{\text{R1}^3\text{N}} $$

11 examples
63-87 %

$$ \underset{\text{[Co2]}}{\text{[Co2] =}} \quad \underset{\text{77 \%}}{\text{[Co2]}} \xrightarrow{\text{67 \%}} \underset{\text{83 \%}}{\text{[Co2]}} \xrightarrow{\text{63 \%}} \underset{\text{83 \%}}{\text{[Co2]}} \xrightarrow{\text{67 \%}} \underset{\text{CF}_3}{\text{[Co2]}} $$

<caption>Scheme 46. Synthesis of pyrroles, pyridines and quinolines.</caption>

complex [Co2], compared to Milstein's cobalt complex [Co1],[66] can be easily prepared as the ligand is not air-sensitive. In addition, more functional groups are tolerated with this methodology.

In 2018, a cobalt-catalyzed synthesis of quinolines, 2-alkylaminoquinolines and quinoxalines was reported by Kundu.[69] Starting from 2-aminobenzylalcohols and secondary alcohols, various 2-arylquinolinones were obtained in 55 to 93% yields (Scheme 47). 2-Aminoquinolines were also obtained in two steps using 2-aminobenzylalcohols and benzonitrile (Scheme 47). As reported by Srimani,[47] an initial Co-catalyzed
<figure><img src="image_47.png" /><figcaption>**Scheme 47.** Cobalt-catalyzed synthesis of quinolines and 2-aminoalkylquinolines by Kundu and collaborators.</figcaption></figure>

dehydrogenation leading to 2-aminoquinoline derivative was then followed by a Co-catalyzed amino-alkylation in a second step.

The same complex [Co3] was applied to the synthesis of quinoxalines using vicinal diols and 2-aminoanilines or 2-nitroanilines. Alkyl- and aryl-substituted quinoxalines were isolated in good to excellent yields (48–96%, Scheme 48).

In 2019, De Sarkar reported the use of Co(acac)₂, without any other ligand, as a catalyst for the synthesis of benzimidazoles.[70] Starting from ortho-nitroanilines and benzyl alcohols in the presence of ¹BuONa (1 equiv.) and Co(acac)₂ (5 mol%) in 1,4-dioxane at 135 °C, the desired heterocyclic compounds were obtained in moderate to good yields (39 to 81%, Scheme 49). Both in terms of group tolerance and reactivity, this simple catalytic system is competitive with the Co-pincer complex described by Milstein.[67] To highlight this reactivity, the 2-cyclopropylbenzimidazole, a heterocyclic compound with a strained and reactive cyclopropyl substituent, was prepared in 59% yield following this procedure.

In 2023 Samanta developed an innovative catalytic system based on cobalt(II) and non-innocent azo-aromatic ligands for the synthesis of quinolines.[71] This reaction, coupling 2-amino-benzyl alcohols with ketones via dehydrogenative oxidation,

<figure><img src="image_48.png" /><figcaption>**Scheme 48.** Synthesis of quinoxalines.</figcaption></figure>

<figure><img src="image_49.png" /><figcaption>**Scheme 49.** Cobalt-catalyzed synthesis of benzimidazoles.</figcaption></figure>

proved highly efficient, achieving moderate to good yields of 55% to 91% (Scheme 50).

Continuing the work exploring the use of phosphine free cobalt complexes, Paul demonstrated the use of Co-complex bearing a 2-(phenyldiazenyl)-1,10-phenanthroline ligand [Co5] could catalyze the synthesis of N-heterocycles via dehydrogenative coupling of amines and alcohols.[72] 1H-Indole was synthesized via an intramolecular dehydrogenative N-alkylation reaction in 82% yield, and various substituted quinolines were
<figure><img src="image_50.png" /><figcaption>**Scheme 50.** Cobalt-catalyzed synthesis of quinolines.</figcaption></figure>

synthesized, yielding 81–92% by coupling of 2-aminobenzyl alcohol and secondary alcohols (Scheme 51).

For the first time in 2024, tetrazene-based cobalt complexes were synthesized via a novel dehydrogenation coupling of amines.[73] These complexes have demonstrated their effectiveness in dehydrogenative coupling reactions for the synthesis of quinolines and quinazolinones in 58–91% yield, and for the preparation of bioactive molecules such as (±)-galipinine (Scheme 52).

