Transition Metal-catalyzed Carbene Coupling Reaction

Transition Metal-catalyzed Carbene Coupling Reaction | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Chinese Journal of Chemistry This is a preprint and has not been peer reviewed. Data may be preliminary. 9 October 2025 V1 Latest version Share on Transition Metal-catalyzed Carbene Coupling Reaction Authors : Ying Li , Ze Li , Shufeng Chen , and Jianbo Wang 0000-0002-0092-0937 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175997169.98464121/v1 Published Chinese Journal of Chemistry Version of record Peer review timeline 428 views 182 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract A brief introduction to the key scientists’ main contributions in the field. (The following part is suggested to be presented in the form of a timeline with a flexible format, which could be redesigned by the authors.) functionalization Cite this paper: Chin. J. Chem. 2025 , 43 , XXX—XXX. DOI: 10.1002/cjoc.202400XXX Transition Metal-catalyzed Carbene Coupling Reaction Ying Li, a Ze Li, a Shufeng Chen,* , a and Jianbo Wang* , a,b a College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P. R. China b Beijing National Laboratory of Molecular Sciences (BNLMS) and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China Transition-metal-catalyzed | Cross-coupling | Carbene |Diazo compounds | N-Tosylhydrazones Comprehensive Summary A brief introduction to the key scientists’ main contributions in the field. (The following part is suggested to be presented in the form of a timeline with a flexible format, which could be redesigned by the authors.) functionalization Content 1. Introduction 3 2. Pd-Catalyzed Carbene Cross-coupling Reactions 3 2.1. The early developments 3 2.2. Further expansion of the Pd-catalyzed carbene coupling reactions 5 2.3. Three-component cross-coupling reactions 6 2.4. Carbene migratory insertion of Pd-Si and Pd-B bonds 7 3. Cu(I)-Catalyzed Carbene Cross-coupling Reactions 8 4. Rh-Catalyzed Carbene Coupling Reactions 10 5. C-H Bond functionalization with carbene coupling 11 5.1. Directing-group-assisted C-H bond functionalizations 11 5.2. C(sp 3 )−H bond functionalization with carbene migratory insertion 12 6. Asymmetric Catalysis in Carbene Coupling 13 7. Pd-Catalyzed Cross Coupling with Difluorocarbene 14 8. Non-Diazo Compounds as Carbene Precursors 15 9. Pd-Catalyzed Carbene Coupling for Polymerization 16 10. Conclusion 17 Acknowledgement 17 References 17 1. Introduction Transition-metal-catalyzed carbene transfer reactions have been well-established as synthetic tools for creating organic molecules for the applications in various fields such as pharmaceutical and functional material industries. 1-6[] The classic Transition-metal-catalyzed carbene transfer reactions mainly include C-H bond and X-H (X = Si, O, N, S etc) bond insertions, cyclopropanations and ylide reactions. The field has been extensively explored over the past decades, in particular, remarkable progresses have been made in the corresponding catalytic asymmetric carbene transfer reactions. In addition to traditional transition-metal catalysis, the carbene transfer reactions have also been found effective by using artificial metalloenzymes, whose activity and selectivity can be optimized by directed evolution. 7[] On the other hand, transition-metal-catalyzed cross-coupling reactions have been indispensable in modern synthetic organic chemistry, enabling effective C-C bond formations needed for building up molecular complexity. 8-10[] The field has been extensively explored over the decades, and one of the endeavors in this arena is the expanding of cross-coupling partners. While diazo compounds and other carbene precursors are highly useful in generating metal carbene intermediates in transition-metal-catalyzed transformations, their uses as cross-coupling partners have seldom been considered until recently. It is evident that if carbene precursors could be commonly utilized in cross-coupling reactions, the coupling chemistry should be significantly enriched. In the past two decades, one has witnessed the rapid evolvement of the research along this line. Now, carbene precursors, in particular diazo compounds and N -tosylhydrazones have been employed in various transition-metal-catalyzed carbene coupling reactions. 11-13[] As shown in Scheme 1, the common feature of this type of cross-coupling reactions is the formation of metal carbene species and the carbene migratory insertion process. Such type of novel coupling reactions have been demonstrated to be general, in which the metal catalysts include Pd, Cu, Rh, Ru, etc. In this article, we briefly summarize the typical advances in this arena by discussing some selected examples. Scheme 1 . General pathway for transition-metal-catalyzed cross-coupling with carbene precursors 2. Pd-Catalyzed Carbene Cross-coupling Reactions 2.1. The early developments In 2001, Van Vranken and co-workers reported a significant advancement in Pd-catalyzed cross-coupling chemistry, demonstrating the coupling of diazo compounds with benzyl halides. Using (trimethylsilyl)diazomethane (TMSCHN₂) as a carbene precursor, the method enabled efficient synthesis of styrene derivatives. 14[] To explain the formation of the olefinic coupling products, the authors proposed a catalytic cycle involving carbene generation followed by a subsequent migratory insertion step (Scheme 2 ). Scheme 2 . Pd-catalyzed cross-coupling reaction between benzyl halides and TMSCHN 2 Subsequently, they further developed a Pd-catalyzed three-component reaction involving vinyl iodides, amines, and TMSCHN 2 . 15[] This methodology generated π-allyl palladium intermediates via carbene migratory insertion, which were then intercepted by external amino nucleophiles to afford TMS-substituted allylamines in moderate to excellent yields. The nucleophilic substitution of amines onto the π-allyl palladium intermediates exhibited regioselectivity toward the distal position of the TMS group, ultimately forming products with vinylsilane structures. Scheme 3 . π-Allyl palladium species trapped by amines They also explored the Pd-catalyzed insertion of α-diazoesters into vinyl halides, establishing a robust method for producing α,β -unsaturated γ -amino acid derivatives (Scheme 3). 16[] Furthermore, they effectively showcased the use of N -tosylhydrazones as carbene precursors in a three-component reaction, employing benzyltriethylammonium chloride as a phase transfer catalyst and lithium tert-butoxide as the base to generate allylamines with notable regioselectivity. 