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The pyrrolyl cation is a 4π electron ring system which is anti-aromatic and unstable. This work reports an experimental procedure to obtain stable pyrrolyl cations. Electron donation from N-heterocyclic carbenes makes the ring stable by converting a 4p electron ring system into a 6p electron ring system. [ABSTRACT FROM AUTHOR]

N‐Heterocyclic carbenes (NHCs) are molecules with exceptional catalytic properties in a wide range of reactions. Recently, there has been a growing interest in using them as catalysts for biomass conversion. In this review, we examine the current state of glycerol valorisation (GV) using several catalytic systems, including metallic complexes bearing NHCs as ligands, polymeric building blocks and organocatalysts. The study covers literature published in the last 15 years, detailing the catalyst scope, reaction conditions and products of GV. This analysis emphasises the importance of varying the steric and electronic properties of NHCs within the reviewed catalytic systems. Furthermore, a critical evaluation of the proposed mechanistic pathways complements these results. A comprehensive understanding of the challenges of using such catalytic systems will emerge by compiling literature on the use of NHCs for the studied reactions. Additionally, this will promote new opportunities for further research in both academic and industrial fields. [ABSTRACT FROM AUTHOR]

Herein, a novel and efficient protocol for the synthesis of polycyclic spiroindolines is realized through a 1,2‐acyloxy migration‐cyclization cascade starting from ester‐tethered 1,2,3‐triazoles. [6,5,5,6] and [6,5,6,6] ring skeleton are constructed in moderate to excellent yields. [ABSTRACT FROM AUTHOR]

Bicyclo[3.2.1]oct-2-yne was generated from the Fritsch–Buttenberg–Wiechell rearrangement of 2-norbornylidene carbene. The rearrangement preferentially involves migration of a tertiary carbon over a secondary carbon, a trend that contrasts with rearrangements of acyclic carbenes and which may be attributable to hyperconjugative effects promoted by the bridged structure of the carbene. [ABSTRACT FROM AUTHOR]

In this work, we present an organocatalytic spiroannulation method that provides access to spirocyclic benzofuran‐2‐ones. The reaction proceeds via the generation of an azolium dienolate from an α‐bromo‐α,β‐unsaturated aldehyde using a chiral N‐heterocyclic carbene, followed by annulation with benzofuran‐2‐one‐derived alkenes. The resulting chiral spirobenzofuranone‐fused cyclohexenones were obtained in 77–99% ee and dr from 1.2:1 to 20:1. The utility of the protocol was underscored by gram‐scale reaction and follow‐up transformations. [ABSTRACT FROM AUTHOR]

In this study, we discuss the synthesis and structural characterization of three N‐heterocyclic carbene isothiocyanate zwitterionic ligands, 1,3‐bis(2,6‐diisopropylphenyl)‐2‐N‐phenyl‐thiocarbamoyl imidazolium (ImDipp‐CSNPh, L1), 1,3‐bis(tert‐butyl)‐2‐N‐phenylthiocarbamoyl imidazolium (ImtBu‐CSNPh, L2) and 1,3‐bis(2,4,6‐trimethylphenyl)‐2‐N‐phenylthiocarbamoyl imidazolium (ImMes‐CSNPh, L3) and their cadmium complexes, [(ImDipp‐CSNPh)2CdCl][CdCl3] (1) and [(ImtBu‐CSNPh)Cd(μ‐Cl)Cl]2 (2). Both bench‐stable cadmium complexes were synthesized by directly reacting the corresponding NHC‐CSNPh adducts with anhydrous cadmium chloride. The solid‐state structures of zwitterionic NHC‐isothiocyanate betaine adducts (L1‐L3) and cadmium complexes (1 and 2) were investigated with the assistance of single crystal X‐ray diffraction analysis. A steric modification of N‐substituent on NHC governs a unique structural change from betaine ImDipp‐CSNPh coordinated cationic cadmium complex to neutral dimeric cadmium complex with ImtBu‐CSNPh. [ABSTRACT FROM AUTHOR]

Reactions of cyclic thioureas (1,3,4,5‐tetramethylimidazol‐2‐thione and 1,3‐dimethylimidazolidin‐2‐thione) and ureas (1,3,4,5‐tetramethylimidazol‐2‐one and 1,3‐dimethylimidazolidin‐2‐one) with an isolable dialkylsilylene were examined. In these reactions, cyclic thioureas served as sulfur and NHC (N‐heterocyclic carbene) sources, and the corresponding silanethione and NHC‐derived products formed via silanethione‐NHC complexes. Reactions of cyclic ureas with the silylene afforded a new NHC and an isolable azomethine ylide. These reactions involved oxygen atom transfer and 1,3‐silyl migrations, which were supported by computational studies. [ABSTRACT FROM AUTHOR]

In this short review, we present applications merging the reactivity of cyclic hypervalent iodine reagents with carbenes and carbenoids developed in the period 2014–2024. The use of more stable cyclic hypervalent iodine reagents has led to major advances in this area. The combination of iodine(III) chemistry with carbenes or carbenoids enables new disconnections that are not possible using classical reactivity. Both the use of hypervalent iodine compounds as partners in reactions with metal carbenes and as reagents combining the reactivity of iodine(III) and carbenes in a single molecule to give carbyne equivalents are discussed in this review. 1 Introduction 2 Transfer of Fluorinated Groups (F, CF3) 3 Oxy- and Aminoalkyn(en)ylation 3.1 Oxy- and Aminoalkynylation with Acceptor-Substituted Diazo Compounds 3.2 Oxyalkenylation with Acceptor-Substituted Diazo Compounds 3.3 Oxyalkynylation with Acceptor-Acceptor-Substituted Diazo Compounds 4 Carbyne Equivalents 4.1 Diazo-Based Reagents 4.2 Sulfur Ylide Based Reagents 5 Conclusion [ABSTRACT FROM AUTHOR]

