Crystalline Monomeric Allenyl/Propargyl Radical

Cyclic (Alkyl)(Amino)Carbene-Iminoboryl Compounds with Three Formal Oxidation States

BN/CC isosterism is an effective strategy to build hybrid functional molecules with unique properties. In contrast to the alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) that feature only one reversible reduction wave, the isoelectronic cationic CAAC-iminoboryl adducts could be singly and doubly reduced smoothly. Both the resultant neutral radical and anionic azaborataallenes bear NBC-mixed allenic structures. The former radical has a high spin-density of 0.55e at CCAAC carbon, yet exhibits formal boron-centered radical reactivity. The latter azaborataallenes feature the nucleophilic CCAAC center and polar N(δ-)═B(δ+)═C(δ-) unit, and readily undergo nucleophilic substitution, isocyanide insertion, dipolar addition and cycloaddition reactions etc. The N-substituents have been shown to have a significant influence on the solid-state structure, thermal stability, and reactivity of azaborataallenes. This work showcases the allenic BN-unsaturated species as versatile building blocks in organic synthesis.

Single electron reduction of NHC-CO2-borane compounds

The carbon dioxide radical anion [CO2˙-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2˙-] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO2-BR3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO2 by N-heterocyclic carbenes (NHCs) and boranes (BR3) - undergo single electron reduction at a less negative potential than free CO2. A net gain of more than one volt was notably measured with a CAAC-CO2-B(C6F5)3 adduct, which was chemically reduced to afford [CAAC-CO2-B(C6F5)3˙-]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO2˙-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2˙-]-like species.

Bis-[cyclic(alkyl)(amino)carbene]-derived diradicals

Crystalline polymeric structures of trans-1,4-cyclohexylene bridged N-tethered bis-CAACs in the form of their LiOTf adducts were synthesized and isolated. These were further used as building blocks for the synthesis of crystalline (amino)(carboxy)-based diradicals. The triplet diradical character of these compounds was unambiguously confirmed by the presence of a half-field signal in their EPR spectra. Theoretical calculations show that the singlet state is marginally more stable than the triplet state.

Reduction of 2-H-substituted pyrrolinium cations: the carbon-carbon single bond in air stable 2,2'-bipyrrolidines as a two-electron-source

Reduction of 2-H-substituted pyrrolinium cations via initially formed secondary radicals results in either dimerisation or H-abstracted products, while the outcome depends on the N-substituents. The resultant central carbon-carbon single bond in the dimerised 2,2'-bipyrrolidine derivatives can be oxidised chemically and electrochemically. The notably air and moisture-stable dimers were subsequently utilised as a source of two electrons in various chemical transformations.

Metal-Free Highly Regioselective 1,4-Sulfonyliodination of 1,3-Enynes

Herein, a novel, practical, and green synthetic method using readily available 1,3-enynes with sulfonyl hydrazides and I₂ through tert-butyl hydroperoxide (TBHP)-mediated 1,4-sulfonyliodination has been developed for synthesizing various tetrasubstituted allenyl iodides under metal-free conditions. Notably, the proposed method exhibits a broad substrate scope, operational simplicity, tolerance to air, high functional-group tolerance, satisfactory yields, and excellent regioselectivity as well as involves the use of cost-effective reagents such as green oxidants.

Luminescent Crystalline Carbon- and Nitrogen-Centered Organic Radicals Based on N-Heterocyclic Carbene-Triphenylamine Hybrids

Developing luminescent radicals with tunable emission is a challenging task due to the limitation of alternative skeletons. Herein, a series of carbene-triphenylamine hybrids were prepared by the direct C2-arylation of N-heterocyclic carbenes with 4-bromo-N,N-bis(4-methoxyphenyl)aniline. These hybrids showed multiple redox-active properties and could be converted to carbon-centered luminescent radicals with blue-to-cyan emissions (λ_max : 436-486 nm) or nitrogen-centered luminescent radicals with orange emissions (λ_max : 590-623 nm) through chemical reduction or oxidation, respectively. The radical species were characterized by electron paramagnetic resonance spectroscopy, ultraviolet-visible spectroscopy, and single-crystal X-ray diffractometry analysis. Notably, the corresponding nitrogen-centered radicals exhibited good stability in atmospheric air, and their thermal decomposition temperatures were determined to be above 200 °C. In addition, spectral and theoretical calculations indicate that all radicals exhibit anti-Kasha emissions.

