Carbenoid Carbon Research Articles

Unveiling the mechanism, regiochemistry and substituent effects of the [3 + 2] cycloaddition reactions of C, N-diaryl nitrile imine to ethylene derivatives from the molecular electron density theory perspective

The [3 + 2] cycloaddition (32CA) reactions of C-(4-methoxyphenyl)-N-phenyl nitrile imine (NI) with a series of ethylene derivatives of increased electrophilic character have been studied within the framework of Molecular Electron Density Theory (MEDT). These 32CA reactions are kinetically controlled with activation Gibbs free energies between 14.8 and 24.0 kcal mol−1, and with the preferred regioselectivity involving the nucleophilic attack of the carbenoid carbon of NI on the non-substituted carbon of the electrophilic ethylenes in agreement with the experimental outcomes. The presence of electron-withdrawing substituents in the ethylene makes the 32CA reaction of forward electron density flux (FEDF) more quickly, relative to that with nucleophilic propene, showing the highest activation parameters and non-polar character. Electron localization function (ELF) and atom-in-molecules (AIM) topological analysis of the electron density at the transition state structures characterize the non-concerted nature of these one-step cb-type 32CA reactions.

A Combination of Computational and Experimental Studies to Correlate Electronic Structure and Reactivity of Donor–Acceptor Singlet Carbenes

AbstractMost of the reactivities of donor–acceptor (D–A) singlet carbenes are similar to metal carbenoids. However, the lone pair at the carbenoid carbon, coordinated with metal, is free in D–A carbene thereby making it nucleophilic as well. Herein, DFT-optimized structural features of D–A carbene has been investigated and is compared with rhodium carbenoid. It was observed that, when a D–A carbene reacts with cyclic-1,3-diones in different ethereal solvents, it is the lone pair at the sp2 orbital of the carbene that abstracts the proton from the enol form (of the cyclic-1,3-diones) to form a benzylic carbocation and an enolate. Subsequently, the carbocation undergoes nucleophilic attack by O of the ether solvents and then by the enolate to afford the desired ether-linked products. Accordingly, herein the reaction in THF, which otherwise had failed to work as a substrate in reported amino etherification reactions, worked well. DFT-calculated orbital energy levels and reaction profile support this reverse reactivity of singlet carbenes. Furthermore, HOMO–LUMO calculations indicated that electron-rich arenes in D–A carbene stabilizes the LUMO and destabilizes the HOMO which increases yield. Additionally, a library of 37 enol ether and 39 ether-linked compounds of potential medicinal relevance have been synthesized with good to excellent yields using numerous cyclic-1,3-diones.

Synthesis, Characterization, and Trapping of a Cyclic Diborylcarbene, an Electrophilic Carbene.

A carbene bearing two geminal boryl substituents, called diborylcarbene (DBC), has been predicted to be highly Lewis acidic in sharp contrast to the well-studied persistent carbenes stabilized by π-donating substituents. Studies on DBC have been limited to either the base-trapping or theoretical calculations. Herein, we developed chemical equivalents for DBC, namely, K/X-diborylcarbenoids 2X (X = F or Cl). Treatment of 2F with Al(C6F5)3 yielded [AlF(C6F5)3]--stabilized DBC 1-FAl, which showed a significant low-field shift of the carbenoid carbon from 169 ppm (doublet, coupling with 19F) to 242 ppm (singlet). The loss of halogen was also detected through electrospray ionization time-of-flight mass spectrometry analysis of 2X only in the presence of Al(C6F5)3. Generated DBC 1 from 1-FAl or 2Cl was successfully trapped with excess amounts of trialkylphosphines (PR3, R = Me or Et), which afforded the corresponding DBC-PR3 adducts. In addition, the Lewis acidity of DBC 1 was evaluated both experimentally and theoretically to reveal that 1 is one of the most Lewis acidic species among neutral molecules.

Understanding the Participation of Fluorinated Azomethine Ylides in Carbenoid-Type [3 + 2] Cycloaddition Reactions with Ynal Systems: A Molecular Electron Density Theory Study.

The carbenoid-type (cb-type) 32CA reaction of 1,1-difluoroated azomethine ylide (DFAY) with phenylpropynal has been studied using the molecular electron density theory (MEDT). Electron localization function (ELF) characterizes DFAY as a carbenoid species participating in cb-type 32CA reactions. The supernucleophilic character of DFAY and the strong electrophilic character of the ynal cause this polar 32CA reaction to have an unappreciable barrier; the reaction, which is highly exothermic, presents total chemo- and regioselectivity. ELF topological analysis of the bonding changes along the reaction establishes its non-concerted two-stage one-step mechanism, in which the nucleophilic attack of the carbenoid carbon of DFAY on the electrophilic carbonyl carbon of the ynal characterizes the cb-type reactivity of this three-atom component (TAC). The presence of two fluorines at DFAY modifies the pseudodiradical structure and reactivity of the simplest azomethine ylide to that of a carbenoid TAC participating in cb-type 32CA reactions toward electrophilic ethylenes.

Regio- and Enantioselective Intramolecular Amide Carbene Insertion into Primary C-H Bonds Using Ru(II)-Pheox Catalyst.

We have established a method for the highly regio- and enantioselective functionalization of tert-butyl groups via intramolecular amide carbene insertion into C-H bonds, yielding γ-lactams with 91% ee in up to 99% yield. This reaction uses a ruthenium(II) phenyl oxazoline (Ru(II)-Pheox) complex. The catalytic intramolecular carbene transfer reaction to the primary C-H bond proceeds rapidly and selectively compared to that with secondary C-H, benzylic secondary C-H, tert-C-H, or sp2C-H bonds in the presence of 1 mol % Ru(II)-Pheox catalyst. This is the first example of a catalytic carbenoid insertion into an unactivated tert-butyl group with enantiocontrol at the carbenoid carbon.

