Carbene Center Research Articles

Catalyst-free reactions of Schiff bases with cyanopropargyl alcohols: synthesis of functionalized 1,3-oxazolidines

Schiff bases react with cyanopropargylic alcohols (80°C, 10-12 h) in the absence of catalysts and solvents to afford functionalized 1,3-oxazolidines in up to 82% yield. The reaction proceeds through a cascade of transformations including the formation of a 1,3-dipolar intermediate via the addition of a neutral N -nucleophile to the activated triple bond, the proton transfer from the hydroxyl group to the carbanionic center of the intermediate, and the intramolecular attack of the carbonyl carbon of the Schiff base by the O -centered anion to close 1,3-oxazolidine cycle.

Reactivities of α‐Oxo BMIDA Gold Carbenes Generated by Gold‐Catalyzed Oxidation of BMIDA‐Terminated Alkynes

AbstractAn 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.

Reactivities of α-Oxo BMIDA Gold Carbenes Generated by Gold-Catalyzed Oxidation of BMIDA-Terminated Alkynes.

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.

L‐Shaped Heterobidentate Imidazo[1,5‐a]pyridin‐3‐ylidene (N,C)‐Ligands for Oxidant‐Free AuI/AuIII Catalysis

AbstractIn the last decade, major advances have been made in homogeneous gold catalysis. However, AuI/AuIII catalytic cycle remains much less explored due to the reluctance of AuI to undergo oxidative addition and the stability of the AuIII intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N‐heterocyclic carbene) ligands through the development of a new class of L‐shaped heterobidentate ImPy (ImPy=imidazo[1,5‐a]pyridin‐3‐ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ‐donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for AuI/AuIII redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

L-Shaped Heterobidentate Imidazo[1,5-a]pyridin-3-ylidene (N,C)-Ligands for Oxidant-Free AuI /AuIII Catalysis.

In the last decade, major advances have been made in homogeneous gold catalysis. However, AuI /AuIII catalytic cycle remains much less explored due to the reluctance of AuI to undergo oxidative addition and the stability of the AuIII intermediate. Herein, we report activation of aryl halides at gold(I) enabled by NHC (NHC=N-heterocyclic carbene) ligands through the development of a new class of L-shaped heterobidentate ImPy (ImPy=imidazo[1,5-a]pyridin-3-ylidene) N,C ligands that feature hemilabile character of the amino group in combination with strong σ-donation of the carbene center in a rigid conformation, imposed by the ligand architecture. Detailed characterization and control studies reveal key ligand features for AuI /AuIII redox cycle, wherein the hemilabile nitrogen is placed at the coordinating position of a rigid framework. Given the tremendous significance of homogeneous gold catalysis, we anticipate that this ligand platform will find widespread application.

Anionic N-heterocyclic carbenes featuring weakly coordinating perfluoroalkylphosphorane moieties.

The anionic 1-methyl-3-(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate 3 and the 1,3-bis(tris(pentafluoroethyl)difluorophosphorane)imidazoline-2-ylidenate dianion 4 were obtained in high yield by deprotonation of {(C2F5)3PF2}-methylimidazole 1 and the {(C2F5)3PF2}2-imidazolate anion 2. Carbenes 3 and 4 are first examples for a novel class of NHCs carrying weakly coordinating anions (WCA-NHCs). First reactions of these new ligands with elemental selenium and chloro(phosphine)gold(I) complexes to result in an anionic selenium adduct (5) and WCA-NHC gold complexes (6 and 7) have been undertaken. The structural and spectroscopic properties of these NHC derivatives in conjunction with data from quantum chemical calculations provide an insight into the electronic and steric properties of the WCA-NHCs 3 and 4. Especially the combination of properties such as weakly coordinating periphery combined with the coordinative carbene center, negative charge, large buried volume (%Vbur), and strong σ-donor as well as efficient π-acceptor ability render NHCs 3 and 4 unique and promising new ligands.

Three-component chemo-selective oxy-allylation of α-diazo carbonyl compounds: Access to α-ternary carboxylic esters

A synergistic Rh(II)/Pd(0) dual-catalyzed strategy that enabled three-component oxy-allylation of α-diazo esters, alcohols, and allylic benzoates was described. Trapping of active protic oxonium ylides with catalytic π-allyl Pd(II) intermediates was proposed for this reaction. More strikingly, water was also a suitable partner in this reaction involving a different allylic migratory insertion of palladium-carbenes. These transformations provided straightforward access to various α-ternary allylated carboxylic esters using simple and readily available starting materials under redox-neutral conditions, in which two new CO and CC bonds were generated successively at the carbene center.

