Brönsted Basicities and Nucleophilicities of N-Heterocyclic Olefins in Solution: N-Heterocyclic Carbene versus N-Heterocyclic Olefin. Which Is More Basic, and Which Is More Nucleophilic?

A tridentate and dianionic N-heterocyclic olefin (NHO) with two unsymmetrically-tethered aryloxide sidearms on bismuth: a fortuitous followed by systematic discovery

Herein, we report our previously developed bifunctional imidaozlium-phenol LH2Br ([HO-4,6-tBu2-C6H2-2-CH2{CH(NCH = CHNAr)}]Br; Ar = Dipp = 2,6-iPr2-C6H3), leading to three distinct outcomes with bismuth by simply altering the relative ratios of the reacting components. Treating LH2Br and Bi(HMDS)3 (HMDS = N(SiMe3)2) in a 1 : 1 ratio gives the NHC-Bi complex [(L)BiBr(HMDS)] (1). In contrast, LH2Br, Bi(HMDS)3, and KHMDS in a 2 : 1 : 1 ratio serendipitously result in an unprecedented NHO-Bi complex [(DippNHO2ArO)BiBr] (2), featuring a novel dianoinic and tridentate NHO ligand with two unsymmetric aryloxide sidearms. A tri-acidic imidazolium salt L'H3I ([HO-4,6-tBu2-C6H2-2-CH2{C(HO-4,6-tBu2-C6H2-2-CH2)(NCH = CHNMe)}]I) is synthesized independently as a potential precursor to such an NHO framework, and the corresponding NHO-BiI complex [(MeNHO2ArO)Bi(μ-I)]2 (32) is made as proof of concept. Lastly, treating LH2Br and Bi(HMDS)3 in a 2 : 1 ratio also unexpectedly leads to an 'abnormal' NHC-Bi complex [(aL)BiBr2(DippImd)] (4; DippImd = Dipp-imidazole). The multi-component and essentially multi-step reactions leading to 2 and 4 are challenging to fully elucidate mechanistically. Still, control experiments indicate 1 as a possible intermediate in both cases. Based on these results and prior insights into LH2Br and an intermediate LH, plausible routes for both 2 and 4 are hypothesized. DFT calculations are also performed to analyze the bonding in 2 and 32 and to justify an NHC to aNHC isomerization towards the formation of 4.

Recent advances in the use of N-heterocyclic carbene adducts of N, P, C elements as supporting ligands in organometallic chemistry

With the rapid development of N-heterocyclic carbene (NHC) coordination chemistry, the potential of NHCs as strong electron-donating ligands has been fully explored. At the same time, an increasing number of p-block element NHC adducts, due to their possession of lone pairs or polarized carbon-element π-bonds, have gradually become important ligands of significant interest in coordination chemistry. Among these, N-heterocyclic imine (NHI), N-heterocyclic phosphinidene (NHCP) and N-heterocyclic olefin (NHO) adducts, formed by linking the 2-position of the N-heterocycle with nitrogen, phosphorus, and carbon elements, respectively, have been widely applied in both main-group and transition metal chemistry. The efficient stabilization of positive charges by the N-heterocycle allows the exocyclic atoms to accumulate negative charges, thereby significantly enhancing the basicity and nucleophilicity of the exocyclic elements. The improvement in nucleophilicity has facilitated the successful synthesis of numerous complexes with unique element-metal bonds. Additionally, by modifying the N-substituents, the kinetic stabilization requirements of highly reactive species can be fulfilled. This review aims to summarize the latest research progress on NHIs, NHCPs, and NHOs in main-group and transition metal compounds. It is important to emphasize that this review focuses on the commonly used five-membered N-heterocyclic carbene adducts, including imidazolin-2-ylidenes, imidazolidin-2-ylidenes, and benzimidazolin-2-ylidenes.

Facile and Regioselective Deuteration of C2-Alkylated Imidazolium Salts in the Presence of Cesium Carbonate

Thirteen imidazolium iodides bearing benzyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms, and C1-C4 alkyl chains on their C2 carbon atom were readily deuterated with D2O as a cheap and non-toxic deuterium source in the presence of Cs2CO3, a weak, innocuous, inorganic base. The isotopic exchange proceeded quickly and efficiently under mild, aerobic conditions to afford a range of aNHC and NHO precursors regioselectively labeled on their C2α exocyclic position and/or C4=C5 heterocyclic backbone. A "carbene-free" mechanism was postulated, in which the carbonate anion acts as a catalyst to activate an exocyclic, acidic C-H bond and ease a deuterium transfer from D2O to the imidazolium salt in a concerted fashion.

