Carbene Species Research Articles

Cu/SaBox‐Catalyzed Photoinduced Coupling of Acylsilanes with Alkynes

AbstractThe transition metal‐catalyzed cross‐coupling reaction with Fischer metal carbene intermediates bearing an electron‐rich alkoxyl or siloxyl group remains a big challenge due to the lack of readily available corresponding carbene precursors. Herein, we report the coupling of alkynes with the Fischer‐type copper carbene species bearing aα‐siloxyl group, which could be in situ generated from acylsilanes catalytically under photoirradiation and redox‐neutral conditions. The side‐arm modified bisoxazoline (SaBox) ligands prove to be crucial for this coupling reaction, which provides the corresponding alkynyl alcohol in high yields with remarkable heterocycle tolerance and broad substrate scope.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters.

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Synthesis of 1,2,3,4‐Tetrahydroisoquinolines via Palladium‐Catalyzed Cyclization of N‐Tosylhydrazones with ortho‐Bromophenethyl Tosylamides

AbstractA Pd‐catalyzed synthesis of 1,2,3,4‐tetrahydroisoquinoline (THIQ) via cyclization of ortho‐bromophenethyl tosylamides with aryl N‐tosylhydrazones in the presence of base was developed. The reaction is believed to proceed via the oxidative addition of palladium with ortho‐bromophenethyl tosylamides, followed by carbene insertion and cyclization. The carbene species were generated in‐situ from the aryl N‐tosylhydrazones and KOtBu.magnified image

Photoinduced Carbene for Effective Photodynamic Therapy Against Hypoxic Cancer Cells

Photoinduced Carbene for Effective Photodynamic Therapy Against Hypoxic Cancer Cells

How to Stabilize Carbenes in Enzyme Active Sites without Metal Ions.

Carbenes are highly reactive compounds with unique value to synthetic chemistry. However, a small number of natural enzymes have been shown to utilize carbene chemistry, and artificial enzymes engineered with directed evolution required transition metal ions to stabilize the carbene intermediates. To facilitate the design of broader classes of enzymes that can take advantage of the rich carbene chemistry, it is thus important to better understand how to stabilize carbene species in enzyme active sites without metal ions. Motivated by our recent studies of the anaerobic ergothioneine biosynthesis enzyme EanB, we examine carbene-protein interaction with both cluster models and QM/MM simulations. The cluster calculations find that an N-heterocyclic carbene interacts strongly with polar and positively charged protein motifs. In particular, the interaction between a guanidinium group and carbene is as strong as ∼30 kcal/mol, making arginine a great choice for the preferential stabilization of carbenes. We also compare the WT EanB and its mutant in which the key tyrosine was replaced by a non-natural analogue (F2Tyr) using DFTB3/MM simulations. The calculations suggest that the carbene intermediate in the F2Tyr mutant is more stable than that in the WT enzyme by ∼3.5 kcal/mol, due to active site rearrangements that enable a nearby arginine to better stabilize the carbene in the mutant. Overall, the current work lays the foundation for the pursuit of enzyme designs that can take advantage of the unique chemistry offered by carbenes without the requirement of metal ions.

A Stereodivergent–Convergent Chiral Induction Mode in Atroposelective Access to Biaryls via Rhodium-Catalyzed C–H Bond Activation

Understanding the reaction mechanisms, particularly the chiral induction mode, is critical for the development of new asymmetric catalytic reactions. Rhodium(III)-catalyzed C–H activation en route to atroposelective [4 + 2] annulative coupling with α-diazo β-ketoesters has been realized, affording axially chiral phenanthrenes in good to excellent enantioselectivity. A combination of experimental and computational studies revealed a nontraditional stereodivergent–convergent chiral induction mode. The reaction proceeded with a rhodafluorene intermediate, followed by competitive, constructive, and stereodivergent migratory insertions of the two Rh–C(aryl) bonds into the carbene species to give β-ketoester intermediates. Then, the other Rh–C(aryl) bond migratorily inserts into the ketone carbonyl group. Following this stereodetermining carbonyl insertion, an ester-chelated rhodium(III) alkoxide species bearing two poorly controlled chiral centers and a well-controlled C(sp2)–C(sp3) chiral axis is generated. The final product is delivered via stereoconvergent elimination of a rhodium(III) species with retention of the well-controlled axial chirality and with loss of the central chirality.

Copper-Catalyzed Ligand-Controlled Selective Borocarbonylation of α-Substituted Styrenes Toward β-Boryl Aldehydes and Cyclopropyl Boronate Esters

Copper-Catalyzed Ligand-Controlled Selective Borocarbonylation of α-Substituted Styrenes Toward β-Boryl Aldehydes and Cyclopropyl Boronate Esters

Reductive Al-B σ-Bond Formation in Alumaboranes: Facile Scission of Polar Multiple Bonds.