Crystalline monometal-substituted free carbenes

<h2>Summary</h2> The rich chemistry of transition-metal (TM) carbyne complexes has fueled their application in a wide range of fields, from synthetic chemistry to materials science. The isomeric forms of these complexes are monometal-substituted carbenes and have so far been detected only in the gas phase. In this article, we describe the synthesis of crystalline monometal-substituted free carbenes. To isolate these molecules, we selected a bulky π-donor substituent and a closed-shell electron-rich TM center as the substituents around the carbene center. This combination allows these metallocarbenes to exhibit a nucleophilic carbenic character with a high-lying highest occupied molecular orbital (HOMO) arising from the metal substituent. Additionally, we report a formal rearrangement of the metallocarbene to a zwitterionic species containing a carbyne anion, which makes these free compounds not only synthetic astonishments but also attractive for the development of novel subvalent carbon-transfer reactions.

From Stable PH‐Ylides to α‐Carbanionic Phosphines as Ligands for Zwitterionic Catalysts

AbstractAlthough ylides are commonly used reagents in organic synthesis, the parent methylphosphine MePH2 only exists in its phosphine form in the condensed phase. Its ylide tautomer H3P+−CH2− is considerably higher in energy. Here, we report on the formation of bis(sulfonyl)methyl‐substituted phosphines of the type (RO2S)2C(H)−PR2, which form stable PH ylides under ambient conditions, amongst the first examples of an acyclic phosphine which only exists in its PH ylide form. Depending on the exact substitution pattern the phosphines form an equilibrium between the PH ylide and the phosphine form or exist as one of both extremes. These phosphines were found to be ideal starting systems for the facile formation of α‐carbanionic phosphines. The carbanion‐functionalization leads to a switch from electron‐poor to highly electron‐rich phosphines with strong donor abilities and high basicities. Thus, the phosphines readily react with different electrophiles exclusively at the phosphorus atom and not at the carbanionic center. Furthermore, the anionic nature of the phosphines allows the formation of zwitterionic complexes as demonstrated by the isolation of a gold(I) complex with a cationic metal center. The cationic gold center allows for catalytic activity in the hydroamination of alkyne without requiring a further activation step.

From Stable PH-Ylides to α-Carbanionic Phosphines as Ligands for Zwitterionic Catalysts.

Although ylides are commonly used reagents in organic synthesis, the parent methylphosphine MePH2 only exists in its phosphine form in the condensed phase. Its ylide tautomer H3P+−CH2 − is considerably higher in energy. Here, we report on the formation of bis(sulfonyl)methyl‐substituted phosphines of the type (RO2S)2C(H)−PR2, which form stable PH ylides under ambient conditions, amongst the first examples of an acyclic phosphine which only exists in its PH ylide form. Depending on the exact substitution pattern the phosphines form an equilibrium between the PH ylide and the phosphine form or exist as one of both extremes. These phosphines were found to be ideal starting systems for the facile formation of α‐carbanionic phosphines. The carbanion‐functionalization leads to a switch from electron‐poor to highly electron‐rich phosphines with strong donor abilities and high basicities. Thus, the phosphines readily react with different electrophiles exclusively at the phosphorus atom and not at the carbanionic center. Furthermore, the anionic nature of the phosphines allows the formation of zwitterionic complexes as demonstrated by the isolation of a gold(I) complex with a cationic metal center. The cationic gold center allows for catalytic activity in the hydroamination of alkyne without requiring a further activation step.

Tricyanoborane-Functionalized Anionic N-Heterocyclic Carbenes: Adjustment of Charge and Stereo-Electronic Properties.

The 1‐methyl‐3‐(tricyanoborane)imidazolin‐2‐ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN)3‐methylimidazole adduct 1. Regarding charge and stereo‐electronic properties, anion 2 closes the gap between well‐known neutral NHCs and the ditopic dianionic NHC, the 1,3‐bis(tricyanoborane)imidazolin‐2‐ylidenate dianion (IIb). The influence of the number of N‐bonded tricyanoborane moieties on the σ‐donating and π‐accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3‐dimethylimidazolin‐2‐ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)4] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V bur) and σ‐donor and π‐acceptor abilities of NHCs can be effectively fine‐tuned via the number of tricyanoborane substituents.