An Acceptor-Substituted N-Heterocyclic ortho-Quinodimethane: Pushing the Boundaries of Polarization in Donor-Acceptor-Substituted Polyenes

We report the synthesis, isolation, and characterization of a stable donor-acceptor substituted ortho-quinodimethane (oQDM). This system with an imidazolidine scaffold as the donor can also be referred to as acceptor-substituted ortho-N-heterocyclic quinodimethane (oNHQ). We have examined the extent of polarization of the conjugated π-system using single-crystal X-ray diffraction, NMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations. The bond lengths in the phenyl linker do not exhibit the alternation typical of oQDMs. In addition, the 13C and 15N NMR shifts suggest significant charge separation, an interpretation supported by the diatropic ring current determined by NICSZZ(r) computations, which is characteristic of aromatic compounds. DFT calculations show that polarization is an electronic effect that is amplified by steric influences. More strikingly, the oxidation and reduction potentials of the push-pull substituted oQDM are virtually identical to those of authenticated anionic and cationic derivatives. The results therefore indicate that an aromatic zwitterionic structure represents the electronic structure more accurately than a neutral quinoidal Lewis structure, which indicates that the acceptor-substituted oNHQ is a rare example of an organic zwitterion in which the centers of charge are in conjugation. The ambiphilic reactivity of the acceptor-substituted oNHQ, which is evidenced by the dehydrogenation of ammonia borane and the addition of phenylacetylene via heterolytic C-H bond cleavage, further supports its notation as an organic zwitterion and is reminiscent of frustrated Lewis pairs (FLPs). Thus, the acceptor-substituted oNHQ can be considered to be an intramolecular carbogenic FLP in terms of its reactivity.

N-Heterocyclic Carbene-Catalyzed Asymmetric SN2 Alkylation via Noncovalent Activation

The field of asymmetric catalysis has been developed by exploring noncovalent interactions, particularly within N-heterocyclic carbene-mediated processes. Despite challenges due to the limited number of compatible electrophiles (predominantly π-acceptors), this study introduces the first asymmetric α-alkylation of 3-aryl oxindoles using Csp3 electrophiles. The innovative protocol integrates diverse oxindoles and alkyl, allyl, and propargyl electrophiles, achieving high yields and enantioselectivities. Preliminary mechanistic explorations support a noncovalent catalytic mechanism, enhancing the tool kit for constructing complex chiral molecules with potential applications.

Mesoionic N-Heterocyclic Olefins as Initiators for the Lewis Pair Polymerization of Epoxides

Mesoionic N-heterocyclic olefins (mNHOs) have recently emerged as a novel class of highly nucleophilic and super-basic σ-donor compounds. Making use of these properties in synthetic polymer chemistry, it is shown that a combination of a specific mNHO and a Mg-based Lewis acid (magnesium bis(hexamethyldisilazide), Mg(HMDS)2) delivers poly(propylene oxide) in quantitative yields from the polymerization of the corresponding epoxide (0.1 mol% mNHO loading). The initiation mechanism involves monomer activation by the Lewis acid and direct ring-opening of the monomer by nucleophilic attack of the mNHO, forming a zwitterionic propagating species. Modulation of the mNHO properties is thereby a direct tool to impact initiation efficiency, revealing a sterically unencumbered triazole-derivative as particularly useful. The joint application of mNHOs together with borane-type Lewis acids is also outlined, resulting in high conversions and fast polymerization kinetics. Importantly, while molar mass distributions remain relatively broad, indicating faster propagation than initiation, the overall molar masses are significantly lower than found in the case of regular NHOs, underlining the increased nucleophilicity and ensuing improved initiation efficiency of mNHOs.

Towards structurally versatile mesoionic N-heterocyclic olefin ligands and their coordination to palladium, gold, and boron hydride

We have developed an efficient and versatile approach for the synthesis of a family of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands and investigated their coordination to palladium, gold, and boron hydride experimentally and computationally. We reacted mNHOs obtained through deprotonation of the corresponding methylated and ethylated 1,3,4-triaryl-1,2,3-triazolium salts with [Pd(allyl)Cl]2 to give the corresponding [Pd(η3-allyl)Cl(mNHO)] coordination complexes. 13C NMR data revealed the strong σ-donor character of the mNHO ligands, consistent with the calculated bond orders and atom-condensed charges. Furthermore, we also synthesized [AuCl(mNHO)] and a BH3-mNHO adduct by reacting the triazolium salts with AuCl(SMe2) and BH3·THF, respectively. The BH3-mNHO adduct was tested in the reduction of select aldehydes and ketones to alcohols.

