Carbene Center Research Articles

Increasing the Donor Strength of Alkenylphosphines by Twisting the C=C Double Bond

Electron‐rich phosphines play a crucial role in transition metal‐based homogeneous catalysis. While alkyl groups have traditionally been employed to increase the phosphine donor strength, recent studies have shown that zwitterionic functional groups such as phosphorus ylides can result in a further enhancement. Herein we report the concept of twisting a C=C double bond to introduce a zwitterionic substituent by the synthesis and application of N‐heterocyclic olefin phosphines with a sulfonyl substituent (sNHOP). This sulfonyl group enables the twisting of the olefin moiety due to steric and electronic stabilization of the carbanionic center. The resulting zwitterionic structure leads to a significant increase of the donor strength of the sNHOP ligands compared to conventional NHOP systems with a planar N‐heterocyclic olefin moiety. The potential of this new ligand platform for catalysis is demonstrated by its application in the gold‐catalyzed hydroamination and cyclo‐isomerization of alkynes. Here, the ligands outperform the original NHOP ligands suggesting favorable properties for future catalysis applications.

Increasing the Donor Strength of Alkenylphosphines by Twisting the C=C Double Bond.

Electron-rich phosphines play a crucial role in transition metal-based homogeneous catalysis. While alkyl groups have traditionally been employed to increase the phosphine donor strength, recent studies have shown that zwitterionic functional groups such as phosphorus ylides can result in a further enhancement. Herein we report the concept of twisting a C=C double bond to introduce a zwitterionic substituent by the synthesis and application of N-heterocyclic olefin phosphines with a sulfonyl substituent (sNHOP). This sulfonyl group enables the twisting of the olefin moiety due to steric and electronic stabilization of the carbanionic center. The resulting zwitterionic structure leads to a significant increase of the donor strength of the sNHOP ligands compared to conventional NHOP systems with a planar N-heterocyclic olefin moiety. The potential of this new ligand platform for catalysis is demonstrated by its application in the gold-catalyzed hydroamination and cyclo-isomerization of alkynes. Here, the ligands outperform the original NHOP ligands suggesting favorable properties for future catalysis applications.

IPr*F - Highly Hindered, Fluorinated N-Heterocyclic Carbenes.

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon-fluorine bonds. Simultaneously, sterically-hindered N-heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition-metal-catalysis. Herein, we report the first class of sterically-hindered, fluorinated N-heterocyclic carbenes. These ligands feature variable fluorine substitution at the N-aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one-pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon-fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers.

Some Terminally Alkenyl-substituted Porphyrins: Synthesis and Attempts of Heck Reaction Leading to Porphyrin–Azulene Dyads

: The synthesis of meso-tetraphenylporphyrin derivatives, double functionalized in one of the phenyl rings (with a nitro group and carbon substituents), is described. 5-(4- Nitroaryl)-10,15,20-triarylporphyrinates react, in the presence of a base (t-BuOK), with carbanions containing a leaving group at the carbanionic center to give products of substitution of hydrogen at ortho-position to the nitro group via vicarious nucleophilic substitution scheme. Their alkylation with allyl bromide or 4-bromobut-1-en (t-BuOK/DMF, at 0-5°C) leads to porphyrins bearing side alkyl chain with a terminal double bond in yields of up to 82%. The above-mentioned double-substituted intermediates are very convenient substrates for Heck cross-coupling reaction. In this study, the synthesis of porphyrin–azulene dyads is reported. In some cases, the formation of highly conjugated systems is observed, with moderate yield. Their structures are interesting from both the porphyrin and azulene point of view.

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations.

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Hydrogen Tag Shifts as Vibrational Reporters for Positional Isomers of Formylphenides: Surprising Mobility of the Carbanion Center upon Collisional Decarboxylation of the Parent Benzoates.

Infrared photodissociation of weakly bound "mass tags" is widely used to determine the structures of ions by analyzing their vibrational spectra. Molecular hydrogen is a common choice for tagging in cryogenic radio-frequency ion traps. Although the H2 molecules can introduce distortions in the target species, we demonstrate an advantage of H2 tagging in the analysis of positional isomers adopted by the molecular anions derived from decarboxylation of formylbenzoates. Attachment of H2 to the carbanion centers of three such isomers yields distinct shifts in the H2 stretch, which can be used to determine the distribution of isomers in an unknown sample. Electronic structure calculations indicate that the position-dependent shifts are due to different reactivities of the carbanion sites with respect to an intracluster proton-transfer reaction with the H2 molecule. We exploit this spectroscopic method to quantify the surprisingly facile migrations of the anionic center that have been previously reported for phenide rearrangements.