A visible-light-induced multicomponent reaction of vinyl sulfoxonium ylide, thiol, and diazo ester to generate tertiary sulfide is described. The present diastereoselective gem -difunctionalization of the diazo ester can be achieved under mild conditions, as it does not require any additives, catalysts, or transition metals and is tolerant of air and moisture. Due to more nucleophilicity, vinyl sulfoxonium ylide undergoes S–H insertion with thiols to generate an allyl sulfide intermediate. Simultaneously, diazo ester undergoes photolysis to generate a carbene intermediate. Subsequently, the coupling of carbene and allyl sulfide intermediates generates sulfonium ylide, which undergoes Doyle–Kirmse rearrangement to generate tertiary sulfide scaffolds. [ABSTRACT FROM AUTHOR]

Since their discovery in 2011, gold-catalyzed transformations of vinyldiazo compounds have become an important synthetic tool, enabling the identification of new reaction patterns that have greatly expanded the versatility of these reagents. In this short review, we showcase the most relevant advances that have been made in this exciting area of research. 1 Introduction 2 Gold-Catalyzed Transformations of Vinyldiazo Compounds Involving Metal Carbene Intermediates 2.1 Liu's Seminal Work: Vinylogous Reactivity of Au(I) Vinyl Carbenes 2.2 Gold-Catalyzed Reactions of Vinyldiazo Compounds with Alkenes 2.3 Gold-Catalyzed Reactions of Vinyldiazo Compounds with Alkynes 2.4 Gold-Catalyzed Reactions of Vinyldiazo Compounds with Allenes 2.5 Gold-Catalyzed Reactions of Vinyldiazo Reagents with Aromatic Compounds 2.6 Gold-Catalyzed Reactions of Vinyldiazo Compounds with Nitriles 2.7 Gold-Catalyzed Diazo Cross-Couplings 3 Gold-Catalyzed Transformations of Vinyldiazo Compounds That Do Not Involve Initial Dinitrogen Extrusion 3.1 Gold-Catalyzed Formal [n+2] Cycloaddition Reactions through the Vinyl Moiety of Vinyldiazo Compounds 3.2 Gold-Catalyzed Transformations of Vinyldiazo Compounds Involving Initial Activation of the Non-Diazo Reagent 4 Conclusions [ABSTRACT FROM AUTHOR]

An efficient N-heterocyclic carbene (NHC)-catalyzed asymmetric conjugate addition reaction to afford synthetically challenging benzofuranone derivatives having vicinal all-carbon quaternary and tertiary stereocenters is presented. The reaction operates solely through noncovalent interaction between a newly designed NHC and the substrates, providing access to a series of functionalized benzofuranones in good yields and with high ee values. The protocol applies to preparative-scale synthesis. A catalytic cycle involving a noncovalent substrate–NHC interaction is implicated in the process, based on a mechanistic control study. [ABSTRACT FROM AUTHOR]

The recently described crystalline cyclic (alkyl)(amino)carbene with a 1,1'‐ferrocenylene (fc) backbone fc(CPh2−C−NMes) (A, Mes=mesityl) is highly reactive due to its particularly pronounced ambiphilicity and is thermally not stable in solution due to an intramolecular insertion of the divalent carbon atom into a methyl C−H bond of the Mes substituent. The closely related congener fc(CPh2−C−N‐p‐C6H4‐tBu) (1) cannot undergo such an insertion reaction. Nevertheless, 1 is too short‐lived for isolation due to a rapid 1,2‐shift of a phenyl group, furnishing the isomeric cyclic enamine fc[C(Ph)=C(Ph)−N‐p‐C6H4‐tBu] (1') in a specific decomposition process unprecedented for CAACs. Trapping of 1 was possible with carbon monoxide, elemental selenium and with [CuBr(SMe2)], respectively affording the aminoketene 1=C=O, the selenoamide 1=Se and the homoleptic CuI complex [Cu(1)2][CuBr2]. 1 is an even stronger ambiphile than A according to NMR spectroscopic data. Similar to A, 1 does not react with H2, because the experimentally observed intramolecular process is kinetically more favourable according to DFT results. [ABSTRACT FROM AUTHOR]

Electrophilic activation of the aromatic cyclo‐P5 ligand in [Cp*Fe(η5‐P5)] is demonstrated to drastically enhance its reactivity towards weak nucleophiles. Unprecedented functionalized, contracted as well as complexly aggregated polyphosphorus compounds are accessed utilizing [Cp*Fe(η5‐P5Me)][OTf] (A), highlighting the great potential of this underexplored mode of reactivity. Addition of carbenes to A affords novel 1,2‐ or 1,1‐difunctionalized cyclo‐P5 complexes [Cp*Fe(η4‐P5(1‐L)(2‐Me)][OTf] (L=IDipp (1), EtCAAC (2), IiPr (3 b)) and [Cp*Fe(η4‐P5(1‐IiPr)(1‐Me)][OTf] (3 a). For the first time, the much smaller IMe4 leads to the contraction of the cyclo‐P5 ligand and formation of [Cp*Fe(η4‐P4(1‐IMe)(4‐Me)] (4). DFT calculations shed light on the delicate mechanism of this type of reaction, which is reinforced by the experimental identification of key intermediates. Even the comparably weak nucleophile IDippCH2 reacts with A to form [Cp*Fe(η4‐P5(1‐IDippCH2)(1/2‐Me)][OTf] (6 a/b), highlighting its explicitly more reactive nature. Moreover, exposure of A to IDippEH (E=N, P) leads to a unique aggregation reaction affording [{Cp*Fe}2{μ2,η4:3:1–P10Me2(IDippN)}][OTf] (8) and [{Cp*Fe}2{μ2,η4:1:1:1–P11Me2(IDipp)}][OTf] (9), respectively. [ABSTRACT FROM AUTHOR]