Computational Design of an Electro-Organocatalyst for Conversion of CO2 into Formaldehyde

Density functional theory calculations employing a hybrid implicit/explicit solvation method were used to explore a new strategy for electrochemical conversion of CO2 using an electro-organocatalyst. A particular structural motif is identified that consists of an electron-rich vicinal enediamine (>N-C═C-N<) backbone, which is capable of activating CO2 by the formation of a C-C bond while subsequently facilitating the transfer of electrons from a chemically inert cathode to ultimately produce formaldehyde. Unlike transition metal-based electrocatalysts, the electro-organocatalyst is not constrained by scaling relations between the formation energies of activated CO2 and adsorbed CO, nor is it expected to be active for the competing hydrogen evolution reaction. The rate-limiting steps are found to occur during two proton-coupled electron transfer (PCET) sequences and are associated with the transfer of a proton from a proton transfer mediator to a carbon atom on the electro-organocatalyst. The difficulty of this step in the second PCET sequence necessitates an electrode potential of -0.85 V vs RHE to achieve the maximum turnover frequency. In addition, it is postulated that the electro-organocatalyst should also be capable of forming long-chain aldehydes by successively carrying out reductive aldol condensation to grow the alkyl chain one carbon at a time.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs)

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

Pushing the Upper Limit of Nucleophilicity Scales by Mesoionic N-Heterocyclic Olefins

A series of mesoionic, 1,2,3-triazole-derived N-heterocyclic olefins (mNHOs), which have an extraordinarily electron-rich exocyclic CC-double bond, was synthesized and spectroscopically characterized, in selected cases by X-ray crystallography. The kinetics of their reactions with arylidene malonates, ArCH=C(CO2 Et)2 , which gave zwitterionic adducts, were investigated photometrically in THF at 20 °C. The resulting second-order rate constants k2 (20 °C) correlate linearly with the reported electrophilicity parameters E of the arylidene malonates (reference electrophiles), thus providing the nucleophile-specific N and sN parameters of the mNHOs according to the correlation lg k2 (20 °C)=sN (N+E). With 21 Collapse

Key Words

• DFT Calculations
• Kinetics
• Mesoionic Compounds
• Methyl Cation Affinities

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MESH Headings Collapse

Grants

• 390677874-RESOLV Deutsche Forschungsgemeinschaft
• HA 8832/1-1 Deutsche Forschungsgemeinschaft
• 101077332 HORIZON EUROPE European Research Council
• J-4592 Österreichische Forschungsförderungsgesellschaft

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Affiliation(s)

Andreas Eitzinger Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377, München, Germany
Justus Reitz Fakultät für Chemie und Chemische Biologie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany
Patrick W Antoni Fakultät für Chemie und Chemische Biologie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany
Herbert Mayr Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377, München, Germany
Armin R Ofial Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377, München, Germany
Max M Hansmann Fakultät für Chemie und Chemische Biologie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany

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Collaborators Collapse

Synthetic Strategies of N-Heterocyclic Olefin (NHOs) and Their Recent Application of Organocatalytic Reactions and Beyond

N-heterocyclic olefin (NHO) derivatives have an electron-rich as well as highly polarized carabon-carbon (C=C) double bond because of the electron-donating nature of nitrogen and sulphur atoms. While NHOs have been developing as novel organocatalysts and ligands for transition-metal complexes in various organic compound syntheses, different research groups are currently interested in preparing imidazole and triazolium-based chiral NHO catalysts. Some of them have been used for enantioselective organic transformations, but were still elusive. N-heterocyclic olefins, the alkylidene derivatives of N-heterocyclic carbenes (NHC), have shown promising results as effective promoters for numerous organic syntheses such as asymmetric catalysis, hydroborylation, hydrosilylation, reduction, CO2 sequestration, alkylation, cycloaddition, polymerization and the ring-opening reaction of aziridine and epoxides, esterification, C-F bond functionalization, amine coupling, trifluoromethyl thiolation, amination etc. NHOs catalysts with suitable structures can serve as a novel class of Lewis/Bronsted bases with strong basicity and high nucleophilicity properties.These facts strongly suggest their enormous chemical potential as sustainable catalysts for a wide variety of reactions in synthetic chemistry. The synthesis of NHOs and their properties are briefly reviewed in this article, along with a summary of the imidazole and triazole core of NHOs' most recent catalytic uses.