An air-stable radical with a redox-chameleonic amide

An air-stable (amino)(amido)radical was synthesized by reacting a cyclic (alkyl)(amino)carbene with carbazoyl chloride, followed by one-electron reduction. We show that an adjacent radical center weakens the amide bond. It enables the amino group to act as a strong acceptor under steric contraint, thus enhancing the stabilizing capto-dative effect.

A bis-NHC-CAAC dimer derived dicationic diradical

The isolation of carbon-centered diradicals is always challenging due to synthetic difficulties and their limited stability. Herein we report the synthesis of a trans-1,4-cyclohexylene bridged bis-NHC-CAAC dimer derived thermally stable dicationic diradical. The diradical character of this compound was confirmed by EPR spectroscopy. The variable temperature EPR study suggests the singlet state to be marginally more stable than the triplet state (2J = -5.5 cm-1 (ΔE ST = 0.065 kJ mol-1)). The presence of the trans-1,4-cyclohexylene bridge is instrumental for the successful isolation of this dicationic diradical. Notably, in the case of ethylene or propylene bridged bis-NHC-CAAC dimers, the corresponding dicationic diradicals are transient and rearrange to hydrogen abstracted products.

A platform for blue-luminescent carbon-centered radicals

Organic radicals, which have unique doublet spin-configuration, provide an alternative method to overcome the efficiency limitation of organic light-emitting diodes (OLEDs) based on conventional fluorescent organic molecules. Further, they have made great breakthroughs in deep-red and near-infrared OLEDs. However, it is difficult to extend their fluorescence into a short-wavelength region because of the natural narrow bandgap of the organic radicals. Herein, we significantly expand the scope of luminescent radicals by showing a new platform of carbon-centered radicals derived from N-heterocyclic carbenes that produce blue to green emissions (444–529 nm). Time-dependent density functional theory calculations and experimental investigations disclose that the fluorescence originates from the high-energy excited states to the ground state, demonstrating an anti-Kasha behavior. The present work provides an efficient and modular approach toward a library of carbon-centered radicals that feature anti-Kasha’s rule emission, rendering them as potential new emitters in the short-wavelength region.

Organic radicals, which have unique doublet spin-configuration, provide an alternative method to overcome the efficiency limitation of organic light-emitting diodes (OLEDs) but extending their fluorescence into a short-wavelength region remains challenging. Here, the authors significantly expand the scope of luminescent radicals by showing a new platform of carbon-centered radicals that produce blue to green emission.

Photo and copper dual catalysis for allene syntheses from propargylic derivatives via one-electron process

Different from the traditional two-electron oxidative addition-transmetalation-reductive elimination coupling strategy, visible light has been successfully integrated into transition metal-catalyzed coupling reaction of propargylic alcohol derivatives highly selectively forming allenenitriles: specifically speaking, visible light-mediated Cu-catalyzed cyanation of propargylic oxalates has been realized for the general, efficient, and exclusive syntheses of di-, tri, and tetra-substituted allenenitriles bearing various synthetically versatile functional groups. A set of mechanistic studies, including fluorescence quenching experiments, cyclic voltammetric measurements, radical trapping experiments, control experiments with different photocatalyst, and DFT calculation studies have proven that the current reaction proceeds via visible light-induced redox-neutral reductive quenching radical mechanism, which is a completely different approach as compared to the traditional transition metal-catalyzed two-electron oxidative addition processes.