Gold(I) α‐Trifluoromethyl Carbenes: Synthesis, Characterization and Reactivity Studies

AbstractAryl trifluoromethyl diazomethanes 2‐R (R=Ph, OMe, CF3) are readily decomposed by the (o‐carboranyl)diphosphine gold(I) complex 1. The resulting α‐CF3 substituted carbene complexes 3‐R have been characterized by multi‐nuclear NMR spectroscopy as well as X‐ray crystallography (for 3‐Ph). The bonding situation was thoroughly assessed by computational means, showing stabilization of the electrophilic carbene center by π‐donation from the aryl substituent and backdonation from Au, as enhanced by the chelating P^P ligand. Reactivity studies under stoichiometric and catalytic conditions substantiate typical carbene‐type behavior for 3‐Ph.

Gold(I) α-Trifluoromethyl Carbenes: Synthesis, Characterization and Reactivity Studies.

Aryl trifluoromethyl diazomethanes 2‐R (R=Ph, OMe, CF3) are readily decomposed by the (o‐carboranyl)diphosphine gold(I) complex 1. The resulting α‐CF3 substituted carbene complexes 3‐R have been characterized by multi‐nuclear NMR spectroscopy as well as X‐ray crystallography (for 3‐Ph). The bonding situation was thoroughly assessed by computational means, showing stabilization of the electrophilic carbene center by π‐donation from the aryl substituent and backdonation from Au, as enhanced by the chelating P^P ligand. Reactivity studies under stoichiometric and catalytic conditions substantiate typical carbene‐type behavior for 3‐Ph.

Strong dispersion interaction of the carbanionic center in methide anion derivatives with halogen atom of metal halides

Strong dispersion interaction of the carbanionic center in methide anion derivatives with halogen atom of metal halides

Divergent stereochemical outcomes in the insertion of donor/donor carbenes into the C-H bonds of stereogenic centers.

Intramolecular C–H insertions with donor/donor dirhodium carbenes provide a concise and highly stereoselective method to set two contiguous stereocenters in a single step. Herein, we report the insertion of donor/donor carbenes into stereogenic carbon centers allowing access to trisubstituted benzodihydrofurans in a single step. This study illuminates, for the first time, the stereochemical impact on the carbene center and delineates the structural factors that enable control over both stereogenic centers. Sterically bulky, highly activated C–H insertion centers exhibit high substrate control yielding a single diastereomer and a single enantiomer of product regardless of the catalyst used. Less bulky, less activated C–H insertion centers exhibit catalyst control over the diastereomeric ratio (dr), where a single enantiomer of each diastereomer is observed with high selectivity. A combination of experimental studies and DFT calculations was used to elucidate the origin of these results. First, hydride transfer from the stereogenic insertion site proceeds with high stereoselectivity to the carbene center, thus determining the absolute configuration of the product. Second, the short lived zwitterionic intermediate can diaster-eoselectively ring-close by a hitherto unreported SE2 mechanism that is either controlled by the substrate or the catalyst. These results demonstrate that donor/donor carbenes undergo uniquely stereoselective reactions that originate from a stepwise reaction mechanism, in contrast to the analogous concerted reactions of carbenes with one or more electron-withdrawing groups attached.

N-Alkoxyimidazolylidines (NOHCs): nucleophilic carbenes based on an oxidized imidazolium core.

We report the first preparation of N-alkoxyimidazolylidene (NOHC), a nucleophilic carbene based on an oxidized imidazolium core. The Arduengo-type analogous carbene center shows the most upfield 13C NMR shift compared to common NHCs. The obtained gold(I) complex of the carbene follows the 13C NMR upfield trend and shows the marked influence the alkoxy substituents. Similarly, the 77Se and 15N NMR shifts of a range of NOHC-selenium adducts show increased σ-donation and decreased π-back donation in the bonding with the nucleophile. This extension of the NHC family provides altered electronic properties for the use of such carbenes as ligands or catalysts.

Cationic Charge-Appended Abnormal Carbenes: Synthesis and Study of Electronically Modified Abnormal N-Heterocyclic Carbenes.