### 4.5. Nickel-Catalyzed Dehydrogenative Synthesis of Heterocycles

In 2017, Paul reported the first nickel-catalyzed synthesis of heterocycle derivatives following a dehydrogenative coupling

<figure><img src="image_51.png" /><figcaption>**Scheme 51.** Synthesis of quinoline by Paul and collaborators.</figcaption></figure>

<figure><img src="image_52.png" /><figcaption>**Scheme 52.** Cobalt-catalyzed synthesis of quinolines and quinazolinones.</figcaption></figure>

methodology.[74] In the optimized reaction conditions, benzyl alcohols reacted with 2-aminobenzamides in the presence of tBuONa (1.5 equiv.) and a nickel complex bearing a tetraaza macrocyclic ligand (5 mol%) in xylene at 100 °C (Scheme 53). Both the yields and the diversity of substituent for the reported small library of quinazolin-4(3H)-ones are similar to those

<figure><img src="image_53.png" /><figcaption>**Scheme 53.** Nickel-catalyzed synthesis of quinazolinones.</figcaption></figure>
described previously by Li[43] in the presence of Cu(OAc)₂ or by Nguyen in the presence of FeCl₃.[56]

The same complex [Ni1] allowed also to open the route to quinolines via either a mono- or double-dehydrogenative coupling process.[75] Various substituted aryl and aliphatic

alcohols or ketones were engaged with 2-aminobenzyl alcohol in the presence of stoichiometric amount of BuOK and catalytic amount of the nickel complex in toluene at 90 °C. Both methods led to the desired heterocycles in moderate to good yields (29–89%, Scheme 54), albeit, the double-dehydrogenative methodology furnished the quinolines products in slightly lower yields.

Paul *et al.* also described the synthesis of quinazolines using 2-aminobenzylamine and alcohols, or 2-aminobenzyl alcohol and benzonitrile.[76] The quinazolines products were isolated in low to good yields using whatever methods (25 to 86%, Scheme 55). Again, the mono-dehydrogenative coupling (i.e., Method B, Scheme 55) led to better results.

In 2019, Paul reported another nitrogen ligand-based nickel complex and its application in dehydrogenative processes, i.e. the synthesis of quinolines, aminoquinolines and quinazolines (Scheme 56).[77] The main feature of these catalytic reactions are the following: (i) less amount of base required,[77] and (ii) the reaction can be carried out under an air atmosphere, otherwise, the reaction conditions and the isolated yields are very similar.

Finally, the activities of [Ni1] and [Ni2] in the synthesis of pyrimidines were compared.[78] Very similar conditions were used for both complexes. Nevertheless, the second [Ni1] led to slightly higher yields in most cases (20–83%, Scheme 57).

In 2018, Banerjee described a very simple and inexpensive 2,2'-bipyridine NiCl₂ complex catalyzed synthesis of pyrroles.[79] Following the same strategy than Barta,[57] various unsaturated 1,4-diols reacted with aromatic and aliphatic amines to provide the pyrrole derivatives in 30 to 80% yields (Scheme 58). Similar results were obtained with Barta or Sundararaju's methodologies.[57,58] Nevertheless, this methodology does not require the synthesis of the catalyst a priori.

<figure><img src="image_54.png" /><figcaption>Scheme 54. Synthesis of quinolines from ketones or secondary alcohols.</figcaption></figure>

<figure><img src="image_55.png" /><figcaption>Scheme 55. Synthesis of quinazolines from benzylamines or benzonitriles.</figcaption></figure>

<figure><img src="image_56.png" /><figcaption>Scheme 56. Synthesis of quinolines and 2-aminoquinolines reported by Paul.</figcaption></figure>
<figure><img src="image_57.png" /><figcaption>**Scheme 57.** Synthesis of pyrimidines reported by Paul and collaborators.</figcaption></figure>

<figure><img src="image_58.png" /><figcaption>**Scheme 58.** Synthesis of pyrroles.</figcaption></figure>

Following the methodology proposed by Kempe,[45] amino-alcohols can be pro-nucleophile/pro-electrophile partners in this process. Thus, they can react with either ketones[80] or secondary alcohols,[81] in the presence of ¹BuOK (1 or 2 equiv.), bipyridine-type ligand (12 mol%) and NiCl₂ (10 mol%) at elevated temperatures. Various non-symmetric pyrroles were obtained in yields ranging from 30 to 70% (Scheme 59).