Synthesis of Tetrasubstituted Allenes via Visible-Light-Promoted Radical 1,3-Difunctionalization of Alkynyl Diazo Compounds

Herein, we described an unprecedented process for generating allenyl radicals through radical addition to alkynyl diazo compounds followed by a 1,2-radical shift with the loss of nitrogen. Using this protocol, radical 1,3-difunctionalization of alkynyl diazo compounds for the synthesis of tetrasubstituted allenes with RSO₂X (X = SeR', SR', and I) as the radical sources was developed. The reactions were promoted by visible light without photocatalyst and any additives.

Metalloradical Activation of In Situ-Generated α-Alkynyldiazomethanes for Asymmetric Radical Cyclopropanation of Alkenes

α-Alkynyldiazomethanes, generated in situ from the corresponding sulfonyl hydrazones in the presence of a base, can serve as effective metalloradicophiles in Co(II)-based metalloradical catalysis (MRC) for asymmetric cyclopropanation of alkenes. With D₂-symmetric chiral amidoporphyrin 2,6-DiMeO-QingPhyrin as the optimal supporting ligand, the Co(II)-based metalloradical system can efficiently activate different α-alkynyldiazomethanes at room temperature for highly asymmetric cyclopropanation of a broad range of alkenes. This catalytic radical process provides a general synthetic tool for stereoselective construction of alkynyl cyclopropanes in high yields with high both diastereoselectivity and enantioselectivity. Combined computational and experimental studies offer several lines of evidence in support of the underlying stepwise radical mechanism for the Co(II)-catalyzed olefin cyclopropanation involving a unique α-metalloradical intermediate that is associated with two resonance forms of α-Co(III)-propargyl radical and γ-Co(III)-allenyl radical. The resulting enantioenriched alkynyl cyclopropanes, as showcased with several stereospecific transformations, may serve as valuable chiral building blocks for stereoselective organic synthesis.

Recent advances in the domain of Cyclic (alkyl)(amino) carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

Copper-Catalyzed Regioselective 1,4-Sulfonylcyanation of 1,3-Enynes with Sulfonyl Chlorides and TMSCN

A novel and practical copper-catalyzed reaction for the 1,4-sulfonylcyanation of 1,3-enynes under mild conditions is described. This protocol provides efficient and straightforward access to a variety of 5-sulfonylpenta-2,3-dienenitrile derivatives with...

The curious case of a sterically crowded Stenhouse salt

We report a peculiar Stenhouse salt. It does not evolve into cyclopentenones upon basification, due to the steric hindrance of its bulky stable carbene patterns. This allowed for the observation and characterization of the transient open-chain neutral derivative, which was isolated as its cyclized form. The latter features an unusually long reactive C–O bond (150 pm) and a rich electrochemistry, including oxidation into an air-persistent radical cation.

Bulky substituents of a Stenhouse salt prevent the usual formation of a cyclopentenone upon deprotonation. For the first time, a transient open-chain neutral derivative was observed; the cyclized form and an air-persistent radical were isolated.

Radical transformations for allene synthesis

Allenes are valuable organic molecules that feature unique physical and chemical properties. They are not only often found in natural products, but also act as versatile building blocks for the access of complex molecular targets, such as natural products, pharmaceuticals, and functional materials. Therefore, many remarkable and elegant methodologies have been established for the synthesis of allenes. Recently, more and more methods for radical synthesis of allenes have been developed, clearly emphasizing the associated great synthetic values. In this perspective, we will discuss recent important advances in the synthesis of allenes via radical intermediates by categorizing them into different types of substrates as well as distinct catalytic systems. The mechanistic studies and synthetic challenges will be highlighted.

Recent important advances in the synthesis of allenes via radical strategies are highlighted.

Air-/Heat-Stable Crystalline Carbon-Centered Radicals Derived from an Annelated N-Heterocyclic Carbene

Organic radicals are open-shell species and have been extensively applied to functional materials due to their unique physicochemical properties with unpaired electrons; however, most of them are highly reactive and short-lived. Herein, a series of stable radicals were readily accessed in two steps from a bis(imino)acenaphthene-supported N-heterocyclic carbene (IPr(BIAN)) through enhancing the delocalization of spin density. The IPr(BIAN)-based radicals 3a-c, obtained by reduction of the corresponding iminium salts 2a-c with KC₈, have been spectroscopically and crystallographically (3a,c) characterized. DFT calculations indicate that increasing the electron-withdrawing properties of the para substituent on the carbene carbon atom results in the spin density evolving from the acenaphthene ring to the phenyl ring. The IPr(BIAN)-based radicals 3a-c show excellent stability: they have half-lives of 1 week in well-aerated solutions and feature a high thermal decomposition temperature up to 200 °C.