Intramolecular photochemical C−H insertion reactions of α‐diazocarbonyl compounds are powerful synthetic transformations leading to carbon‐carbon bond formation. While there are limitations in the scope of C−H bonds amenable to C−H insertion under photochemical conditions relative to metal catalysed C−H insertions, the benefits of conducting these synthetically powerful transformations in a metal‐free approach offers "green" advantages. Recent advances in technology have revolutionised the potential for use of photochemical transformations in synthesis, including the wavelength control enabled by LEDs and the benefits associated with continuous flow processing which overcome many of the traditional challenges associated with synthetic photochemistry including scale‐up. The potential for telescoping the synthesis of α‐diazocarbonyl compounds and downstream photochemical reactions is clear. This review summarises the development of intramolecular photochemical C−H insertions and highlights the benefits of recent technological advances. [ABSTRACT FROM AUTHOR]

The terphenyl amido stannylene‐NHC adducts, HMDS(MesTer)Sn(NHC), (MesTer = 2,6‐Mes2C6H3, Mes = 2,4,6‐Me3‐C6H2, HMDS = N(SiMe3)2, NHC = IMe4, IEt, IMe4 = 1,3,4,5‐tetramethylimidazol‐2‐ylidene, Iet = 1,3‐diethyl‐4,5‐dimethylimidazol‐2‐ylidene), were reacted with benzaldehyde resulting in the formation of HMDS(MesTer)Sn(OCH(Ph)NHC). In the presence of acetophenone, deprotonation occurs to afford the tin enolate (MesTer)Sn(NHC)(OPhC=CH2), via the release of hexamethyldisilamine. This stannylene‐NHC adduct is a catalyst for the homogeneous hydrosilylation of aldehydes and ketones under ambient conditions. [ABSTRACT FROM AUTHOR]

Diradicals based on a meta‐quinodimethane (m‐QDM) scaffold generally have a triplet ground state and are rather scarce. Herein, m‐QDM‐based non‐Kekulé diradicals [3,3′‐(NHC)2BP] (3‐NHC) (NHC = SIPr = C{N(Dipp)CH2}2; IPr = C{N(Dipp)CH}2, Me‐IPr = C{N(Dipp)CMe}2; Dipp = 2,6‐iPr2C6H3; BP = 1,1′‐C6H4C6H4) featuring N‐heterocyclic carbene (NHC) pendants are reported as crystalline solids. The EPR spectra of 3‐NHC show both allowed (Δms = 1) and forbidden (Δms = 2; 'half‐field') transitions characteristic for triplet diradicals. Variable temperature EPR studies however reveal a singlet ground state for 3‐SIPr. Consistent with the EPR spectra, calculations predict a remarkably small singlet‐triplet energy gap (ΔEST ≤ 0.26 kcal/mol) for the 3‐NHC compounds. The calculated singlet diradical character for the ground states of the 3‐NHC compounds amounts to ~99 %. [ABSTRACT FROM AUTHOR]

Is the 1JC−H coupling constant for protonated carbene a relevant measure of its σ‐donation ability? This paper addresses this question by comparing calculated 1JC−H values with various experimental and theoretical approaches across a broad spectrum of carbene compounds. We examined Arduengo‐type NHCs based on the 2‐imidazolylidene scaffold and many other derivatives with modified frameworks, including carbenes with extended, saturated, or conjugated rings, reduced heteroatom stabilization, alternative heteroatoms, permanently charged carbenes, acyclic carbenes, amidocarbenes, and cyclic amino(alkyl/aryl) carbenes, carbodicarbenes and carbodiphosphoranes. Our findings reveal a nuanced relationship between different parameters associated with σ‐interaction, such as 1JC−H, Huynh electronic parameter (HEP), σ‐donation from ETS‐NOCV, and lone pair energy. Notably, the best correlation was observed between 1JC−H and the ETS‐NOCV method, particularly for mono‐ and diaminocarbenes, highlighting the utility of 1JC−H in comparing σ‐donation among structurally similar carbene types. However, the use of 1JC−H as a universal measure across all carbene classes appears limited, especially when considering carbenes with significantly different structural frameworks. While HEP is less effective for carbenes with diverse structural backbones, our study suggests that 1JC−H has potential across a broader range of systems. Additionally, the analysis demonstrates that lone pair energy reflects basicity rather than σ‐donor ability. [ABSTRACT FROM AUTHOR]