We present facile access to an alumaborane species with electron precise Al-B σ-bond. The reductive rearrangement of 1-(AlI2 ), 8-(BMes2 ) naphthalene (Mes=2,4,6-Me3 C6 H2 ) affords the alumaborane species cyclo-(1,8-C10 H6 )-[1-Al(Mes)(OEt2 )-8-B(Mes)] with a covalent Al-B σ-bond. The Al-B σ-bond performs the reductive scission of multiple bonds: S=C(NiPrCMe)2 affords the naphthalene bridged motif B-S-Al(NHC), NHC=N-heterocyclic carbene, while O=CPh2 is deoxygenated to afford an B-O-Al bridged species with incorporation of the remaining ≡CPh2 fragment into the naphthalene scaffold. The reaction with isonitrile Xyl-N≡C (Xyl=2,6-Me2 C6 H4 ) proceeds via a proposed (amino boryl) carbene species; which adds a second equivalent of isonitrile to ultimately form the Al-N-B bridged species cyclo-(1,8-C10 H6 )-[1-Al(Mes)-N(Xyl)-8-B{C(Mes)=C-N-Xyl}] with complete scission of the C≡N triple bond. The latter reaction is supported with isolated intermediates and by DFT calculations.

Rhodium‐Catalyzed Transannulation of N‐Sulfonyl‐1,2,3‐triazoles with Carboxylic Esters

AbstractA rhodium‐catalyzed transannulation of N‐sulfonyl‐1,2,3‐triazoles with carboxylic esters followed by acid‐mediated aromatization has been developed, affording a variety of 2,5‐disubstituted oxazole derivatives in good to high yields under mild reaction conditions. The present transformation is proposed to involve the intermolecular carbonyl ylide formation between in situ generated α‐imino rhodium carbene species and carboxylic esters, which undergoes the nucleophilic attack of the neighboring α‐imino moiety.magnified image

Condition-Dependent Selective Synthesis of Indolo[1,2-c]quinazolines and Indolo[3,2-c]quinolines from 2-(1H-Indol-2-yl)anilines and Sulfoxonium Ylides.

In this paper, a selective synthesis of indolo[1,2-c]quinazolines and indolo[3,2-c]quinolines through the cascade reactions of 2-(1H-indol-2-yl)anilines with sulfoxonium ylides is presented. The formation of products involves the generation of a carbene species from sulfoxonium ylide and its N-H bond insertion reaction with 2-(1H-indol-2-yl)aniline followed by deoxygenative imine formation, intramolecular N- or C- nucleophilic addition and deoxygenative aromatization. This switchable synthesis was condition-dependent. In the presence of K2CO3 in CH3CN, the reaction mainly furnished indolo[1,2-c]quinazolines. In the presence of HOAc in dioxane, it selectively afforded indolo[3,2-c]quinolines. In addition, direct C-H/N-H functionalization of the products obtained provides a convenient and direct access to polycyclic heteroaromatic compounds. These novel protocols have advantages such as readily accessible substrates, easily tunable selectivity, good compatibility with diverse functional groups, and the use of air as a cost-free and sustainable oxidant.

Free N‐heterocyclic carbenes from Brønsted acidic ionic liquids: Direct detection by electrospray ionization mass spectrometry

RationaleThe occurrence of N‐heterocyclic carbenes in imidazolium‐based ionic liquids has long been discussed, but no spectroscopic evidence has been reported yet due to their transient nature. The insertion of an ionizable acid group into the cation scaffold of an ionic liquid which acts as a charge tag allows for the direct detection of free carbenes by mass spectrometry.MethodsThree different Brønsted acidic ionic liquids were synthesized: 1‐methyl‐3‐carboxymethylimidazolium chloride (MAICl), 1‐methyl‐3‐carboxymethylimidazolium acetate (MAIAc) and the corresponding 2‐(3‐methyl‐1H‐imidazol‐3‐ium‐1‐yl)acetate zwitterion (MAI − H). The speciation of these compounds was then analysed by electrospray ionization ion‐trap mass spectrometry in the negative ion mode.ResultsThe C2‐H deprotonation of the imidazolium cation leading to the formation of the corresponding carbene is highly affected by the basic properties of the counter‐anion. In the case of MAICl and MAI − H ionic liquids, no charged species corresponding to the free N‐heterocyclic carbene was detected. On the contrary, in the presence of a sufficiently basic anion, such as acetate of MAIAc ionic liquid, an intense signal related to the free carbenic species was observed without the addition of an external base.Conclusions In situ formation of free N‐heterocyclic carbenes from Brønsted acidic ionic liquids was demonstrated, highlighting the crucial role of anion basicity in promoting the C2‐H proton abstraction from imidazolium cations with a carboxylic side chain.