Alkene Difunctionalization Triggered by a Stabilized Allenyl Radical: Concomitant Installation of Two Unsaturated C-C Bonds

Radical-mediated difunctionalization of alkenes provides a promising approach to introduce one alkenyl or alkynyl group to target compounds. However, simultaneous installation of two unsaturated C-C bonds via alkene difunctionalization remains elusive, attributable to the high instability and transient lifetimes of alkenyl and alkynyl radicals. Herein, we report the photocatalytic 1,2-alkynylalkenylation and 1,2-enynylalkenylation of alkenes for the first time, triggered by the intermolecular addition of a stabilized allenyl radical to an alkene. A portfolio of strategically designed, easily accessible dual-function reagents are applied to a radical docking-migration cascade. The protocol has broad substrate scope and efficiently increases the degree of unsaturation.

Highly Stable 1,2-Dicarbonyl Radical Cations Derived from N-Heterocyclic Carbenes

Stable organic radicals have been of great academic interest not only in the context of fundamental understanding of reactive intermediates but also because of their numerous applications as functional materials. Apart from the early examples of triphenylmethyl and TEMPO derivatives, reports on air- and water-stable organic radicals are scarce, and their development remains a challenge. Herein, we present the design and synthesis of a novel organic radical based on a 1,2-dicarbonyl scaffold supported by N-heterocyclic carbenes (NHCs). The presented radical cations exhibit remarkable stability toward various harsh conditions, such as the presence of reactive chemicals (reductants, oxidants, strong acids, and bases) or high temperatures, by far exceeding the stability of triphenylmethyl and TEMPO radicals. In addition, physiological conditions including aqueous buffer and blood serum are tolerated. The steric and electronic stabilization provided by the two NHC moieties enabled the successful design of the highly stable radical.

C(sp2) – H functionalization of aldimines and related compounds: advances and prospects

This is the first systematic review of the most relevant approaches to direct C(sp2)–H bond functionalization of azomethine derivatives. The scope of the applicability of various transformations is analyzed. The review assesses prospects of the application of this functionalization strategy in the multistep synthesis of valuable compounds for use in medicinal chemistry, materials science and related areas.

The bibliography includes 124 references.

A Bottleable Imidazole-Based Radical as a Single Electron Transfer Reagent

Reduction of 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenyl-1H-imidazol-3-ium chloride (1) resulted in the formation of the first structurally characterized imidazole-based radical 2. 2 was established as a single electron transfer reagent by treating it with an acceptor molecule tetracyanoethylene. Moreover, radical 2 was utilized as an organic electron donor in a number of organic transformations such as in activation of an aryl-halide bond, alkene hydrosilylation, and in catalytic reduction of CO₂ to methoxyborane, all under ambient temperature and pressure.

Copper-Catalyzed Asymmetric Coupling of Allenyl Radicals with Terminal Alkynes to Access Tetrasubstituted Allenes

In contrast to the wealth of asymmetric transformations for generating central chirality from alkyl radicals, the enantiocontrol over the allenyl radicals for forging axial chirality represents an uncharted domain. The challenge arises from the unique elongated linear configuration of the allenyl radicals that necessitates the stereo-differentiation of remote motifs away from the radical reaction site. We herein describe a copper-catalyzed asymmetric radical 1,4-carboalkynylation of 1,3-enynes via the coupling of allenyl radicals with terminal alkynes, providing diverse synthetically challenging tetrasubstituted chiral allenes. A chiral N,N,P-ligand is crucial for both the reaction initiation and the enantiocontrol over the highly reactive allenyl radicals. The reaction features a broad substrate scope, covering a variety of (hetero)aryl and alkyl alkynes and 1,3-enynes as well as radical precursors with excellent functional group tolerance.

cAAC-Stabilized 9,10-diboraanthracenes-Acenes with Open-Shell Singlet Biradical Ground States

Narrow HOMO-LUMO gaps and high charge-carrier mobilities make larger acenes potentially high-efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open-shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO-LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10-diboraanthracenes, which are shown to feature disjointed, open-shell singlet biradical ground states.