Reactions of bipyridyl‐functionalized imidazole‐thiones and selones with MeX (X=I, OTf) afforded sulfenyl and selenenyl cations [(NNC)EMe]X (2/3, E=S, Se). Further reactions of these main‐group cations with [Cu(CH3CN)4]BF4, Cu(OTf) furnished dicationic [{Cu(μ‐I)(NNC)EMe}2][Y]2 (5/6, Y=BF4, OTf) and tricationic copper(I) complexes [Cu{(NNC)EMe}2](OTf)2BF4 (7 a/7 b) when employed [(NNC)EMe]I and [(NNC)EMe]OTf respectively. All these cationic complexes were characterized by various spectroscopic techniques, including X‐ray diffraction analysis. The solid‐state structures revealed novel bonding modes of the cations. The cationic nature of new complexes was analyzed by the 77Se NMR spectroscopy, which indicated different electronic environments around the selenium centers. The cations [(NNC)EMe]X (X= I, OTf), and (NNC)SMe bearing copper complex [{Cu(μ‐I)(NNC)EMe}2][Y]2 proved as potential candidates for alkylation of various Lewis bases and as molecular catalyst in aldehyde‐alkyne‐amine coupling reactions, respectively. The latter catalytic reactions yielded a range of three‐component products in good to excellent yields with low catalyst loading under solvent‐free conditions, which demonstrate the potential utility of group‐16 cations as ancillary ligands in homogeneous catalysis. [ABSTRACT FROM AUTHOR]

Past one decade has witnessed a tremendous growth in the field of carbene stabilized low‐valent silicon compounds unravelling very exciting properties of these molecules. Herein, we have employed a bicyclic (alkyl)(amino)carbene, (MeBICAAC) to explore the low‐valent chemistry of silicon. The reduction of bicyclic (alkyl)(amino)carbene‐SiCl4 complex, [(MeBICAAC)SiCl4] (1) with KC8 afforded low‐valent Si complexes, including Si(III) radical [(MeBICAAC)SiCl3] (2) and a complex with silicon center in a formal zero‐valent state, [(MeBICAAC)2Si] (3). Similarly, the reduction of in‐situ generated MeBICAAC adduct of Me2SiCl2 with one equivalent of KC8 led to the formation of [(MeBICAAC)SiMe2Cl] (4) complex having an unpaired electron. These complexes have been characterized by IR, UV‐Vis. NMR, HRMS, EPR and their solid‐state structures were also elucidated by single crystal X‐ray crystallography. Further, DFT calculations revealed the lower energy singlet state for complexes 1, 3 and doublet state for complexes 2, 4. [ABSTRACT FROM AUTHOR]

Single-atom skeletal editing is an increasingly powerful tool for scaffold hopping-based drug discovery. However, the insertion of a functionalized carbon atom into heteroarenes remains rare, especially when performed in complex chemical settings. Despite more than a century of research, Ciamician-Dennstedt (C-D) rearrangement remains limited to halocarbene precursors. Herein, we report a general methodology for the Ciamician-Dennstedt reaction using α-halogen-free carbenes generated in situ from N-triftosylhydrazones. This one-pot, two-step protocol enables the insertion of various carbenes, including those previously unexplored in C-D skeletal editing chemistry, into indoles/pyrroles scaffolds to access 3-functionalized quinolines/pyridines. Mechanistic studies reveal a pathway involving the intermediacy of a 1,4-dihydroquinoline intermediate, which could undergo oxidative aromatization or defluorinative aromatization to form different carbon-atom insertion products. Ciamician–Dennstedt rearrangement remains limited to halocarbene precursors. Herein, the authors report a general methodology for the Ciamician-Dennstedt reaction using α-halogen-free carbenes generated in situ from N-triftosylhydrazones. [ABSTRACT FROM AUTHOR]

The development of Michael addition reactions to conjugated cyclopropenes is a challenge in organic synthesis due to the fleeting and reactive nature of such strained Michael acceptor systems. Herein, the development of a photochemical approach towards such conjugated cyclopropenes is reported that serves as a strategic entry point to densely functionalized cyclopropanes in a diastereoselective fashion. The process involves the light‐mediated generation of transient cyclopropenyl α,β‐unsaturated esters from vinyl diazo esters, followed by an organic base catalyzed nucleophilic addition of N‐heterocycles to directly access β‐N‐heterocyclic cyclopropanoic esters. With this synergistic approach, various trisubstituted cyclopropanes bearing N‐heteroaryl and N‐heterocyclic rings such as indole, pyrrole, benzimidazole, isatin, pyridinone, and quinolinone were accessed efficiently in good yield and decent to good diastereoselectivities. Further, β‐indolyl cyclopropanoic acids have been synthesized and were successfully evaluated as FABP‐4 inhibitors. Theoretical calculations have been performed to elucidate the mechanism, which was further supported by experimental findings. [ABSTRACT FROM AUTHOR]

Skeletal editing offers a unique route to assemble complex architectures from simple feedstocks that are otherwise difficult to obtain. However, the asymmetric version of skeletal editing has not been widely studied. Herein, we present a modular rhodium/boron asymmetric catalytic system that enables ring‐expansion of phenols with cyclopropenes to synthesize highly functionalized cycloheptadienones in excellent chemo‐ and regioselectivities. This unique protocol features with low‐catalyst loading, atom and step‐economies, and mild neutral reaction conditions. Isotope‐labelling experiments and DFT calculations have been conducted to reveal that boron reagent plays a vital role in the whole catalytic cycle. [ABSTRACT FROM AUTHOR]

The catalytic performance of a series of mono‐ and bimetallic Ir(I) and Rh(I) complexes bearing the (COC) bis‐triazolylidene ligand, coordinated in a bridging or chelating fashion, was evaluated in the cyclization of 4‐pentynoic acid. The chelated mononuclear cationic complex [Ir(cod)(COC)]PF6 (cod=1,5‐cyclooctadiene; COC=biscarbene ether) displayed the highest reaction rate for 4‐pentynoic acid to form γ‐methyl‐ene‐γ‐butyrolactone (5‐methylenedihydrofuran‐2(3H)‐one). Density Functional Theory calculations were carried out to rationalize the reaction mechanism which proceeds in three steps, namely coordination of the alkyne to the active catalyst followed by intramolecular cyclization and final reductive elimination. Its catalytic performance was further assessed against different substituted 4‐pentynoic acid derivatives. [ABSTRACT FROM AUTHOR]