Amide C–N bonds activation by A new variant of bifunctional N-heterocyclic carbene

We report an organocatalyst that combines a triazolium N-heterocyclic carbene (NHC) with a squaramide as a hydrogen-bonding donor (HBD), which can effectively catalyze the atroposelective ring-opening of biaryl lactams via a unique amide C–N bond cleavage mode. The free carbene species attacks the amide carbonyl, forming an axially chiral acyl-azolium intermediate. Various axially chiral biaryl amines can be accessed by this methodology with up to 99% ee and 99% yield. By using mercaptan as a catalyst turnover agent, the resulting thioester synthon can be transformed into several interesting atropisomers. Both control experiments and theoretical calculations reveal the crucial role of the hybrid NHC-HBD skeleton, which activates the amide via H-bonding and brings it spatially close to the carbene centre. This discovery illustrates the potential of the NHC-HBD chimera and demonstrates a complementary strategy for amide bond activation and manipulation.

A stable rhodium-coordinated carbene with a σ0π2 electronic configuration.

Isolable singlet carbenes have universally adopted a σ2π0 electronic state, making them σ-donors and π-acceptors. We present a rhodium-coordinated, cationic cyclic diphosphinocarbene with a σ0π2 ground state configuration. Nuclear magnetic resonance spectroscopy studies show a carbene carbon chemical shift below -30.0 parts per million. X-ray crystallography reveals a planar RhP2C configuration. Quantum chemical calculations rationalize how σ-electron delocalization/donation and π-electron negative hyperconjugation together stabilize the formally vacant σ orbital and the filled π orbital at the carbene center. In contrast to traditional carbene counterparts this carbene can undergo synthetic transformations with both a Lewis base and a silver salt, producing a Lewis acid/base adduct and a silver π-complex, respectively. Exhibiting ambiphilic reactivity, it can also form a ketenimine through reaction with an isocyanide.

Pentacycloundecanylidene and pentacycloundecanone - hyperconjugatively stabilized carbene and ketone.

Pentacycloundecanylidene was spectroscopically identified during the photolysis of the corresponding aziridine and its aerial oxidation results in the corresponding ketone. Here we report the role of hyperconjugative interactions in stabilizing the singlet pentacycloundecanylidene and its corresponding ketone pentacycloundecanone. The pentacycloundecanylidene possesses a singlet ground state with two possible geometrical isomers based on the orientation of the carbene bridge (U1 and U2). The energy difference between U1 and U2 is minimal (0.9 kcal mol-1) and the triplet state is 5.6 kcal mol-1 energetically higher than the more stable singlet state U1. The proximal C-C bonds of the carbene bridge in the singlet state are significantly elongated as compared to the distal C-C bonds. The bending of the carbene bridge in the triplet state and the carbonyl group in the ketone are minimal as compared to the parent pentacycloundecane. The molecular orbital (MO), natural bond orbital (NBO) and energy decomposition analysis (EDA) show that both Cieplak-type hyperconjugation viz. donation of electrons from the proximal C-C σ bonds to the empty p-orbital on the carbene centre and Felkin-Anh type hyperconjugation viz. donation of the lone pair of carbene carbon to the distal C-C σ* orbitals exist in carbene systems. The bending of the carbene bridge in singlet carbene is to enhance the Cieplak-type hyperconjugative interaction. The ketone is also stabilized by Cieplak-type hyperconjugative interaction but to a lower extent as compared to the singlet carbene. The reactivity study suggests that the singlet pentacycloundecanylidene is ambiphilic in nature.

Ambiphilicity of ring-expanded N-heterocyclic carbenes.

N-heterocyclic carbenes, such as imidazole-2-ylidenes and imidazolin-2-ylidenes, the popular class of singlet carbenes introduced by Arduengo in 1991 have not been shown to be ambiphilic owing to the two σ-withdrawing, π-donating amino groups flanking the carbene centre. However, our experimental data suggest that ring-expanded N-heterocyclic carbenes (RE-NHCs), especially the seven and eight membered rings, are significantly ambiphilic. Our results also show that the steric environment in RE-NHCs can become a determining factor for controlling the E-H bond activation.