The NiCl₂(bpy) complex was then applied for the synthesis of quinolines and from amino-alcohols and ketones as also observed by Paul.[75] The quinoline products were obtained in moderate to good yields (48–90%, Scheme 60). Few substituents were described in this work, and consequently the scope appeared limited compared to other works.

Concerning the NiCl₂(1,10-phenanthroline) complex, it was further used for the synthesis of benzimidazoles (Scheme 61).[82] Contrary to the work of Wang[41] or Srimani,[48] for which the nature and the loading of the base changed the selectivity for the final product, the selectivity for the mono- or disubstituted benzimidazole was only driven by the amount of the alkylating agent.

<figure><img src="image_59.png" /><figcaption>**Scheme 59.** Synthesis of pyrroles from amino-alcohols.</figcaption></figure>

<figure><img src="image_60.png" /><figcaption>**Scheme 60.** Synthesis of quinolines and pyridines.</figcaption></figure>

In 2020, Kundu described the synthesis of quinoxalines using Ni(II)/phen complex.[83] The desired heterocycles were obtained following a similar methodology previously reported by this group using a cobalt catalyst.[69] Lower yields were
obtained with the nickel system and more specifically with alkyl substituents (Scheme 62).

The same year, the synthesis of pyrimidines and triazines was reported by Adhikari.[84] Following previously reported methods for the synthesis of pyrimidines[41,42,64] or 1,3,5-triazines[37] and using a new nickel(II) complex, various heterocyclic products were isolated in good to very good yields (65–91%, Scheme 63). Nevertheless, this reaction was limited to benzamidines and benzyl alcohols.

In 2024, Balakrishna synthesized a series of cationic Ni(II) complexes based on aminophosphine triazolyl-pyridine ligands for the production of substituted quinolines via acceptorless dehydrogenative coupling at 110 °C under microwave irradiation with 20 mol% of KOH (Scheme 64).[85] These complexes provided a wide range of derivatives (62 examples) in good to excellent yields (75–98%).

<figure><img src="image_61.png" /><figcaption>**Scheme 62.** Nickel-catalyzed synthesis of quinazolines.</figcaption></figure>

<figure><img src="image_62.png" /><figcaption>**Scheme 63.** Synthesis of pyrimidines and triazines.</figcaption></figure>

<figure><img src="image_63.png" /><figcaption>**Scheme 64.** Nickel-catalyzed synthesis of quinoline.</figcaption></figure>

## 5. Summary and Outlook

The dehydrogenative coupling reaction is not only an effective tool for accessing many heterocyclic frameworks, but also a means to open the door to developing new methodologies that expand the synthetic toolbox for heterocycle chemistry,[86] with special interest on N-heterocycles. Through the use of novel catalysts, reagents, and reaction conditions, researchers continue to unlock new opportunities for the synthesis of bioactive and functionalized heterocyclic compounds.

## Acknowledgements

We gratefully acknowledge financial support from the "Ministère de la Recherche et des Nouvelles Technologies", Normandie Université, University of Caen Normandie, CNRS, "Région Normandie", the LABEX SynOrg (ANR-11-LABEX-0029), and European Union (HORIZON-MSCA-2022-DN-01 Project: 101119574). C. M. thanks Sorbonne University Alliance for a PhD grant. We thank the Spanish Ministerio de Ciencia e Innovación for project PID2021-127423NB-I00 and the Generalitat de Catalunya for project 2021SGR623. A. P. is a Serra Húnter Fellow.

## Conflict of Interests

The authors declare no conflict of interest.

**Keywords:** Acceptorless Dehydrogenative Coupling · alcohol · nitrogen containing heterocyclic derivatives · coupling · quinolin-2(1H)-one
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Manuscript received: December 1, 2024

Revised manuscript received: February 16, 2025

Accepted manuscript online: February 18, 2025

Version of record online: March 10, 2025

[Correction added on 2 June 2025, after first online publication: The copyright line was changed.]