Among the new complexes, metal N-heterocyclic carbene (NHC) complexes have recently gained remarkable attention as they are entirely appropriate prerequisites for effective drug design and quick optimization. Furthermore, N-heterocyclic carbenes (NHCs) like phosphines contain strong σ-donating properties, which can bind to metals and create stable complexes. This article reports a general theoretical discussion on the structures and nature of C(carbene/alkenyl) → M, P → M and C≡C bonds. Also, the influence of changing L and Rʹ groups in some adducts of [RʹC≡C → ML], (M=Cu (I), Ag (I), Au (I); R'=C10H7, C9NH12SO2; L=NHC (R), P (R)3; and R=F, Cl, Br, H, CH3, SiH3, Ph) has been studied. In this context, DFT calculations by PBE-D3/def2-TZVP level of theory have been used. The nature of C(carbene/alkenyl) → M bonds in [RʹC≡C → MNHCR] and also P → M and C(alkenyl) → M bonds in [RʹC≡C → MPR3] complexes was surveyed. This was done using natural bond orbital (NBO), atoms in molecules (AIM), energy decomposition analysis (EDA), and energy decomposition analysis natural orbital for chemical valence (EDA-NOCV). The data have shown that σ donation from C(alkenyl) to M atom in [RʹC≡C → MPR3] complexes was greater than corresponding [RʹC≡C → MNHCR] complexes. Also, the C(alkenyl) → M bonds in corresponding complexes were predominantly electrostatic. In addition, the C≡C bond has been also investigated by applying AIM, EDA, and ETS-NOCV analysis. The outcomes indicate that the highest percentage of interaction energy for C≡C bond is related to covalent interaction. [ABSTRACT FROM AUTHOR]

We report improved protocols for the synthesis of thiazolium precatalysts from primary amines, carbon disulfide, and α-halo ketones. For N -alkyl-substituted derivatives, yields of the corresponding thiazolethiones can be dramatically improved by isolating the intermediate dithiocarbamates. In most cases, meta -chloroperbenzoic acid can advantageously replace H2 O2 in acetic acid for the oxidation of thiazolethiones into thiazoliums. This approach was applied to the synthesis of a thiazolium featuring a 2-adamantyl N -substituent, the corresponding persistent carbene, and its dimer. [ABSTRACT FROM AUTHOR]

Previous carbene insertion to C-H bonds of 1,3-azoles relied on metal carbene species. Herein, we report a metal-free C(sp2)-H bond functionalization of 1,3-azoles with trifluoroacetylsilanes. The reaction features mild conditions, broad substrate scope and wide functional group tolerance. The mechanistic study supports that the success of the reaction is probably attributed to the dual roles of trifluoroacetylsilanes under the photocatalyzed conditions: generating carbenes which undergo cyclopropanation and generating biradicals which promote ring-opening aromatization of the in situ generated fused cyclopropanes. [ABSTRACT FROM AUTHOR]

This perspective highlights recent developments of the use of N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (cAACs) in alane and aluminum organyl chemistry. Especially in the last few years this flourishing research field led to some remarkable discoveries including various substitution patterns at the central aluminum atom, different oxidation states, neutral and charged compounds with varying coordination numbers and unique reactivities. Thereby NHCs play a vital role in the stabilization of these otherwise highly reactive compounds, which would not be realizable without the use of this intriguing class of ligands. Nevertheless, main group hydrides and especially NHC ligated alanes also tend to undergo NHC decomposition reactions, which are part of ongoing research and provide important information for NHC research in general. [ABSTRACT FROM AUTHOR]

The small, lipophilic tetrafluoroborate anion is ubiquitous in the imidazol(in)ium and iminium precursors to N‐heterocyclic carbenes (NHCs), including cyclic (alkyl)(amino)carbenes (CAACs). Its symmetrical distribution of charge over four highly electronegative fluorine atoms confers high stability, attenuating basicity while shielding the boron center. Here we report that this inertness does not withstand the nucleophilicity of free carbenes, when combined with a polarizing lithium cation. Reaction of CAAC ⋅ HBF4 salts with LiN(SiMe3)2, a protocol widely used to liberate free CAACs, generates zwitterionic CAAC‐BF3 adducts within minutes at 80 °C, or hours at room temperature, even for bulky N‐diisopropylphenyl (DIPP) CAACs. Imidazol(in)ium tetrafluoroborates likewise form NHC‐BF3 adducts in excellent yields. [ABSTRACT FROM AUTHOR]

Catalytic activation of Caryl‐O bonds is considered as a powerful strategy for the production of aromatics from lignin. However, due to the high reduction potentials of diaryl ether 4‐O‐5 linkage models, their single electron reduction remains a daunting challenge. This study presents the blue light‐induced bifunctional N‐heterocyclic carbene (NHC)‐catalyzed one‐electron reduction of diaryl ether 4‐O‐5 linkage models for the synthesis of trivalent phosphines. The H‐bond between the newly devised bifunctional NHC and diaryl ethers is responsible for the success of the single electron transfer. Furthermore, this approach demonstrates selective one‐electron reduction of unsymmetric diaryl ethers, oligomeric phenylene oxide, and lignin model. [ABSTRACT FROM AUTHOR]