17[] Again, the stereochemical outcomes in the nucleophilic substitution process were notably influenced by the steric hindrance of the carbene moiety. In particular, Z -configured vinyl iodides proved more effective than their E -configured counterparts in these transformations. In 2007, Barluenga and co-workers first applied N -tosylhydrazones in Pd-catalyzed cross-coupling with aryl halides under basic conditions (Scheme 4) 18[] . In this transformation, Pd₂(dba)₃ combined with XPhos (2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl) serves as an efficient catalyst. Tosylhydrazones derived from cyclic ketones, aryl ketones, or alkyl carboxyaldehydes undergo coupling with aryl bromides and chlorides under these conditions. The reaction exhibits broad functional group tolerance, highlighting its versatility. Scheme 4 . Pd-catalyzed cross-coupling of N -tosylhydrazones Later, they reported an efficient one-pot, single-step transformation in which carbonyl compounds are directly employed as coupling partners. In this protocol, the reaction proceeds through the in situ formation of N -tosylhydrazones in the presence of tosylhydrazine (Scheme 5). 19[] This strategy enables the synthesis of a wide range of polysubstituted olefins. Notably, this approach grants access to 4-aryltetrahydropyridines, which serve as important scaffolds in medicinal chemistry. Such characteristics emphasize its synthetic value and suggest significant potential in drug discovery applications. Scheme 5 . Pd-catalyzed one-pot conversion of carbonyls Meanwhile, our research focused on the exploration of Pd-catalyzed cross-coupling reactions involving diazo compounds. In initial studies, Pd catalysis was employed to couple ethyl diazoacetate with common electrophiles such as aryl and vinyl iodides. Notably, the cross-coupling of vinyl iodides with ethyl diazoacetate, using Pd(PPh₃)₄ as the catalyst and triethylamine as an additive, led to the formation of α,β-unsaturated α-diazoesters (Scheme 6). 20[] This outcome was unexpected, as it challenged the prevailing notion that diazo groups are highly reactive and readily decompose upon exposure to transition metals. Moreover, the successful coupling with aryl iodides highlighted the broader applicability of this strategy across diverse electrophiles. This work expanded the synthetic use of diazo compounds and challenged assumptions about their stability in transition-metal catalysis. Scheme 6 . Pd-catalyzed cross-coupling of vinylhalides with ethyl diazoacetate Notably, this C-H bond coupling reaction represents a new type of methods to access diazo compounds, which have many applications in organic synthesis. Since its publication, this coupling reaction has attracted significant attentions and application. 21-30[] For an example, Davies and Williams found this coupling reaction is crucial in their synthesis of functionalized bicyclo[5.3.0]decane frameworks, which exist in many natural products (Scheme 7). 24[] Scheme 7 . Application of the Pd-catalyzed C-H functionalization of diazoacetate 2.2. Further expansion of the Pd-catalyzed carbene coupling reactions We subsequently developed a Pd-catalyzed oxidative cross-coupling between arylboronic acids and α-diazocarbonyl compounds, providing direct access to α-aryl α,β-unsaturated carbonyl derivatives (Scheme 8). 31[] The reaction utilized benzoquinone (BQ) as a stoichiometric oxidant to regenerate Pd(II) while notably preventing the oxidation of the diazo group. Mechanistic studies support a pathway involving: (i) transmetallation to form the aryl-Pd(II) intermediate A ; (ii) dediazoniation to generate the Pd-carbene complex B ; (iii) aryl migratory insertion into the carbene center gives intermediate C ; (iv) β-hydride elimination to release the product and Pd(0); (v) oxidation of Pd(0) by BQ to close the catalytic cycle. Scheme 8 . Pd-catalyzed oxidative cross-coupling between boronic acids and α-diazocarbonyl compounds Allyl and aryl halides have been well-established as versatile electrophilic coupling partners in Pd-catalyzed transformations with α-diazocarbonyl compounds. Accordingly, we developed a Pd-catalyzed cross-coupling of α-diazocarbonyl compounds with allyl halides, affording polysubstituted 1,3-dienes in good yields and with high E/Z selectivity (Scheme 9). 32[] Otherwise, the trifluoromethyl- and phosphonate-substituted diazo compounds also proved to be effective partners. Based on these findings , the proposed reaction mechanism involves a π-allyl palladium intermediate. 3334[,] Scheme 9 . Pd-catalyzed couplings of diazo compounds with allyl halides Subsequently, the carbene coupling with π-allylic palladium species was employed by Gong and co-workers in their π-allylic C–H bond functionalization system, enabling the direct olefination of allylic substrates with α -diazo esters. 35[] Additionally, Szabó and co-workers reported the use of allylboronic acids in a Pd-catalyzed cross-coupling with α-diazoketones (Scheme 10). Notably, this reaction proceeded in the absence of an external oxidant, thereby distinguishing it from conventional Pd-catalyzed allylic functionalizations. The absence of β-hydride elimination in the catalytic cycle indicates that the Pd(II) catalyst remains in its original oxidation state throughout the transformation. 36[] Scheme 10 . Allylboronic acids in Pd-catalyzed cross-coupling with α-diazoketones Furthermore, we have extended the Pd-catalyzed oxidative coupling strategy to N -tosylhydrazones, thereby enabling the synthesis of di-, tri-, and tetrasubstituted alkenes from arylboronic acids (Scheme 11). 37[] The optimized reaction conditions utilize Pd(PPh₃)₄ as the catalyst, LiO t Bu as the base, CuCl as an additive, and O₂ as a synergistic oxidant for the regeneration of Pd(II). This method exhibits broad tolerance for a wide range of electronic and steric variations in both coupling partners, delivering olefins in moderate to good yields. Scheme 11 . Pd-catalyzed oxidative coupling of arylboronic acids with N -tosylhydrazones Moreover, we have reported a Pd-catalyzed oxidative coupling reaction between terminal alkynes and N -tosylhydrazones, providing a highly efficient approach to conjugated enynes with exceptional stereoselectivity (Scheme 12). The reaction demonstrates broad substrate tolerance toward alkynes bearing aliphatic, aryl, heteroaryl, and silyl substituents, delivering exclusively Z -enyne products. 