First Synthesis of DBU-Conjugated Cationic Carbohydrate Derivatives and Investigation of Their Antibacterial and Antifungal Activity

The emergence of drug-resistant bacteria and fungi represents a serious health problem worldwide. It has long been known that cationic compounds can inhibit the growth of bacteria and fungi by disrupting the cell membrane. The advantage of using such cationic compounds is that the microorganisms would not become resistant to cationic agents, since this type of adaptation would mean significantly altering the structure of their cell walls. We designed novel, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene)-derived amidinium salts of carbohydrates, which may be suitable for disturbing the cell walls of bacteria and fungi due to their quaternary ammonium moiety. A series of saccharide-DBU conjugates were prepared from 6-iodo derivatives of d-glucose, d-mannose, d-altrose and d-allose by nucleophilic substitution reactions. We optimized the synthesis of a d-glucose derivative, and studied the protecting group free synthesis of the glucose-DBU conjugates. The effect of the obtained quaternary amidinium salts against Escherichia coli and Staphylococcus aureus bacterial strains and Candida albicans yeast was investigated, and the impact of the used protecting groups and the sugar configuration on the antimicrobial activity was analyzed. Some of the novel sugar quaternary ammonium compounds with lipophilic aromatic groups (benzyl and 2-napthylmethyl) showed particularly good antifungal and antibacterial activity.

Synthesis of the Three Most Expensive l-Hexose Thioglycosides from d-Glucose

The biologically important l-hexoses, which are less widespread than d-hexoses, cannot be obtained from natural sources or can only be extracted very costly. Due to the complexity of their synthesis, their commercially available derivatives (which are sold mostly in free form) are also very expensive, which is further exacerbated by the current rapid rise in prices. In the present work, starting from the cheapest d-hexose, d-glucose, using inexpensive and readily available chemicals, a reaction pathway was developed in which the three most expensive l-hexoses (l-idose, l-altrose, and l-talose) were successfully prepared in orthogonally protected thioglycoside form, ready for glycosylation. The l-ido and l-talo derivatives were synthesized by C-5 epimerization of the corresponding 5,6-unsaturated thioglycosides. From the l-ido derivatives, the orthogonally protected thioglycosides of l-altrose were then prepared by C-4 epimerization. Different approaches to the preparation of the key intermediates, 5,6-unsaturated thioglycoside derivatives, were systematically investigated in the presence of various protecting groups (ether and ester) and using commercially available reagents.

Recent advances of mesoionic N-heterocyclic olefins

This Perspective article highlights the recent development of mesoionic N-heterocyclic olefins (mNHOs), where the exo-cyclic olefinic carbon is not bonded to strongly electron-withdrawing groups. The unquenched basicity and nucleophilicity of the exo-cyclic olefinic carbon make mNHOs strong σ-donors and enable unique reactivity patterns.

Synthesis of Four Orthogonally Protected Rare l-Hexose Thioglycosides from d-Mannose by C-5 and C-4 Epimerization

l-Hexoses are important components of biologically relevant compounds and precursors of some therapeuticals. However, they typically cannot be obtained from natural sources and due to the complexity of their synthesis, their commercially available derivatives are also very expensive. Starting from one of the cheapest d-hexoses, d-mannose, using inexpensive and readily available chemicals, we developed a reaction pathway to obtain two orthogonally protected l-hexose thioglycoside derivatives, l-gulose and l-galactose, through the corresponding 5,6-unsaturated thioglycosides by C-5 epimerization. From these derivatives, the orthogonally protected thioglycosides of further two l-hexoses (l-allose and l-glucose) were synthesized by C-4 epimerization. The preparation of the key intermediates, the 5,6-unsaturated derivatives, was systematically studied using various protecting groups. By the method developed, we are able to produce highly functionalized l-gulose derivatives in 9 steps (total yields: 21-23%) and l-galactose derivatives in 12 steps (total yields: 6-8%) starting from d-mannose.

Stable Singlet Carbenes as Organic Superbases

A simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol-4-ylidenes, a type of mesoionic carbene, also named cyclic-bentallenes (CBA). They are more basic (pK_aH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P₄ (^t Bu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

Nucleophilic Activation of Sulfur Hexafluoride by N-Heterocyclic Carbenes and N-Heterocyclic Olefins: A Computational Study

Sulfur hexafluoride (SF₆ ) is considered as a potent greenhouse gas, whose effective degradation is challenging. Here we report a computational study on the nucleophilic activation of sulfur hexafluoride by N-heterocyclic carbenes and N-heterocyclic olefins. The result shows that the activation of SF₆ is both thermodynamically and kinetically favorable at mild condition using NHOs with fluoro-substituted azolium and sulfur pentafluoride anion being formed. The Gibbs free energy barrier during the activation of SF₆ has a linear relationship with the energy of HOMO of substrates, which could be a guideline for applying those compounds that feature higher energy in HOMO to activate SF₆ in high efficiency.

A comparison of zwitterionic and anionic mechanisms in the dual-catalytic polymerization of lactide

Two different polymerization mechanisms for lactide are selectivity addressed to illuminate the respective role of organobase and Lewis acid component.

Structure and activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems: application in ring-opening polymerization of lactones

Structure–activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems were performed and applied to ROP of lactones.