Breaking aromaticity by inserting additional atoms within the skeleton of heteroaromatic rings has gained significant attention over the years. As part of the emerging concept of 'skeletal editing', this short review retraces the recent progress made on dearomative enlargement reactions of both five- and six-membered heterocycles. 1 Introduction 2 Dearomative Enlargement of Five-Membered Rings 2.1 Pyrroles, Furans, Thiophenes and Their Fused Analogues 2.2 Pyrazoles, Isoxazoles, Isothiazoles and Their Fused Analogues 3 Dearomative Enlargement of Six-Membered Rings 4 Conclusion and Perspectives [ABSTRACT FROM AUTHOR]

Multidentate bis‐NHCs ligands with various spacers are expected to combine the advantages of coordination chemistry and catalysis. These offer opportunities to construct various organometallic frameworks involving transition metals and main group elements. Therefore, developing a general procedure for synthesizing a variety of carbene salt precursors using a convenient technique is key in this context. The extended protocol of a solvent‐free approach for synthesizing various bridged bis‐imidazolium carbene salts, including tris and tetrakis‐imidazolium precursors, is reported here. This method can be performed in the laboratory, leading to high yields (80–95 %) and isolated as analytically pure, multigram, bench‐stable compounds. [ABSTRACT FROM AUTHOR]

The intricate σ and π-bonding of N-heterocyclic carbenes (NHCs) to metals and the need to quantify their electronic properties to rationalize reactivity of complexes have resulted in the creation of numerous methodologies to understand the NHC–metal interaction which are, as we now show, flawed. Our search for a unified, easily accessible system to gauge these fundamental properties has resulted in the discovery of two systems that highlight the flaws present in existing systems and provide a more accurate measure of the NHC ligand electronic properties. [ABSTRACT FROM AUTHOR]

ABSTRACT NHC pincers (NHC = N‐heterocyclic carbene), which combine the structural benefits of both carbenes and pincer platforms, have shown diverse applications, spanning from fundamental organometallic chemistry to homogeneous catalysis. Although aryl‐bridged bis(NHCs) represent the earliest developed and most studied type of NHC pincers, such [C^C^C]‐platforms have been underutilized in the synthesis of rhodium complexes. In this study, we present several less explored organorhodium(III) complexes featuring [C^C^C]‐pincers. Their synthetic route via a convenient oxidative addition approach has been explored, and the obtained cyclorhodium(III) complexes show versatile coordination geometry (square pyramidal or octahedral). Additionally, these cyclorhodium(III) complexes exhibit very different regioselectivity in catalytic alkyne hydrosilylations compared to known Rh(III) NHC catalytic systems. Finally, a few mechanistic studies have also been conducted, and a plausible mechanism was proposed. [ABSTRACT FROM AUTHOR]

The selective, transition metal‐free hydrosilylation of CO2 to CH2(OSiEt3)2 has been achieved under mild conditions and in high isolated yields (up to 90%) by using Et3SiH and the simple, easily prepared borohydride catalyst Li+[HB(C6F5)3]−. The resulting CO2‐derived bis(silyl)acetal product—whose mechanism of formation has been interrogated through detailed computational and experimental studies—can be rapidly valorized through the facile synthesis of N‐heterocyclic carbenes, via their corresponding imidazolium salts. By using relatively inexpensive, isotopically enriched 13CO2 this protocol can be exploited to prepare NHC isotopologues that are selectively 13C labelled at the key, ligating C2 position. This provides an electronically responsive 13C NMR spectroscopic handle with dramatically enhanced sensitivity, which can directly benefit reactivity studies in both organo‐ and organometallic catalysis, where NHC use is ubiquitous. [ABSTRACT FROM AUTHOR]

The design approach for case teaching on carbenes and their complexes integrates basic theory with frontier research. By analyzing and discussing research findings reported in the literatures through fundamental theories, this approach helps students solidify their foundational knowledge while fostering their innovative thinking and capabilities. [ABSTRACT FROM AUTHOR]

Inherently chiral medium-ring derivatives have important applications in many research fields, such as materials science, molecular recognition, and asymmetric catalysis. However, the enantioselective assembly of these molecules, especially by organocatalytic strategies, remains a formidable challenge, and few methods are available. Here, we report the enantioselective NHC-catalyzed (NHC: N-heterocyclic carbenes) formal high-order (5 + 3) annulation of 1-(2-indolyl)naphthalen-2-ols with ynals. In the presence of an NHC pre-catalyst, base, Lewis acid and oxidant, this protocol enables the catalytic formation of C–C and C-O bonds, providing practical and facile access to an array of inherently chiral saddle-shaped eight-membered lactones featuring an oxocin-2-one scaffold with structural diversity in good efficiency and excellent enantiocontrol. Moreover, the scale-up preparation and representative late-stage transformations of the eight-membered lactones further demonstrate the application potential of this synthetic technology. Inherently chiral medium-ring derivatives have important applications in many research fields, but few enantioselective methods are available. Herein, the authors report the enantioselective N-heterocyclic carbenes-catalyzed formal (5 + 3) annulation of 1-(2-indolyl)naphthalen-2-ols with ynals for the synthesis of eight-membered lactones. [ABSTRACT FROM AUTHOR]