38[] The mechanism involves in situ diazo formation from N -tosylhydrazones, alkynyl-Pd(II) complex formation, migratory insertion into a Pd-carbene intermediate, and syn β-hydride elimination, regenerating Pd(II) with benzoquinone (BQ) as the oxidant. Notably, this methodology obviates the need for prefunctionalized alkenes or toxic organometallic reagents, providing a more efficient and environmentally benign approach to enyne synthesis. Scheme 12 . Pd-catalyzed oxidative coupling of N -tosylhydrazones with terminal alkynes 2.3. Three-component cross-coupling reactions Prior to Pd–carbene formation, the organopalladium intermediate generated via oxidative addition may undergo cascade transformations, such as carbonyl insertion, carbopalladation of unsaturated bonds or related processes, affording a new organopalladium species. This newly formed intermediate can subsequently react with a diazo compound to initiate the carbene cross-coupling sequence. Furthermore, the organopalladium species resulting from carbene migratory insertion may undergo alternative transformations before completing the catalytic cycle. These observations underscore the potential for developing Pd-catalyzed cascade coupling reactions, wherein carbene formation and migratory insertion represent key mechanistic steps. 39[] Scheme 13 . Pd-catalyzed four-component coupling of aryl iodide, CO, α-diazoester and triethylsilane In 2010, we reported a discovery involving a Pd-catalyzed four-component coupling reaction. This reaction involved aryl iodide, CO, α -diazoester and triethylsilane, resulting in the synthesis of a range of 1,3-dicarbonyl compounds (Scheme 13). 40[] Mechanistically, the pathway involves carbonylation to generate a novel acyl-Pd(II) species, which actively participates in the catalytic cycle. Subsequent migratory insertion of Pd-carbene and transmetalation with Et 3 SiH result in a H-Pd(II) species, ultimately undergoing reductive elimination to produce the products. The intermolecular carbopalladation of unsaturated bonds can also be incorporated into the reaction. For instance, we reported a Pd-catalyzed cascade involving aryl iodides, allenes, and diazo compounds, enabling the efficient synthesis of multisubstituted 1,3-dienes (Scheme 14). 41[] The transformation proceeds through intermolecular carbopalladation of the allene, generating an allylic-Pd(II) intermediate that subsequently undergoes migratory insertion with a Pd–carbene species. In a related study, we also developed a cascade reaction wherein a π-allyl palladium intermediate, generated via carbopalladation of an allene with an aryl iodide, subsequently reacts with a diazoacetate to furnish 1,3-diene products. These findings highlight the synthetic utility and mechanistic diversity of Pd-catalyzed cascade reactions for the construction of complex conjugated olefins. Scheme 14 . Pd-catalyzed cascade reaction of aryl iodide, allenes and α-diazoester When β-H elimination is not feasible, the reaction can proceed through alternative pathways involving transmetalation and reductive elimination, leading to the formation of multicomponent products. For an example, we incorporated terminal alkynes as a third component in Pd-catalyzed cross-coupling reactions of N -tosylhydrazones with aryl halides, resulting in the synthesis of benzhydryl acetylene derivatives (Scheme 15). 42[] The reaction mechanism involves carbene migratory insertion, followed by the generation of an alkynyl-Pd(II) species via transmetalation, which then undergoes reductive elimination to yield the final products. Scheme 15 . Pd-catalyzed cross-couplings of N -tosylhydrazones with aryl halides and terminal alkynes 2.4. Carbene migratory insertion of Pd-Si and Pd-B bonds The traditional carbene σ-bond insertions are typically confined to X-H insertions (X = C, Si, O, N, S…). These insertions adhere to concerted or ylide mechanisms, which are unsuitable for carbene C-C or C-Si bond insertions. 43[] Therefore, leveraging the carbene migratory process, we have introduced a novel approach to σ-bond insertion. This strategic pathway entails a series of steps including oxidative addition, carbene formation, migratory insertion, and reductive elimination (Scheme 16). Scheme 16 . Stepwise strategy for carbene insertion into σ bonds We have demonstrated that this strategy can be applied to carbene-mediated Si–C bond insertion. Accordingly, a highly efficient Pd-catalyzed carbene insertion into the strained Si–C bonds of silacyclobutanes with excellent enantioselectivity has been developed. This reaction provides a straightforward approach to access silacyclopentanes bearing three or four-substituted stereocenters in an asymmetric manner (Scheme 17). Mechanistic studies employing DFT calculations support a catalytic cycle involving oxidative addition, carbene migratory insertion, and reductive elimination. Scheme 17 . Pd-catalyzed carbene C-Si bond insertion of silacyclobutane In the aforementioned Pd-catalyzed carbene coupling reactions, the carbene migratory insertion into Pd–C bonds is the pivotal step leading to C–C bond formation. Considering the broad utility of such couplings, extending the migratory insertion pathway to other Pd–X bonds (X = B, Si, N, O, etc.) is of significant interest. In this regard, we reported a Pd-catalyzed reaction of N -tosylhydrazones with bis(pinacolato)diboron (B₂pin₂). This method efficiently converts ketones into di-, tri-, and tetrasubstituted alkenylboronates, which are versatile intermediates in organic synthesis (Scheme 18). 