Monoanionic C^N^N Luminophores and Monodentate C-Donor Co-Ligands for Phosphorescent Pt(II) Complexes: A Case Study Involving Their Photophysics and Cytotoxicity

A family of Pt(II) complexes bearing monoanionic C^N^N ligands as luminophoric units as well as a set of monodentate ligands derived from allenylidene and carbene species were synthesized and characterized in terms of structure and photophysical properties. In addition, we present the extraordinary molecular structure of a phosphorescent complex carrying an allenylidene ligand. Depending on the co-ligand, an effect can be observed in the photoluminescence lifetimes and quantum yields as well as in the radiative and radiation less deactivation rate constants. Their correlation with the substitution pattern was analyzed by comparing the photoluminescence in fluid solution at room temperature and in frozen glassy matrices at 77 K. Moreover, in order to gain a deeper understanding of the electronic states responsible for the optical properties, density functional theory calculations were performed. Finally, the cytotoxicity of the complexes was evaluated in vitro, showing that the cationic complexes exhibit strong effects at low micromolar concentrations. The calculated half-maximum effective concentrations (EC50 values) were 4 times lower in comparison to the established antitumor agent oxaliplatin. In contrast, the neutral species are less toxic, rendering them as potential bioimaging agents.

Capturing Reactive Carbanions by Microdroplets.

Carbanions appear in many organic or biological reactions as fleeting intermediates, prohibiting direct observation or spectroscopic measurement. An aqueous environment is known to rapidly annihilate a carbanion species, reducing its lifetime to as short as picoseconds. We report that aqueous microdroplets can capture and stabilize reactive carbanion intermediates isolated from four classic organic reactions, aldol and Knoevenagel condensations, alkyne alkylation, and the Reimer-Tiemann reaction, enabling the detection of their carbanion intermediates by desorption electrospray ionization mass spectrometry. This is accomplished in real time of the reaction, allowing new insights into reaction mechanisms to be obtained. The efficacy of microdroplets in capturing such elusive species was examined by varying the solvent and the microdroplet negative charge density. We observed that microdroplets composed of water-methanol outperform other solvents, such as pure water, in capturing carbanions, which is in contrast to the earlier report that presented the highest performance of pure water microdroplets in capturing carbocations. We offer some mechanistic insights to explain the discriminatory behavior of these two oppositely charged species in microdroplets.

Dichotomy of platinum(II) and gold(III) carbene intermediates switching from N- to O-selectivity

Pt(II) and Au(III)-mediated intermolecular divergent annulations of benzofurazans and ynamides highlighted the N- to O-selectivity of tunable metal carbene intermediates. PtCl2 with a bulky phosphite ligand resulted in the specific synthesis of six-membered quinoxaline N-oxides and successfully suppressed the in-situ deoxygenation of N-oxides. On the other hand, an unique gold(III) catalyst (2,6-di-MeO-PyrAuCl3) led to the five-membered ring products, benzimidazoles. A broad scope of functional groups was well compatible, delivering better yields and selectivities in contrast to conventional gold(I) catalysts. The different behavior of presumed platinum(II) and gold(III) carbenes with respect to chemoselectivity was intensively examined by experiments and DFT calculations. A detailed mechanistic study, based on DFT calculations, revealed that the highly electrophilic carbocation-like gold(III) carbene triggers an oxophilic cyclization, followed by a cascade ring contraction and acyl migration. On the contrary, the Pt carbene species is less cationic, favoring the formation of the six-membered ring via N-attack.

A Reinterpretation of the Imidazolate Au(I) Cyclic Trinuclear Compounds Reactivity with Iodine and Methyl Iodide with the Perspective of the Inverted Ligand Field Theory.

Coinage metal cyclic trinuclear compounds (CTCs) are an emerging class of metal coordination compounds that are valuable for many fine optoelectronic applications, even though the reactivity dependence by the different bridging ligands remains somewhat unclear. In this work, to furnish some hints to unravel the effect of substituents on the chemistry of Au(I) CTCs made of a specific class of bridging ligand, we have considered two imidazolate Au(I) CTCs and the effect of different substituents on the pyrrolic N atoms relative to classic metal oxidations with I2 or by probing electrophilic additions. Experimental suggestions depict a thin borderline between the addition of MeI to the N-methyl or N-benzyl imidazolyl CTCs, which afford the oxidized CTC in the former and the ring opening of the CTC and the formation of carbene species in the latter. Moreover, the reactions with iodine yield to the oxidation of the metal centers for the former and just of a metal center in the latter, even in molar excess of iodine. The analysis of the bond distances in the X-ray crystal structures of the oxidized highlights that Au(III)-C and Au(III)-N bonds are longer than observed for Au(I)–C and Au(I)–N bonds, as formally not expected for Au(III) centers. Computational studies converge on the attribution of these discrepancies to an additional case of inverted ligand field (ILF), which solves the question with a new interpretation of the Au(I)–ligand bonding in the oxidized CTCs, which furnishes a new interpretation of the Au(I)-ligand bonding in the oxidized CTCs, opening a discussion about addition/oxidation reactions. Finally, the theoretical studies outputs depict energy profiles that are compatible with the experimental results obtained in the reaction of the two CTCs toward the addition of I2, MeI, and HCl.