Tunable Redox-Active Triazenyl-Carbene Platforms: A New Class of Anolytes for Non-Aqueous Organic Redox Flow Batteries

Non-aqueous all organic redox flow batteries (NORFBs) are one of the promising options for large-scale renewable energy storage systems owing to their scalability with energy and power along with the affordability. The discovery of new redox-active organic molecules (ROMs) for the anolyte/catholyte would bring them one step closer to the practical application, thus it is highly demanded. Here, we report a new class of ROMs based on cationic triazenyl systems supported by N-heterocyclic carbenes (NHCs) and demonstrate, for the first time, that the triazenyl can serve as a new redox motif for ROMs and could be significantly stabilized for the use in NORFBs by the coupling with NHCs even at radical states. A series of NHC-triazenyl ROM families were successfully synthesized via the reaction of a synthon, N-heterocyclic carbene azido cation, with various Lewis bases including NHCs. Remarkably, it is revealed that NHCs substituted on the triazenyl fragments can serve as a versatile platform for tailoring the electrochemical activity and stability of triazenyl-based compounds, introducing various ROMs exploiting triazenyl redox motif, as demonstrated in the full cell of NORFBs for an anolyte.

How to Bend a Cumulene

Allenes (carbodicarbenes) and [3]cumulenes are linear carbon chains that can be bent when the terminal group has a strong carbene nature. This bending can be quite pronounced in allenes but not in [3]cumulenes. In this study, how N-heterocyclic or cyclic (alkyl)(amino) carbene (NHC and CAAC, respectively) terminal groups can modify the linear structure of [n]cumulenes has been analyzed. A low π acidity of the terminal carbene affects the linearity of [2n]cumulenes. Indeed, it has been found that the NHC [4]cumulene is extremely bent, contrary to classical [4]cumulenes. The predicted NHC [4]cumulene or tricarbodicarbene has two lone pairs and the π electrons are delocalized over the whole molecule. More significantly, DFT calculations have shown that this bent [4]cumulene is very stable, considerably more so than the corresponding [3]cumulene, which has been elusive to synthesize. Remarkably, calculations have shown that all the NHC [2n]cumulenes are more than 25 kcal mol^-1 more stable than the [2n-1]cumulenes.

Saturated N-Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker

The synthesis of a SIPr [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C-F bond of C₆ F₆ by SIPr in the presence of BF₃ led to double C-F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C₆ F₆ in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single-crystal X-ray diffraction studies. Experimental and computational studies support the cumulenic closed-shell singlet state of 3 with a singlet-triplet energy gap (ΔE_S-T ) of 23.7 kcal mol^-1 .

Oxygen atom transfer: a mild and efficient method for generating iminyl radicals

Treating iminoxyl species with oxygen acceptors such as PPh₃ resulted in oxygen atom transfer and afforded the corresponding iminyl radicals. DFT and other calculations revealed that association between the oxygen atom acceptors and the iminoxyl species results in the formation of key intermediates during the reaction. Subsequent dissociation is accompanied with homolytic cleavage of the N-O bond and generates iminyl radicals with spin densities that are localized on exocyclic nitrogen atoms.

Synthesis of cAAC stabilized biradical of "Me2Si" and "Me2SiCl" monoradical from Me2SiCl2 - an important feedstock material

The cyclic alkyl(amino) carbene (cAAC) coordinated biradical of dimethylsilicon was isolated as (cAAC)2Me2Si (1), (cAAC = C(CH2)(CMe2)2N-2,6-i-Pr2C6H3), synthesized from the reduction of Me2SiCl2 using two equivalents of KC8 in the presence of two equivalents of cAAC. The reduction of Me2SiCl2 by one equivalent of KC8 in the presence of one equivalent of cAAC resulted in the stable dimethylsiliconchloride monoradical (cAAC)Me2SiCl (2).