44[] Scheme 18 . Synthesis of alkenylboronates by Pd-catalyzed carbene coupling As illustrated in Scheme 19, the proposed mechanism commences with the transmetalation between bis(pinacolato)diboron and Pd(OAc)₂ to afford the B–Pd(II) species B . This intermediate reacts with the diazo compound, which is generated in situ from N -tosylhydrazone. This reaction forms the palladium carbene species C . Subsequently, boryl migratory insertion produces intermediate D . Intermediate D then undergoes cis -H elimination to furnish the final product along with intermediate E . Intermediate E undergoes reductive elimination to form the Pd(0) species F . Then, 2,5-DMBQ oxidizes F to regenerate the Pd(II) species A , completing the catalytic cycle. DFT calculations support this proposed pathway. Scheme 19 . Proposed rection mechanism for alkenylboronate synthesis 3. Cu(I)-Catalyzed Carbene Cross-coupling Reactions The organocopper carbene complexes have attracted significant attention due to their unique reactivity and ability to undergo migratory insertion, leading to new organocopper species. These intermediates participate in various reaction pathways, including protonation, β-fluoride elimination, and electrophilic coupling, thereby providing access to a wide range of functionalized products. Understanding the formation and reactivity of Cu–carbene intermediates is crucial for the development of novel catalytic methodologies and expanding the synthetic utility of organocopper chemistry. In 2011, we reported a Cu(I)-catalyzed carbene coupling of N -tosylhydrazones with terminal alkynes using Cu(MeCN)₄PF₆ and a bisoxazoline ligand, enabling the efficient synthesis of trisubstituted allenes under mild conditions (Scheme 20). 45[] Subsequently, we developed a CuI-catalyzed one-pot, two-step protocol for synthesizing 1,3-disubstituted allenes, which N -tosylhydrazones were generated in situ directly from the corresponding aldehydes. 46[] Mechanistically, these transformations proceed via migratory insertion of a carbene moiety into the Cu(I)–alkynyl bond, followed by protonation of the resulting organocopper intermediate at the terminal alkynyl carbon to furnish the allene products. Scheme 20 . Cu-catalyzed coupling of N-tosylhydrazones with terminal alkynes This straightforward allene synthesis strategy has garnered substantial attention and has been employed as a pivotal step in the assembly of structurally complex natural products. As depicted in Scheme 21, Lee and Han have described the divergent synthesis of the fused tetracyclic core of conidiogenone natural products, which exhibit pronounced cytotoxic, anti-inflammatory, antimicrobial, and anti-allergic activities. The key-step in the Lee’s synthetic strategy to construct the core structure of the fused ring system is the intramolecular [3+2] cycloaddition of the diazo moiety with allene moiety. 47[] Scheme 21 . Synthesis of natural products with Cu-catalyzed allene formation as the key step Alternatively, these coupling reactions can be extended to the enantioselective construction of axially chiral allenes, which represents a challenging topic in asymmetric catalysis. Feng and co-workers demonstrated this approach through a Cu-catalyzed asymmetric synthesis of chiral 2,4-disubstituted allenoates from α-diazoesters, employing chiral guanidinium salts to induce enantioselectivity. 48[] Subsequently, we realized the enantioselective synthesis of axially chiral trisubstituted allenes using a Cu(MeCN)₄PF₆/chiral dioxazoline catalytic system with diazoalkanes as carbene precursors. The reaction featured a broad substrate scope, showing good compatibility with various terminal alkynes and diazoalkane derivatives. 49[] Under appropriately tailored conditions, Cu-catalyzed carbene coupling with terminal alkynes can be directed toward the synthesis of internal alkynes. Building on this reactivity, we developed a Cu-catalyzed synthesis of internal alkynes via the cross-coupling of N -tosylhydrazones with trialkylsilyl substituted terminal alkynes. 50[] Mechanistically, the transformation involves carbene migratory insertion to generate a propargyl–Cu(I) species, which undergoes protonation at the carbenic carbon to yield the final products. Notably, the reaction can be conducted in a one-pot protocol directly from aldehydes or ketones. The trialkylsilyl substituent has shown to be critical for promoting selective C(sp)–C(sp³) bond formation within this methodology. Scheme 22 . Cu-catalyzed coupling of N -tosylhydrazones with trialkylsilyl substituted terminal alkynes In 2012 , Ma and co-workers reported a Cu(I)-catalyzed coupling of terminal alkynes with trifluoromethyl diazoethane (CF₃CHN₂) as the carbene precursor, enabling the direct synthesis of trifluoroethyl-substituted alkynes (Scheme 23). 51[] Notably, the protocol offers operational simplicity and enables the parallel synthesis of trifluoroethylated alkynes. Furthermore, the authors devised an efficient recycling system to utilize CF₃CHN₂ gas generated in situ from CF₃CH₂NH₂·HCl. Scheme 23 . Cu(I)-catalyzed trifluoroethylation of terminal alkynes with gaseous CF 3 CHN 2 Beyond terminal alkynes, arenes bearing acidic C(sp²)–H bonds have also been identified as suitable coupling partners in Cu-catalyzed carbene cross-coupling reactions. In 2011, we reported a Cu-catalyzed C–H benzylation and allylation of 1,3-azoles using N -tosylhydrazones as carbene precursors, enabling efficient access to C2-functionalized azole derivatives (Scheme 24). 52[] This work established a transition-metal-catalyzed strategy for the direct C–H functionalization of heteroarenes with secondary benzylic substrates. It enables efficient functionalization of electron-deficient C–H bonds, contrasting with conventional carbene insertion into electron-deficient C–H systems. Bis(trimethylsilyl)diazomethane was subsequently employed as the carbene precursor, enabling access to gem-disilylated oxazole products that could be further transformed into methylated or alkenylated derivatives. 53[] Subsequently, Miura and co-workers reported a Ni- and Co-catalyzed direct C–H alkylation of azoles employing unactivated alkyl N -tosylhydrazones. This methodology enables the efficient synthesis of alkylated heteroarenes. 54[] Scheme 24 . Cu-catalyzed C–H bond functionalization of 1,3-azoles with N -tosylhydrazones 4. Rh-Catalyzed Carbene Coupling Reactions Rh(II) complexes are well established as highly efficient catalysts in carbene transfer reactions, particularly in C–H insertions, cyclopropanations, and ylide formations. In 2011, Yu and co-workers reported the first Rh(I)-catalyzed one-pot, three-component coupling for the synthesis of quaternary α,α-heterodiaryl carboxylic esters (Scheme 25). 