Uncovering Nitrosyl Reactivity at N-Heterocyclic Carbene Center.

N-heterocyclic carbenes (NHCs) have garnered much attention due to their unique properties, such as strong σ-donating and π-accepting abilities, as well as their transition-metal-like reactivity toward small molecules. In 2015, we discovered that NHCs can react with nitric oxide (NO) gas to form radical adducts that resemble transition metal nitrosyl complexes. To elucidate the analogy between NHC and transition metal NO adducts, here we have undertaken a systematic investigation of the electron- and proton-transfer chemistry of [NHC-NO]• (N-heterocyclic carbene nitric oxide radical) compounds. We have accessed a suite of compounds, comprised of [NHC-NO]+, [NHC-NO]-, [NHC-NOH]0, and [NHC-NHOH]+ species. In particular, [NHC-NO]- was isolated as potassium and lithium ion adducts. Most interestingly, a monomeric potassium [NHC-NO]- compound was isolated with the assistance of 18-crown-6, which is the first instance of a monomeric alkali N-oxyl compound to the best of our knowledge. Our results demonstrate that [NHC-NO]• exhibits redox behavior broadly similar to metal nitrosyl complexes, which opens up more possibilities for utilizing NHCs to build on the known reactivity of metal complexes.

A 3D metal hydrogen-bonded organic framework based on a trans-dichloropalladium complex with bis(N-heterocyclic carbene) ligands

In this study, a trans-PdCl2(NHC)2 complex, where NHC is 1-(4-fluorobenzyl)-3-vinyl-1,3-dihydro-2H-imidazolin-2-ylidene, was synthesized with a 60 % of yield. Crystallographic analysis revealed a significant intermolecular non-classical hydrogen bonding interaction of the type CH···Cl between an aryl proton adjacent to the electronegative F atom and a Cl ligand on the palladium carbene center. Additionally, a slightly longer CH···Cl interaction was identified through normalized contact distance calculation in Hirshfeld surface analysis. These interactions acted as connectors, linking the metal complex nodes to form a 3D 6-connected metal hydrogen-bonded organic framework (M−HOF). The stabilizing energy of 1.02 kcal/mol for the major CH···Cl interaction was confirmed through natural bond orbital (NBO) analysis. Hirshfeld surface analysis further revealed other intermolecular interactions, including the significance of CH···F and CH···π contacts in stabilizing the crystal packing. This work presents a rare example of a 3D M−HOF based on a palladium NHC complex and highlights the potential of utilizing FCC—H···Cl—Pd supramolecular synthons to assemble multi-dimensional M−HOFs.

Dimetal-Binding Scaffold 2-(Pyridin-2-yl)imidazo[1,5-b]pyridazine-7-ylidene: Synthesis of Trinuclear Heterobimetallic Complexes Involving Gold-Metal Interactions.

As a dimetal-binding rigid scaffold, 2-(pyridin-2-yl)imidazo[1,5-b]pyridazine-7-ylidene was introduced. The scaffold was first converted into a meridional Au,N,N-tridentate ligand through binding of a Au(I)Cl moiety at the carbene center. The Au(I) center and the N,N-chelating moiety were expected to function as metallophilic and 4e-σ-donative interaction sites, respectively, in the binding of the second metal center. In this manner, various trinuclear heterobimetallic complexes were synthesized with different 3d-metal sources, such as cationic CuI, CuII, NiII, and CoII salts. SC-XRD analysis showed that the mono-3d-metal di-gold(I) trinuclear heterobimetallic complexes were constructed through gold(I)-metal interactions. Metallophilic interactions were also investigated by quantum chemical calculations including the AIM and IGMH methods.

Influences of Both N,N,N- and N,N,C-Coordination Modes of Tolyl-terpyridine on the Photophysical Properties of Cyclometalated Ru(II) Complexes: Combined Experimental and Theoretical Investigations on Acid/Base-Dependent Reversible Cyclometalation.