Stable cyclic (alkyl)(amino)carbene (cAAC) radicals with main group substituents

Recent attempts to isolate cyclic (alkyl)(amino)carbene stabilized radicals of p-block elements have been described here.

Isolation and characterization of stable radicals has been a long-pursued quest. While there has been some progress in this field particularly with respect to carbon, radicals involving heavier p-block elements are still considerably sparse. In this review we describe our recent successful efforts on the isolation of stable p-block element radicals particularly those involving aluminum, silicon, and phosphorus.

Copper-catalyzed 1,4-alkylarylation of 1,3-enynes with masked alkyl electrophiles

Classical 1,4-dicarbofunctionalization of 1,3-enynes employs organometallic reagents as nucleophiles to initiate the reaction. We report a copper-catalyzed 1,4-alkylarylation of 1,3-enynes with alkyl diacyl peroxides as masked alkyl electrophiles and aryl boronic acids as nucleophiles, selectively affording structurally diversified tetrasubstituted allenes under mild conditions. Mechanistic studies suggest that an allenyl radical might be involved.

Direct access to 2-aryl substituted pyrrolinium salts for carbon centre based radicals without pyrrolidine-2-ylidene alias cyclic(alkyl)(amino)carbene (CAAC) as a precursor

A new strategy to synthesise 2-substituted pyrrolinium salts.

The synthesis of organic radicals is challenging due to their inherent instability. In recent years, cyclic(alkyl)(amino)carbene (CAAC)-derived 2-substituted pyrrolinium salts have been used as synthons for the synthesis of isolable carbon-based radicals. Herein, we report a direct, easy and convenient method for the synthesis of 2-aryl substituted pyrrolinium salts without using CAAC as a precursor. These cations can be reduced to the corresponding radicals. The influence of the aryl substituent at the C-2 position on radical stabilization and dimerization has been investigated. Because of the large scope of our strategy (capability to modulate different substituents at all the C- and N-centres of the pyrrolinium salts), it has the merit to be an extremely effective and productive route for generating carbon-based radicals whose stability as well as reactivity can be varied.

A [C1 + C2] route to propargylidyne complexes

The reactions of [W(CBr)(CO)₂(Tp*)] with a range of terminal alkynes (RCCH), mediated by [Pd(PPh₃)₄] and CuI, afford new propargylidynes [W(C–CCR)(CO)₂(Tp*)] [R = ^tBu, C₆H₄X (X = H, NH₂, NO₂), APh₃ (A = C, Si, Ge), B(O₂CCH₂)₂NMe].

"Abnormal" Addition of NHC to a Conjugate Acid of CAAC: Formation of N-Alkyl-Substituted CAAC

The addition reactions of N-heterocyclic carbenes (NHCs) are mostly known to occur through the carbenic centre (C2), which leads to a "normal" adduct. Herein, we report the "abnormal" addition of NHC^Dip 1 (1,3-(2,6-iPr₂ C₆ H₃ )-imidazole-2-ylidene) to a conjugate acid of cyclic (alkyl)(amino)carbene 2 (CAAC^iPr =1-iPr-3,3,5,5-Me₄ -pyrrolinium triflate). Mechanistic study revealed that this reaction proceeded through the in situ formation of 1,3-(2,6-iPr₂ C₆ H₃ )-imidazolium cation 4 and N-iPr-substituted CAAC 5 followed by the oxidative addition of compound 5 across the C4-H bond (alias backbone C-H) of compound 4. The in situ formation of compound 5 was also proven by the oxidative addition of it to the N-H group of iPrNH₂ . DFT calculations also supported the mechanistic findings. A different methodology for the in situ generation of compound 5 by using TMPLi is also described.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.