55[] The reaction begins with the formation of an arylrhodium intermediate, which undergoes migratory insertion of the diazo-derived carbene to generate a key allylrhodium species. Subsequent reaction with electrophiles under basic conditions was performed. Control experiments confirmed that the presence of t BuOK is essential for the alkylation step, with mechanistic evidence suggesting that it facilitates a cation exchange process leading to the formation of a key potassium enolate intermediate. The reaction exhibits excellent compatibility with a broad range of electrophiles: beyond halobenzyl reagents, simple alkylating agents such as methyl iodide, allyl bromide, and even N -chloromorpholine participate effectively. Notably, when water was employed as a proton source, the reaction delivered diaryl acetate esters in high yields, underscoring the operational versatility of the transformation. Scheme 25 . Rh-catalyzed carbene coupling reaction In 2014, we reported the first Rh(I)-catalyzed formal carbene insertion into C–C bonds of benzocyclobutenols using α-diazoesters, enabling the regio- and diastereoselective synthesis of highly substituted indanols bearing all-carbon quaternary centers under mild conditions (Scheme 26). 56[] This study highlighted carbene coupling as an effective strategy for C–C bond functionalization. Scheme 26 . Rh(I)-catalyzed formal carbene insertion into C−C bond As shown in Scheme 27, the mechanism involves β-carbon elimination to form the arylrhodium(I) intermediate, followed by carbene formation, migratory insertion, and intramolecular aldol cyclization. This sequence delivers indanol products bearing an all-carbon quaternary center at the carbene-derived position. Scheme 27 . Proposed reaction mechanism for Rh(I)-catalyzed C-C carbene insertion Later, Chen and co-workers employed this novel ring-expansion reaction in their synthesis of haouamine A, a structurally complex and highly strained cyclophane alkaloid (Scheme 28). 57[] The unique properties of haouamine A stem from its distorted aromatic ring system, rendering its synthesis particularly challenging. In this strategy, a Rh(I)-catalyzed ring expansion of a cyclobutanol derivative served as a pivotal step, highlighting the utility of this transformation as a powerful tool in the synthesis of structurally complex natural products. Scheme 28 . Synthesis of Haouamine A based on Rh(I)-catalyzed carbene C-C bond Insertion 5. C-H Bond functionalization with carbene coupling 5.1. Directing-group-assisted C-H bond functionalizations In recent years, directed group-assisted C–H metalation has emerged as a powerful strategy for C–H functionalization. Among the various coupling partners, diazo compounds and N -tosylhydrazones have proven particularly efficient and synthetically valuable. In this context, Yu and co-workers in 2012 reported the first Rh(III)-catalyzed carbonylative coupling of α-diazophosphonates with aromatic C–H bonds. 58[] This transformation proceeds efficiently with a variety of directing groups, including oximes, pyridines, carboxylic acids, and amines, which facilitate C–H activation followed by coupling. The catalytic system comprising [Cp*RhCl₂]₂ and AgOAc effectively promotes C–H functionalization, enabling the construction of soquinoline scaffolds in excellent yields via a sequential carbene coupling/amidation process (Scheme 29a). The proposed mechanism involves electrophilic C–H activation to generate an aryl–Rh(III) intermediate, which reacts with a diazo compound to form an Rh(III) carbene via nitrogen extrusion, followed by migratory insertion and protonation to furnish the product. Meanwhile, a concurrent 1,2-aryl migration during nitrogen extrusion to the key intermediate cannot be ruled out (Scheme 29b). Stoichiometric experiments with benzo[h]quinoline as the substrate afforded a cyclometalated Rh(III) complex, which reacted with α-diazoacetate to yield a σ-alkyl–Rh(III) complex in high yield, supporting the proposed mechanism (Scheme 29c). Scheme 29 . Rh(III)-catalyzed carbene coupling via C-H activation Furthermore, significant progress has recently been achieved in Rh(III)-catalyzed carbene coupling via C–H activation. In 2013, Li and co-workers reported an Rh(III)-catalyzed coupling of azacycle-directed aromatic C–H bonds with α-diazomalonates, enabling regioselective ortho-alkylation of a broad range of (hetero)arenes under mild conditions. 59[] Various nitrogen-containing heterocycles, including pyrazole, pyrimidine, pyridine, and oxazole, have been demonstrated as effective directing groups for Rh(III)-catalyzed C–H functionalization. The reaction efficiency can be enhanced by increasing the amount of α-diazoacetate, enabling dialkylation with good yields. In addition, using α-diazo Meldrum’s acid as a carbene precursor allows the introduction of a CH₂CO₂Et fragment via ortho-functionalization followed by alcoholysis and decarboxylation to furnish the desired product (Scheme 30). Scheme 30 . Rh(III)-catalyzed carbene coupling via azacycle-directed C-H activation In 2013, the Rovis group pioneered the Rh(III)-catalyzed directed coupling of N -pivaloyloxybenzamides with donor/acceptor diazo reagents, providing a new strategy for C–H functionalization. 60[] A variety of diazo compounds, including 1-aryl-2,2,2-trifluorodiazoethanes, undergo efficient coupling with benzamides under mild conditions, providing straightforward access to isoindolinone frameworks bearing a tetrasubstituted carbon center (Scheme 31a). Similarly, Cui and co-workers reported that when vinyl diazonium compounds are employed as carbene precursors, the reaction proceeds exclusively via a formal [4+3] cycloaddition, affording azetidinones under comparable conditions (Scheme 31b). 61[] Scheme 31 . Rh(III)-catalyzed carbene cyclization triggered by C-N reductive elimination Glorius and co-workers developed a Rh(III)-catalyzed cyclization of oxime with diazo compounds for the synthesis of isoquinoline and pyridine N -oxides (Scheme 32). 62[] In this transformation, the initial C–H carbene coupling product undergoes oxime–ketone condensation or enol formation, followed by 6π-electrocyclization and H₂O elimination to afford the target product. Notably, the formation of pyridine N-oxides is proposed to proceed via vinyl C–H activation rather than aryl C–H activation. Scheme 32 . Rh(III)-catalyzed construction of pyridine N-oxides from oximes and diazo compounds In 2014, Cramer and co-workers reported a Rh(III)-catalyzed enantioselective coupling of N-pivaloyloxy benzamides with α-diazoesters using a chiral cyclopentadienyl ligand, providing chiral isoindolinones in high yields with excellent enantioselectivities. 