The 2,7‐fluorenone‐linked bis(6‐imidazo[1,5‐a]pyridinium) salt H2‐1(PF6)2 reacts with Ag2O in CH3CN to yield the [2]catenane [Ag4(1)4](PF6)4. The [2]catenane rearranges in DMF to yield two metallamacrocycles [Ag2(1)2](PF6)2. 2,7‐Fluorenone‐bridged bis‐(imidazolium) salts H2‐L(PF6)2 (L=2 a, 2 b) react with Ag2O in CH3CN to yield metallamacrocycles [Ag2(L)2](PF6)2 with interplanar distances between the fluorenone rings too small for [2]catenane formation. Intra‐ and intermolecular π⋅⋅⋅π interactions between the fluorenone groups were observed by X‐ray crystallography. The strongly kinked 2,7‐fluorenone bridged bis(5‐imidazo[1,5‐a]pyridinium) salt H2‐4(PF6)2 reacts with Ag2O to yield [Ag2(4)(CN)](PF6), while the tetranuclear assembly [Ag4(4)2(CO3)](PF6)2 was obtained in the presence of K2CO3. [ABSTRACT FROM AUTHOR]

A new approach to the alkylation of azoles with alkenes catalyzed by the nickel complexes with N-heterocyclic carbenes (Ni/NHC) was developed. The catalytically active Ni/NHC complexes are formed in situ from the air-stable precursors: the nickel chloride complex with pyridine (NiCl2Py2), imidazolium salt IMes•HCl (IMes is 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) as the carbene source, and sodium tert-butoxide (ButONa). The advantages of the developed approach are availability of the components of the catalytic system, no necessity to apply special agents for the reduction of NiII to Ni0, and simplification of the synthesis procedure. [ABSTRACT FROM AUTHOR]

The synthesis of two stable monoylide‐substituted stannylenes of type YCNSnR [R=Cl (1) or N(SiMe3)2 (2), and YCN=Ph3P(CN)C] starting from the cyanido‐substituted α‐metallated ylide YCN–K and SnCl2 or (SiMe3)2NSnCl is reported. Coordination of the cyano group to the tin center results in the dimerization of both stannylenes 1 and 2 in the solid state as well as in solution, where syn‐ and anti‐isomers are present. X‐ray diffraction analyses in combination with DFT calculations revealed that the π‐electron density of the ylide ligand is predominantly shifted into the cyano moiety rather than towards the tin center. This results in a strong preference of the dimers over the monomers, which is further emphasized by the reaction with 2,5‐di‐tert‐butylhydroquinone, which undergoes a [1+4] addition to the tin center to yield a spiro compound while preserving the cyclic core of the dimer. In contrast, phenyl isocyanate and dicyclohexylcarbodiimide insert into the Sn−Cylide bond, ultimately leading to the cleavage of the dimer and the formation of monomeric stannylenes with an ylide‐substituted thioamidato and amidinato ligand, respectively. [ABSTRACT FROM AUTHOR]

Metal carbenes are widely acknowledged as a category of highly effective intermediates that facilitate otherwise inaccessible transformations. In recent decades, carbene chemistry has made considerable advances and has demonstrated remarkable abilities in the formation of diverse chemical bonds and the synthesis of structurally distinctive molecules. Nevertheless, the majority of research within this field has concentrated on α-carbon-substituted carbenes, with comparatively little investigation of carbenes that have been functionalized with a wider structural variety, particularly those that have been substituted with heteroatoms (e.g., O, N, P, S, Si, Ge, Sn and B). The objective of this review is to elucidate the advancements in enantioselective transfer reactions involving metal carbenes substituted with these elements, thereby highlighting their contribution to the expansion of the structural diversity and synthetic utility of carbenes in contemporary chemistry. [ABSTRACT FROM AUTHOR]

Design of metal cofactor ligands is essential for controlling the reactivity of metalloenzymes. We investigated a carbene transfer reaction catalyzed by myoglobins containing iron porphyrin cofactors with one and two trifluoromethyl groups at peripheral sites (FePorCF3 and FePor(CF3)2, respectively), native heme and iron porphycene (FePc). These four myoglobins show a wide range of Fe(II)/Fe(III) redox potentials in the protein of +147 mV, +87 mV, +42 mV and −198 mV vs. NHE, respectively. Myoglobin reconstituted with FePor(CF3)2 has a more positive potential, which enhances the reactivity of a carbene intermediate with alkenes, and demonstrates superior cyclopropanation of inert alkenes, such as aliphatic and internal alkenes. In contrast, engineered myoglobin reconstituted with FePc has a more negative redox potential, which accelerates the formation of the intermediate, but has low reactivity for inert alkenes. Mechanistic studies indicate that myoglobin with FePor(CF3)2 generates an undetectable active intermediate with a radical character. In contrast, this reaction catalyzed by myoglobin with FePc includes a detectable iron–carbene species with electrophilic character. This finding highlights the importance of redox‐focused design of the iron porphyrinoid cofactor in hemoproteins to tune the reactivity of the carbene transfer reaction. [ABSTRACT FROM AUTHOR]

Molecules with chiral boron atoms have been scarcely studied due to limited synthetic access. Herein, we report a new method for their synthesis via asymmetric insertion of arydiazoacetates into the B–H bonds of prochiral carbene–boranes NHC–BH2R. The reaction is catalyzed by the rhodium(I) complex with the chiral diene ligand tBu2-TFB, which can be conveniently prepared by diastereoselective coordination of the racemic diene with (S-Salox)Rh(CO)2. The target boranes were typically obtained in 75–90% yields with 90–95% ee. [ABSTRACT FROM AUTHOR]