A range of palladium complexes featuring electronically modified, imidazole-based abnormal N-heterocyclic carbene (aNHC) ligands have been prepared in the hopes of accessing a new class of cationic aNHC ligands electronically distinct from normal NHCs and aNHCs. These palladium complexes represent the first examples of transition metal-ligated aNHC complexes featuring a cationic moiety adjacent to the abnormal carbene center. It was anticipated that these design principles could facilitate electron transfer between the imidazolinylidene and the cationic heterocycle, thus reducing the electron density at the abnormal carbene center. However, this case study suggests that greater conformational restrictions that allow for heterocycle coplanarity are necessary to achieve significant electron transfer and enable access to a new class of cationic charge-appended aNHCs with unique electronic properties.

Buchwald-Hartwig Amination of Coordinating Heterocycles Enabled by Large-but-Flexible Pd-BIAN-NHC Catalysts.

A new class of large-but-flexible Pd-BIAN-NHC catalysts (BIAN=acenaphthoimidazolylidene, NHC=N-heterocyclic carbene) has been rationally designed to enable the challenging Buchwald-Hartwig amination of coordinating heterocycles. This robust class of BIAN-NHC catalysts permits cross-coupling under practical aerobic conditions of a variety of heterocycles with aryl, alkyl, and heteroarylamines, including historically challenging oxazoles and thiazoles as well as electron-deficient heterocycles containing multiple heteroatoms with BIAN-INon (N,N'-bis(2,6-di(4-heptyl)phenyl)-7H-acenaphtho[1,2-d]imidazol-8-ylidene) as the most effective ligand. Studies on the ligand structure and electronic properties of the carbene center are reported. The study should facilitate the discovery of even more active catalyst systems based on the unique BIAN-NHC scaffold.

The effects of halogen substituents on structure, stability, and electronic properties of bicyclo[1.1.1]pentanylene at density functional theory

AbstractIn a quest to identify novel bicyclic carbenes, the effects of halogen substituents on the stability and reactivity of singlet (s) and triplet (t) forms of carbenes with one, two, and three carbenic centers (1‐15) are compared and contrasted, at B3LYP/6‐311++G** level of theory. All carbene scrutinized turn out as minima for showing no negative force constant on their energy surfaces. The highest stability (ΔEs‐t) and band gap (ΔEHOMO–LUMO) belongs to carbene 11 (37.36 and 133.45 kcal/mol, respectively) with three carbenic centers. Such higher stabilization attributed to three coordinate covalent bonds with their carbenic centers. The nucleophilicity (N) for each series (1‐5, 6‐10, and 11‐15) appears inverse correlation with our results of chemical potential (μ) values. Also, the N of the carbenic center is decreased due to the higher electronegativity of substituted halogens. In addition, the electrostatic potential (ESP) maps qualitatively confirm the relative N for our bicyclic carbenes.The aim of the present work was to assess the effects of halogen substituents on the stability (ΔEs‐t), ΔΕHOMO–LUMO, N, electrophilicity (ω), and proton affinity (ΔEPA). Finally, our investigation introduces carbene 1s with high stability (ΔEs‐t = 30.38 kcal/mol), N (3.438 eV), and negative ΔEPA (−244.94 kcal/mol) that can be applied as accumulated multidentate ligands.

Density Functional Theory Study of Ultrashort Metal−Metal Distances in Diberyllium Complexes Bearing Carbene Ligands

AbstractDiberyllium complexes with ultrashort metal−metal distances (USMMDs, defined as dM−M&lt;1.900 Å) are fascinating for the nature of the valence electronic structure of Be atoms. In this paper a family of diberyllium complexes [L−BeH3Be−L]+ (1–7), in which L was an N‐heterocyclic carbene (NHC) or a mesoionic carbene (MIC), were studied by Density Functional Theory (DFT) at the B3LYP/cc‐pVTZ level. It was found that complexes 1–7 possessed ultrashort Be−Be distances of 1.754 Å–1.779 Å and strong covalent bonds between a Be atom and its adjacent carbene center (Ccarb) at once. Electronic structures and chemical bonding analyses ascribed USMMDs between the Be atoms to the Be−H−Be three‐center two‐electron (3c‐2e) bonds. The strong Be−Ccarb bonds were determined by combining effects of covalent interaction and electrostatic interaction between the Be and the Ccarb atoms. These complexes exhibited a great stability with large highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gaps, high vertical detachment energies (VDEs) and low vertical electron affinities (VEAs) and are potential targets in future experiments.