63[] The high enantioselectivity observed in this reaction was attributed to the stereocontrol provided by the C₂-symmetric cyclopentadienyl ligand featuring a binaphthyl backbone, as well as the steric effect of the bulky ester group on the diazo compound (Scheme 33). Scheme 33 . Asymmetric Rh(III)-catalyzed carbene cyclization triggered by C-N reductive elimination 5.2. C(sp 3 )−H bond functionalization with carbene migratory insertion Pd-catalyzed C(sp³)–H activation can also be incorporated into cross-coupling reactions. For example, Martin and co-workers reported a Pd-catalyzed cascade coupling of aryl bromides with diazo compounds to access polysubstituted indenes (Scheme 34). 6465[,] The proposed mechanism involves oxidative addition, followed by intramolecular C(sp³)–H activation to generate a palladacycle, which then undergoes migratory insertion to afford the final product. Scheme 34 . Pd-catalyzed C(sp 3 )−H bond activation/carbene coupling reactions More recently, Chang and co-workers reported the first example of a catalytic strategy for the carbene-involved regioselective remote C-H alkylation of internal olefins by synergistically combining two iridium-mediated reactivities of olefin chain walking and carbenoid migratory insertion (Scheme 35). 66[] This transformation employs sulfoxonium ylides as bench-stable and robust carbene precursors and has been demonstrated to be effective for a range of olefins bearing alkyl chains, heteroatom substituents, and complex bio-relevant moieties. Combined experimental and computational studies indicate that reversible iridium hydride-mediated chain walking generates a terminal alkyl–Ir intermediate, which subsequently forms a carbenoid species that undergoes migratory insertion, ultimately affording regioselective terminal-alkylated products. Scheme 35 . Remote catalytic C(sp3)−H alkylation via relayed carbenoid transfer upon olefin chain walking 6. Asymmetric Catalysis in Carbene Coupling Asymmetric catalysis in carbene coupling reactions have been explored in recent years. Gu and co-workers realized a Pd-catalyzed enantioselective cross-coupling of aryl bromides with N -formylhydrazones, employing an axially chiral phosphine ligand to construct axially chiral vinyl arenes (Scheme 36a). 67[] The resulting 1-vinylnaphthalen-2-yl phosphine oxides were obtained in high yields and enantioselectivities, and could be readily transformed into biaryl atropisomers or chiral phosphine ligands. In 2021, Hu and Xia reported a Pd-catalyzed cross-coupling of cyclobutanone-derived N -tosylhydrazones with bromobenzene (Scheme 36b). 68[] The catalytic system comprising Pd₂(dba)₃ and a spiro-phosphoramidite ligand efficiently facilitated the formation of cyclobutenes, methylenecyclobutanes, and conjugated dienes with moderate enantioselectivities. Scheme 36 . Enantioselective carbene coupling reactions In 2022, Zhang and co-workers reported a Pd-catalyzed asymmetric carbenylative amination of N -tosylhydrazones with I-vinyl iodides bearing pendant amines, providing a series of chiral pyrrolidines and piperidines in good yields and high enantioselectivities (Scheme 37). 69[] Mechanistic studies suggest that the reaction proceeds via migratory carbene insertion followed by a Tsuji–Trost-type allylic substitution. The synthetic utility of this strategy was further demonstrated by its successful application in the total synthesis of (–)-norruspoline and the formal synthesis of (–)-indolizidine 201. Scheme 37 . Asymmetric carbenylative amination In 2021, Yang and Zhang developed a Pd-catalyzed enantioselective three-component coupling of N -tosylhydrazones, aryl bromides, and terminal alkynes using GF-Phos under mild conditions (Scheme 38a). 70[] This one-step protocol afforded enantioenriched diarylmethyl alkynes in moderate to good yields with high enantioselectivity. Aldehyde-derived hydrazones were well tolerated, whereas ketone-derived hydrazones failed due to β-hydride elimination. The protocol was also applied to the late-stage modification of bioactive molecules, including menthol, glucose, and cholesterol. Subsequently, they reported a Pd-catalyzed asymmetric three-component coupling of N -tosylhydrazones, aryl bromides, and silylboronic esters using the newly developed Ming-Phos ligand, enabling the direct synthesis of chiral gem-diarylmethine silanes with high enantioselectivity (Scheme 38b). 71[] Mechanistic studies indicate that the reaction proceeds via transmetalation of an in situ generated Pd–carbene intermediate with Cu–SiMe₂Ph, followed by migratory insertion and stereoretentive reductive elimination. Scheme 38 . Asymmetric three-component coupling reaction 7. Pd-Catalyzed Cross Coupling with Difluorocarbene As a unique class of carbene species, difluorocarbene has attracted significant attentions in recent years. Its diverse reactivity has enabled the development of numerous synthetic methods valuable for the preparation of fluorine-containing molecules. 72[] While free difluorocarbene shows unique reactivities, the corresponding transition-metal catalyzed reactions, including carbene couplings, have also been explored by Zhang and co-workers. 73[] In 2017, they reported the first Pd-catalyzed carbene coupling using ClCF₂H as a difluoromethylating reagent. As a low-cost and readily available CF₂H source, ClCF₂H had not been previously applied in catalytic difluoromethylation. Under a [Pd₂(dba)₃]/Xantphos/hydroquinone system, a wide range of (hetero)arylboronic acids and esters were efficiently transformed into difluoromethylated (Scheme 39). 74[] Notably, the method proved suitable for the late-stage difluoromethylation of various pharmaceuticals and agrochemicals, and could be combined with a sequential C–H/C–CN borylation and difluoromethylation strategy to achieve site-selective CF₂H incorporation. Scheme 39 . Pd-catalyzed difluoromethylation of arylboronic acids with ClCF 2 H Preliminary mechanistic studies suggest the involvement of a Pd–difluorocarbene intermediate in the Pd-catalyzed difluoromethylation using ClCF₂H. The formation of this species was supported by stoichiometric reactions with difluorocarbene precursors (TMSCF₂Br) and confirmed through mass spectrometric analysis. Complementary fluoride-abstraction and deuterium-labeling experiments further established ClCF₂H as the difluorocarbene source, with arylboronic acids, hydroquinone, and water acting as potential proton donors. Collectively, these results are consistent with two plausible catalytic pathways: a Pd(II)/Pd(II) cycle and a Pd(0)/Pd(II) cycle (Scheme 40) Scheme 40 . Proposed mechanisms for Pd-catalyzed difluoromethylation of arylboronic acids with ClCF 2 H Furthermore, Zhang and co-workers established a versatile Pd-catalyzed difluorocarbene transfer strategy employing diethyl bromodifluoromethylphosphonate (BrCF₂PO(OEt)₂) as a readily available fluorine source (Scheme 41). 