A simplified and stepwise synthetic method for producing sterically congested unsymmetrical 1,2-dicarbonyl radicals was successfully demonstrated including detailed characterization of each radical cation. Using this approach, an aryl- and N-heterocyclic carbene-substituted 1,2-dicarbonyl radical in its neutral form is generated, revealing the stabilizing role of N-heterocyclic carbenes. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Herein, we present a method for the homogeneous hydrogenation of nitroarenes to produce anilines using low catalyst loading (1 mo%) of copper N‐heterocyclic carbene complexes as the catalyst and ammonia borane as the source of hydrogen. A wide range of nitroarenes, featuring diverse functional groups, were selectively transformed into their corresponding primary aromatic amines with high yields. This process can be readily scaled up and exhibits compatibility with various sensitive functional groups, including halogen, trifluoromethyl, aminomethyl, alkenyl, cyano, ester, amide, and hydroxyl. Notably, this catalytic methodology finds application in the synthesis of essential drug compounds. Mechanistic investigations suggest that the in‐situ‐generated Cu‐H species may serve as active intermediates, with reduction pathways involving species such as azobenzene, 1,2‐diphenylhydrazine, nitrosobenzene, and N‐phenylhydroxylamine. [ABSTRACT FROM AUTHOR]

Thanks to the ability of diazo derivatives to react either as 1,3‐dipoles and as carbenes after dinitrogen extrusion, combinations of oxa or aza benzonorbornadienes and diazomalonates afford polycyclic pyrazolidines via a three‐step sequence of (i) a highly diastereoselective [3+2]‐cycloaddition, (ii) a CpRu‐catalyzed carbene addition, and (iii) a second dipolar cycloaddition. Of importance, step (II) represents a unique access to novel bench‐stable N,N‐cyclic azomethine imines, which behave as effective 1,3‐dipoles in combination with electron‐poor dipolarophiles. Each step proceeds efficiently and the 3‐step process can be performed in one‐pot to yield a polycyclic pyrazolidine in excellent overall yield (90 %). [ABSTRACT FROM AUTHOR]

An oxidative strategy is reported to access α-oxo BMIDA gold carbenes directly from BMIDA-terminated alkynes. Besides offering expedient access to seldom studied boryl metal carbenes, these BMIDA gold carbene species undergo facile insertions into methyl, methylene, methine, and benzylic C-H bonds in the absence of the Thorpe-Ingold effect. They also undergo efficient OH insertion, cyclopropanation, and F-C alkylations. This chemistry provides rapid access to structurally diverse α-BMIDA ketones, which are scarcely documented. In combination with DFT studies, the role of BMIDA is established to be an electron-donating group that attenuates the high electrophilicity of the gold carbene center.

Diarylacetylenes ArC≡CAr featuring condensed aromatic hydrocarbon fragments (Ar) such as naphthalene, anthracene, phenanthrene and pyrene were converted into vinylidene ligands by 1,2‐migration reactions within the coordination sphere of half‐sandwich complexes [MII(dppe)Cp]+ (MII = RuII, FeII). Comparison of the extent of conversion of the alkyne substrates to the vinylidene complexes [Ru{=C=CAr2}(dppe)Cp]+ with those obtained from acetylenes functionalized by smaller groups (H, CH3, Ph) show that the molecular volume (VM) of the migrating group and relief of steric congestion plays a role during the rearrangement process. Conversely, the H‐atoms from the larger condensed ring aryl groups that are in close proximity to the migrating sites also have a significant influence on the efficacy and extent of the reaction by restricting access of the alkyne to the metal center, resulting in a less effective migration reaction. This combination of competing steric factors (acceleration due to relief of steric congestion and restricted access of the alkyne moiety to the reaction site) is exemplified by the facile migration of 1‐pyryl entities and the low yields of vinylidene products formed from 1,2‐bis(9‐anthryl)acetylene. [ABSTRACT FROM AUTHOR]

A formal [4+2] annulation reaction of phthalaldehydes and 3‐ylideneoxindoles through a tandem process of a Stetter–aldol reaction was accomplished by the application N‐heterocyclic carbenes as effective catalysts. Under mild conditions, this unprecedented cascade reaction readily occurs in good yield, enabling straightforward access to functionalized tetrahydronaphthalene‐fused spirooxindoles. For the first time, the feasibility of an asymmetric Stetter–aldol reaction has also been briefly explored by employing chiral organic bases to promote the asymmetric transformation. [ABSTRACT FROM AUTHOR]

Ferrocene-substituted carbenes have emerged as attractive, redox-active ligands. However, among the compounds studied to date, ferrocenylated 1,2,4-triazol-5-ylidenes, which are closely related to the archetypal imidazol-2-ylidenes, are still unknown. Here, we demonstrate that the triazolium salt [CHN(Me)NCHN(Fc)]I (2; Fc = ferrocenyl), obtained by alkylation of 4-ferrocenyl-4H-1,2,4-triazole (1) with MeI, reacts selectively with metal alkoxide/hydroxide precursors [(cod)Rh(OMe)]2 and [(IPr)Au(OH)] (cod = cycloocta-1,5-diene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to produce the ferrocene-substituted 1,2,4-triazol-5-ylidene complexes [(cod)RhI{CN(Me)NCHN(Fc)}] and [(IPr)Au{CN(Me)NCHN(Fc)}]I in good yields. The complexes were characterised by NMR and IR spectroscopy, mass spectrometry, cyclic voltammetry, and single-crystal X-ray diffraction analysis. Density function theory (DFT) calculations were used to rationalise the electrochemical behaviour of the carbene complexes and to elucidate the bonding situation in these compounds. An analysis using intrinsic bond orbitals (IBOs) revealed that the 1,2,4-triazol-5-ylidene ligand exerted a strong trans influence and showed a synergistic stabilisation by the negative inductive and positive π-donor effects of the nitrogen atoms adjacent to the carbene carbon atom; these effects were enhanced by conjugation with the CH＝N bond at the exterior, similar to that in imidazol-2-ylidenes. [ABSTRACT FROM AUTHOR]