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6 F5 )3 Towards Carbene Formation.

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N2 release. Recently, tris(pentafluorophenyl)borane [B(C6F5)3] has been shown to be an alternative transition metal‐free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α‐aryl α‐diazocarbonyl compounds using catalytic amounts of B(C6F5)3 is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.

Generation of organo-alkaline earth metal complexes from non-polar unsaturated molecules and their synthetic applications.

Organomagnesium compounds, represented by the Grignard reagents, are one of the most classical yet versatile carbanion species which have widely been utilized in synthetic chemistry. These reagents are typically prepared via oxidative addition of organic halides to magnesium metals, via halogen–magnesium exchange between halo(hetero)arenes and organomagnesium reagents or via deprotonative magnesiation of prefunctionalized (hetero)arenes. On the other hand, recent studies have demonstrated that the organo-alkaline earth metal complexes including those based on heavier alkaline earth metals such as calcium, strontium and barium could be generated from readily available non-polar unsaturated molecules such as alkenes, alkynes, 1,3-enynes and arenes through unique metallation processes. Nonetheless, the resulting organo-alkaline earth metal complexes could be further functionalized with a variety of electrophiles in various reaction modes. In particular, organocalcium, strontium and barium species have shown unprecedented reactivity in the downstream functionalization, which could not be observed in the reactivity of organomagnesium complexes. This perspective will focus on the newly emerging protocols for the generation of organo-alkaline earth metal complexes from non-polar unsaturated molecules and their applications in chemical synthesis and catalysis.

Forging Three Single Bonds on One Carbon via Metal Carbynes or Carbyne Equivalents

Metal carbyne (M≡C) complexes have been discovered for nearly half a century, however, they gained far less study in comparison with metal‐carbon single bond (M—C) and metal carbene (M=C) species in synthetic chemistry. Up to three sigma bonds are possibly forged on a single carbon through metal carbynes or carbyne equivalents, which holds great potentials in organic synthesis but also requires high efficiency of bond formation and precise control of reactivity and selectivity. Along this line, two systems based on stoichiometric transformations of manganese carbynes and ruthenium/rhodium‐catalyzed reactions of diazo hypervalent iodine reagents, carbyne equivalents, have emerged recently, which will be highlighted herein. Future potentials and challenges in this intriguing area are also briefly discussed.

Enantioselective formal carbene insertion into C–N bond of aminal as a concise track to chiral α-amino-β2,2-amino acids and synthetic applications

Chiral α-amino-β2,2-amino acid is an ubiquitous fragment that presents in natural products, pharmaceuticals, and bioactive molecules. However, the catalytic enantioselective synthesis of α-amino-β2,2-amino acids with structural complexity and diversity remains a significant challenge. Herein, a highly enantioselective formal C–N bond insertion of aminals with metal carbene species that was generated in situ from diazo compound has been developed by employing a cooperative catalysis of achiral dirhodium complex with chiral phosphonic acid, providing a practical and efficient protocol for expeditious access to optically pure α-amino-β2,2-amino acid derivatives bearing an α-tetra-substituted stereo center with broad substrate scope. The reaction was proposed to proceed through an enantioselective reassembly of enolate intermediate generated in situ from rhodium carbene with methylene iminium ion via chiral ionic pair interaction. Further synthetic applications of these generated poly-functionalized products furnish different types of key intermediates for the formal synthesis of many bioactive molecules. In addition, these amino acid derivatives could also serve as novel potential tail chains of resorcinol dibenzyl ether type PD-1/PD-L1 small molecule inhibitors.

Reductive Cleavage of Propargylic Ethers with Alkali Metal: Application to the Synthesis of Allenylboronates.

Treatment of propargylic ethers with sodium dispersion in the presence of lithium iodide results in the generation of the corresponding carbanion species via cleavage of the propargylic C-O bond. The anionic species react with trimethoxyborane to yield the allenylboronates including highly substituted ones that are difficult to synthesize.