75[] This method enables the selective transformation of arylboronic acids into four distinct classes of fluorinated products, including difluoromethylated and tetrafluoroethylated arenes, as well as their corresponding fluoroalkylated ketones. The protocol is applicable to late-stage modification of pharmaceutical and agrochemical compounds and allows for one-pot diversification of fluorinated molecules. Mechanistic and computational studies revealed that the reactivity of the Pd–CF₂ species depends on the palladium valence state: Pd(0) is nucleophilic, whereas Pd(II) is electrophilic. This switchable electronic property enables controlled difluorocarbene transfer and precise product selectivity. Scheme 41 . Proposed mechanisms for Pd-catalyzed difluoromethylation of arylboronic acids with ClCF 2 H Subsequently, Zhang and co-workers developed a formal Pd-catalyzed reductive difluorocarbene transfer reaction, employing chlorodifluoromethane (ClCF₂H) as an inexpensive and industrially accessible difluorocarbene source (Scheme 42). 76[] This strategy enables efficient synthesis of difluoromethylated (hetero)arenes from aryl halides or triflates and proton sources without requiring organometallic reagents and shows broad functional group tolerance. Mechanistic studies revealed an unusual Pd(0)/Pd(II) catalytic cycle, in which the Pd(0)=CF₂ species undergoes oxidative addition with aryl electrophiles to generate an aryldifluoromethyl–Pd(II) intermediate, followed by hydroquinone-mediated reductive transfer. This study introduces a new mode of difluorocarbene transfer, in which the carbene couples with two electrophilic partners. Scheme 42 . Pd-catalyzed reductive difluorocarbene transfer Based on mechanistic studies, they proposed a Pd(0/II) catalytic cycle for this reductive difluorocarbene transfer (Scheme 43). In the initial step, ClCF₂H undergoes base-promoted dehydrohalogenation to generate difluorocarbene, which is trapped by the Pd(0) complex to form a key Pd–CF₂ intermediate [Pd⁰(Lₙ)=CF₂] ( A ). This species exists in equilibrium with its protonated form [HCF₂Pd(II)(Lₙ)X] ( B ), although B is not involved in the productive cycle. The Pd–CF₂ complex A undergoes oxidative addition with aryl bromides to afford an aryldifluoromethyl–Pd(II) species ( E ), which serves as the branching point of two possible pathways. In the dominant pathway (Path I), ligand exchange with the in situ generated phenoxide from HQ (hydroquinone) gives intermediate G , which eliminates benzoquinone to form an anionic Pd complex H . Protonation of H yields a Pd–H species I , which undergoes reductive elimination to deliver the difluoromethylated product and regenerate Pd(0). An alternative route (Path II) involving direct protonation of H to release product and Pd(0) cannot be excluded. Scheme 43 . Proposed mechanisms for the Pd-catalyzed reductive difluorocarbene transfer Later, they further investigated Pd(II)-catalyzed difluorocarbene coupling reactions, achieving efficient coupling of benzyl chlorides and allyl trifluoroacetates with difluorocarbene. 77[] The reaction mediated by Pd(II)-promoted difluorocarbene release and rapid migratory insertion demonstrated excellent functional group compatibility and enabled efficient synthesis of various gem-difluoroalkenes. Besides, They also reported a Pd-catalyzed cross-coupling of chlorodifluoromethane with terminal alkynes to produce difluoromethylated alkynes, providing a new method for difluoromethylation. 78[] 8. Non-Diazo Compounds as Carbene Precursors While diazo compounds and hydrazones are well established as carbene precursors, alternative carbene sources have also been explored in carbene transfer reactions. 79[] Metal carbene species generated via 5-exo-dig cyclization of conjugated enynones have emerged as versatile intermediates for a variety of classical carbene-type transformations. In 2013, We reported the first Pd-catalyzed cross-coupling of conjugated enynones with benzyl, aryl, and allylic halides, providing access to a series of furan-substituted olefins (Scheme 44). 80[] Mechanistically, oxidative addition generates an organopalladium(II) species, which coordinates to the enynone substrate and induces a 5-exo-dig cyclization to form a Pd–carbene intermediate. This reactive species then undergoes migratory insertion followed by β-hydride elimination to afford the final products. Scheme 44 . Pd-catalyzed coupling of conjugated enynones with organic halides We also developed a Pd-catalyzed oxidative cross-coupling of enynones with aryl or vinyl boronic acids, efficiently affording furan-based olefins with high E selectivity (Scheme 45a). 81[] This strategy was further extended to oxidative couplings with terminal alkynes and diboron reagents, providing access to furan-functionalized enynes and alkenylboronates. 8283[,] These results highlight the synthetic utility of enynones as carbene precursors and their broad applicability in oxidative Pd-catalyzed transformations for the efficient construction of furan-containing scaffolds. Additionally, allenyl ketones have also been employed as effective carbene precursors in Pd(II)-catalyzed oxidative cross-coupling reactions with organoboronic acids (Scheme 45b). In this transformation, cycloisomerization generates a Pd–carbene intermediate, which undergoes migratory insertion followed by β-hydride elimination to afford the final products. 84[] Scheme 45 . Pd-catalyzed oxidative coupling of enynones or allenyl ketones with organo boronic acids Alkynes can also serve as metal–carbene precursors in the presence of external nucleophiles. In 2018, we reported a Pd(II)-catalyzed oxygenative cross-coupling of ynamides with benzyl bromides, using 8-methylquinoline N -oxide as the oxidant (Scheme 46). 85[] The reaction delivered α,α-disubstituted amides after hydrogenation. Mechanistically, the benzyl–Pd(II) species activates the ynamide toward nucleophilic addition, followed by γ-elimination to generate a Pd–carbene intermediate that enters the catalytic cycle. Scheme 46 . Pd-catalyzed oxygenative coupling of ynamides with benzyl bromides Among various organometallic reagents, chromium(0) carbene complexes have been widely employed as versatile intermediates in Pd-catalyzed transformations. In 2017, we reported a Pd-catalyzed [3+3] annulation between vinyl chromium(0) carbenes and 2-iodophenols, delivering a range of substituted flavonones (Scheme 47a). Notably, when 2-iodoanilines were employed instead of phenol derivatives, the reaction pathway shifted to afford diverse quinolone frameworks. 