With the goal of developing a new strategy for the synthesis of luminescent Ru(II) complexes, we have prepared herein a new set of bis-tridentate complexes of the type [(py-bpy-Ph-X)Ru(tpy-PhCH3)]ClO4 (X = -CH3, -CH2Br, and -CHO) incorporating both non-cyclometalated and cyclometalated coordination motifs of two isomeric forms of methylphenyl-terpyridine (tpy-PhCH3). Thorough characterization of the synthesized complexes is carried out using standard analytical tools and single crystal X-ray diffraction. Detailed investigations on their photophysical and electrochemical behaviors are carried out in MeCN. The presence of a carbanionic center in the cyclometalating unit increases the absorption spectral window of the complexes into a longer-wavelength region. The complexes also exhibit room-temperature luminescence in the NIR domain with enhanced excited-state lifetimes (up to 20.1 ns) compared to their non-cyclometalated counterpart, [Ru(tpy-PhCH3)2]2+. In the presence of acid, the non-coordinated nitrogen atom in the secondary coordination sphere of the complexes allows fine-tuning of the absorption and emission spectral properties. Excess acid induces de-coordination of the Ru-C bond, which is signaled by a remarkable alteration of their spectral profiles. Cleavage of the Ru-C bond is also possible upon treating the acidified solution of the complexes with visible light. Restoration of the Ru-C bond is again feasible upon treating the solution with base at an elevated temperature (∼70 °C). In essence, "on-off" and "off-on" switching of emission is facilitated upon alternating treatment of the Ru(II) complexes with acid, base, and temperature. DFT and TD-DFT calculations are also performed for assignments of the spectral bands as well as to understand structural changes associated with the switching behaviors of the complexes.

1,3-Imidazole-Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N-Heterocyclic Carbenes.

Classical N-heterocyclic carbenes (NHCs) featuring the carbene center at the C2-position of 1,3-imidazole framework (i.e. C2-carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ-donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so-called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ-donors than C2-carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5-anionic dicarbenes (ADCs), also based on an 1,3-imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main-group heterocycles, radicals, molecular catalysts, ligands sets, and more.

1,3‐Imidazol‐basierte mesoionische Carbene und anionische Dicarbene: Erweiterung der Grenzen klassischer N‐heterocyclischer Carbene

AbstractKlassische N‐heterocyclische Carbene (NHCs) mit dem Carbenzentrum an der C2‐Position des 1,3‐Imidazolgerüsts (d.h. C2‐Carbene) sind als vielseitige neutrale Liganden sowohl in der Molekularforschung als auch in den Materialwissenschaften bekannt. Die Effizienz und der Erfolg der NHCs in verschiedenen Bereichen ist im Wesentlichen auf ihre überzeugenden stereoelektronischen Eigenschaften zurückzuführen, insbesondere auf die starke σ‐Donoreigenschaft. Die NHCs mit dem Carbenzentrum an der ungewöhnlichen C4‐ (oder C5‐) Position, die sogenannten abnormalen NHCs (aNHCs) oder mesoionischen Carbene (iMICs), sind jedoch deutlich stärkere σ‐Donatoren als C2‐Carbene. Deshalb haben iMICs ein erhebliches Potenzial als Liganden in der nachhaltigen Synthese und Katalyse. Das Haupthindernis auf diesem Weg ist die anspruchsvolle synthetische Zugänglichkeit zu iMICs. Das Ziel dieses Übersichtsartikels ist es, die aktuellen Fortschritte, insbesondere der Forschungsgruppe des Autors, im Hinblick auf den Zugang zu stabilen iMICs, die Quantifizierung ihrer Eigenschaften sowie die Erforschung ihrer Anwendungen in Synthese und Katalyse aufzuzeigen. Darüber hinaus werden die synthetische Eignung und die Verwendung vicinaler C4,C5‐anionische Dicarbene (ADCs), die ebenfalls auf einem 1,3‐Imidazolgerüst basieren, vorgestellt. Wie auf den folgenden Seiten deutlich wird, haben iMICs und ADCs das Potenzial, die Grenzen der klassischen NHCs zu erweitern, indem sie den Zugang zu konzeptionell neuartigen Hauptgruppenelement‐Heterocyclen, Radikalen, molekularen Katalysatoren, Ligandensätzen und mehr ermöglichen.