86[] Mechanistically, the reaction involves carbene transfer from the vinyl chromium(0) complex to the aryl–Pd(II) species, generating a vinyl–Pd carbene intermediate. Migratory insertion then produces an allyl–Pd(II) species, which undergoes intramolecular trapping by an HO- or H 2 N- groups to afford the corresponding flavonone or quinolone product. Subsequently, we developed a Pd-catalyzed reductive coupling between aryl chromium(0) complexes and aryl iodides using hydrosilanes as terminal reductants (Scheme 47b), providing access to a series of diarylmethyl ethers. 87[] Both experimental and computational studies support a mechanistic pathway in which a Pd–carbene intermediate is generated via carbene transfer, followed by migratory insertion. Scheme 47 . Pd-catalyzed coupling of chromium(0) carbenes with organic halides 9. Pd-Catalyzed Carbene Coupling for Polymerization In addition to organic synthesis, Carbene coupling reactions have been employed in polymerization. In 2019, we reported the Pd-catalyzed synthesis of cross-conjugated polymers via carbene coupling (Scheme 48). Both AB- and A₂+B₂-type polymerizations of N -tosylhydrazones with aryl bromides were used to access cross-conjugated poly(arylene-1,1-vinylidene)s (iso-PAVs), which exhibit distinct thermal stability, optical properties, and post-functionalization potential. 88[] In 2018, Chen and co-workers developed a related A₂+B₂ strategy using heteroaryl bromides. 89[] Scheme 48 . Pd-catalyzed synthesis of cross-conjugated polymers via carbene coupling reactions C1 polymerization has emerged as a powerful strategy for constructing all-carbon backbone polymers, with particular interest in the Pd-catalyzed polymerization of diazoacetates. 90[] In this process, migratory insertion of palladium carbenes constitutes the key step in chain propagation. We demonstrated that (cyclopentadienyl)(π-allyl)Pd(II) [Cp(π-allyl)Pd] is highly effective for this transformation, enabling the efficient synthesis of homo- and copolymers bearing ester substituents on every main-chain carbon from a broad range of alkyl and aryl diazoacetates (Scheme 49). 91[] Detailed kinetic studies revealed that the polymerization proceeds via slow initiation followed by rapid propagation. Polymerization experiments and MALDI-TOF-MS analysis further confirmed that alcohols serve as effective chain transfer agents, providing a practical strategy for controlling the molecular weight of the resulting polymers. As shown in Scheme 47, the proposed mechanism involves initiation by a mixture of Pd–OEt, Pd–OH and Pd–Cp species in the presence of EtOH, with Pd–OH likely generated from trace water. Among these, Pd–OEt is considered the active initiator and can be regenerated via chain transfer between the growing polymer chains and excess EtOH. Consequently, the polymerization is primarily initiated by Pd–OEt, leading to the formation of polymer with OEt group at the chain end. Scheme 49 . Polymerization of various diazoacetates using Cp(π-allyl)Pd 10. Conclusion As discussed in this review, transition-metal-catalyzed carbene coupling reactions have emerged as a powerful and versatile strategy for the construction of structurally diverse molecules and macromolecules. These transformations generally proceed via theformation of metal carbene intermediates followed by migratory insertion processes, enabling efficient bond formation under redox-neutral or oxidative conditions. The broad compatibility of various carbene precursors and coupling partners has significantly expanded the synthetic utility of this approach, facilitating its application not only in complex molecule synthesis but also in the development of functional polymeric materials. Nevertheless, several challenges remain to be addressed. Compared to well-established diazo compounds and N -tosylhydrazones, alternative carbene precursors often exhibit limited reactivity and scope. The discovery of new carbene sources and the development of novel catalytic systems, particularly those enabling enantioselective processes, are essential for further expanding the field. Moreover, a deeper mechanistic understanding, especially in the context of polymerization, will be crucial for achieving precise control over polymer microstructure and properties. With continued innovation, carbene coupling chemistry is expected to continue making impactful contributions to diverse fields, including materials science, pharmaceuticals, and sustainable synthesis. Acknowledgement Acknowledgements are extended to the individuals, laboratories, or organizations that have provided financial support and assistance for this work. 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Left to Right: Author 1, Author 2, Author 3 Entry for the Table of Contents Transition Metal-catalyzed Carbene Coupling Reaction Ying Li, Ze Li, Shufeng Chen,* and Jianbo Wang* Chin. J. Chem. 2024 , 42 , XXX—XXX. DOI: 10.1002/cjoc.202400XXX Text for Table of Contents to summarize the article is required in 1‒3 lines. (The height of this row is fixed at 6.2 cm. Please adjust the image and text to fit this height.) Information & Authors Information Version history V1 Version 1 09 October 2025 Peer review timeline Published Chinese Journal of Chemistry Version of Record 6 Feb 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection Chinese Journal of Chemistry Keywords carbene cross-coupling diazo compounds n-tosylhydrazones transition-metal-catalyzed Authors Affiliations Ying Li Inner Mongolia University College of Chemistry and Chemical Engineering View all articles by this author Ze Li Inner Mongolia University College of Chemistry and Chemical Engineering View all articles by this author Shufeng Chen Inner Mongolia University College of Chemistry and Chemical Engineering View all articles by this author Jianbo Wang 0000-0002-0092-0937 [email protected] Inner Mongolia University College of Chemistry and Chemical Engineering View all articles by this author Metrics & Citations Metrics Article Usage 428 views 182 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Ying Li, Ze Li, Shufeng Chen, et al. Transition Metal-catalyzed Carbene Coupling Reaction. Authorea . 09 October 2025. 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