Construction of Chiral Quaternary Carbon Centers via Asymmetric Metal Carbene gem‐Dialkylation

AbstractThe catalytic asymmetric construction of all‐carbon quaternary stereocenters has received tremendous interest over the past decades, and numerous efficient protocols have been disclosed based on the formation of one C−C bond between two assembling reactants. However, the use of asymmetric multi‐component reactions that build two C−C bonds on the same carbonic center with concomitant assembly of quaternary stereocenters is rare and remains challenging. Herein, we disclose an enantioselective three‐component reaction of α‐diazo ketones with alkenes and 1,3,5‐triazines under dirhodium/chiral phosphoric acid cooperative catalysis, which leads to a practical and atom‐economic synthesis of poly‐functionalized chiral ketones that bear a α‐quaternary stereocenter in generally good to high yields with excellent enantioselectivities. In comparison to the previous method for the construction of tertiary and quaternary stereocenters via carbene gem‐difunctionalization reactions, this reaction features an unprecedented gem‐dialkylation process via sequential installation of two C−C bonds on the carbene center in one reaction, providing an essential complement to the asymmetric construction of all‐carbon quaternary stereocenters using common and readily available starting materials.

Construction of Chiral Quaternary Carbon Centers via Asymmetric Metal Carbene gem-Dialkylation.

The catalytic asymmetric construction of all-carbon quaternary stereocenters has received tremendous interest over the past decades, and numerous efficient protocols have been disclosed based on the formation of one C-C bond between two assembling reactants. However, the use of asymmetric multi-component reactions that build two C-C bonds on the same carbonic center with concomitant assembly of quaternary stereocenters is rare and remains challenging. Herein, we disclose an enantioselective three-component reaction of α-diazo ketones with alkenes and 1,3,5-triazines under dirhodium/chiral phosphoric acid cooperative catalysis, which leads to a practical and atom-economic synthesis of poly-functionalized chiral ketones that bear a α-quaternary stereocenter in generally good to high yields with excellent enantioselectivities. In comparison to the previous method for the construction of tertiary and quaternary stereocenters via carbene gem-difunctionalization reactions, this reaction features an unprecedented gem-dialkylation process via sequential installation of two C-C bonds on the carbene center in one reaction, providing an essential complement to the asymmetric construction of all-carbon quaternary stereocenters using common and readily available starting materials.

Four-Membered Bimetallic Carbon Complex [(MCp*)2(μ-NPh)(μ-C)] (M = Fe, Ru, Os) - An Ambiphilic Carbene with Lone Pair and σ-Hole Reactivity.

Singlet carbenes are extensively studied compounds capable of electrophilic, nucleophilic or ambiphillic behaviour. The ambiphilic reactivity of singlet carbenes have been conventionally observed in orthogonal planes. Here, we report a detailed bonding and reactivity study of a homobimetallic carbon complex [(MCp*)2(μ-NPh)(μ-C)] (1M, M = Fe, Ru, Os) that shows ambiphilicity in the same direction. The structure of this complex can be considered as two fused three-membered M-C-M and M-N-M rings. The bonding analysis suggests that these 17VE homobimetallic complexes have one formal M-M bond with a bridging carbene centre featuring a high lying spn-hybridised lone pair. Accordingly, the carbene centre shows high proton affinity and act as a good 2e- donor to Lewis acids and transition metal fragment. Apart from the transition metal non-bonding electrons, the π-framework of M-C-M and M-N-M arms can be best described as 3c-2e- bonds. The two transition metals in the four-membered skeleton generate many low-lying, virtual orbitals. These low-lying virtual orbitals induces electron excitation from the spn-hybrid orbital in presence of H- and other 2e- donor ligands such as PMe3, NHC and CO. Hence, the spn-hybrid lone pair orbital shows σ-hole reactivity in presence of Lewis bases.

Ultrafast Photoelimination of Nitrogen from Upper Excited States of Diazoalkanes and the Fate of Carbenes Formed in the Reaction

The photochemical reactivity of diphenyldiazomethane 1 and phenyl 1- and 2-adamantyl diazomethanes 2 and 3, respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination of N2 upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of 1 and 2, detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet car-1 in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet car-2 does not undergo ISC but reacts in the intermolecular insertion reactions into C-H bonds. Car-3 has an α-C-H bond next to the carbene center and reacts rapidly in the intramolecular C-H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from 3 is highly indicative of triplet car-3's intermediate formation. The TA spectra from the S1-S3 states of 1-3 were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.