Transition Metal‐Catalysed Intramolecular Carbenoid C−H Insertion for Pyrrolidine Formation by Decomposition of α‐Diazoesters

AbstractThe use of Pd‐, Rh(II)‐ and Ru(II)‐based catalysts has been explored in the transition metal‐catalysed intramolecular carbenoid C−H insertion of α‐diazoesters leading to pyrrolidines. Although the outcome of the reaction was highly substrate‐dependent, in general, it was possible to control the chemoselectivity of the process towards pyrrolidines by adequate catalyst selection. The Pd(0)‐catalysts were as efficient as [Rh(Ph3CCO2)2]2 in promoting the C(sp3)−H insertion of ortho‐substituted anilines. In contrast, for anilines bearing meta‐ and para‐substituents, the Rh(II)‐catalyst provided the best chemoselectivities and reaction yields. On the other hand, [Ru(p‐cymene)Cl2]2 was the most efficient catalyst for the insertion reaction of the N‐benzyl‐N‐phenyl and N,N‐dibenzyl α‐diazoesters, while the C(sp3)−H insertion of the N‐benzylsulfonamide substrate was only promoted by [Rh(Ph3CCO2)2]2. According to density functional theory (DFT) calculations, the mechanism involved in the Pd(0)‐ and Ru(II)‐catalysed C(sp3)−H insertions differs considerably from that typically proposed for the Rh(II)‐catalysed transformation. Whereas the Pd(0)‐catalysed reaction involves a Pd‐mediated 1,5‐H migration from the C(sp3)−H bond to the carbenoid carbon atom leading to the formal oxidation of the transition metal, a Ru(II)‐promoted Mannich type reaction involving a zwitterionic intermediate seems to be operative in the Ru(II)‐catalysed transformation.magnified image

ChemInform Abstract: A Reaction of Triazoles with Thioesters to Produce β‐Sulfanyl Enamides by Insertion of an Enamine Moiety into the Sulfur—Carbonyl Bond.

N-Sulfonyl-1,2,3-triazoles react with thioesters in the presence of a rhodium(II) catalyst to produce β-sulfanyl enamides in a stereoselective manner. The reaction proceeds through generation of an α-imino rhodium carbene complex, nucleophilic addition of the sulfur atom of a thioester onto the carbenoid carbon atom, and subsequent intramolecular migration of the acyl group from the sulfur atom to the imino nitrogen atom. The method is successfully applied to a ring-expansion reaction of thiolactones, thus leading to the formation of sulfur-containing lactams.

A Reaction of Triazoles with Thioesters to Produce β-Sulfanyl Enamides by Insertion of an Enamine Moiety into the Sulfur-Carbonyl Bond.

N-Sulfonyl-1,2,3-triazoles react with thioesters in the presence of a rhodium(II) catalyst to produce β-sulfanyl enamides in a stereoselective manner. The reaction proceeds through generation of an α-imino rhodium carbene complex, nucleophilic addition of the sulfur atom of a thioester onto the carbenoid carbon atom, and subsequent intramolecular migration of the acyl group from the sulfur atom to the imino nitrogen atom. The method is successfully applied to a ring-expansion reaction of thiolactones, thus leading to the formation of sulfur-containing lactams.

A Reaction of Triazoles with Thioesters to Produce β‐Sulfanyl Enamides by Insertion of an Enamine Moiety into the Sulfur–Carbonyl Bond

AbstractN‐Sulfonyl‐1,2,3‐triazoles react with thioesters in the presence of a rhodium(II) catalyst to produce β‐sulfanyl enamides in a stereoselective manner. The reaction proceeds through generation of an α‐imino rhodium carbene complex, nucleophilic addition of the sulfur atom of a thioester onto the carbenoid carbon atom, and subsequent intramolecular migration of the acyl group from the sulfur atom to the imino nitrogen atom. The method is successfully applied to a ring‐expansion reaction of thiolactones, thus leading to the formation of sulfur‐containing lactams.

Mechanism of magnesium carbenoid 1,3-C-H insertion: a DFT study

The 1,3-C–H insertion of magnesium carbenoid and related species was investigated via density functional theory (DFT) calculations. The 1,3-C–H insertion occurred according to an SN2-like mechanism wherein the nucleophilic C–H bond attacked the electrophilic carbenoid carbon atom. The activation energies for the 1,3-C–H insertion of (1-chloropropyl)magnesium chloride, (1-methoxypropyl)magnesium chloride, and [1-(methylthio)propyl]magnesium chloride were 20.0, 33.8, and 47.1 kcal/mol, respectively. Copyright © 2015 John Wiley & Sons, Ltd.

Mechanisms and stereoselectivities of the Rh(I)-catalyzed carbenoid carbon insertion reaction of benzocyclobutenol with diazoester.

In this study, a density functional theory (DFT) study has been carried out to investigate the mechanisms of Rh(I)-catalyzed carbenoid carbon insertion into a C-C bond reaction between benzocyclobutenol (R1) and diazoester (R2). The calculated results indicate that the reaction proceeds through five stages: deprotonation of R1, cleavage of the C-C bond, carbenoid carbon insertion, intramolecular aldol reaction, and protonation of the alkoxyl-Rh(I) intermediate. We have suggested and studied two possible pathways according to different coordination patterns (including ketone-type and enol-type coordination forms) in the fourth stage and found that the enol-type pathway is favorable, making the coordination mode of the Rh(I) center in the oxa-π-allyl Rh(I) intermediate clear in this reaction system. Moreover, four possible protonation channels have been calculated in the fifth stage, and the computational results show that the H2O-assisted proton transfer channel is the most favorable. The first step of the third stage is rate-determining, and the first steps in stages 3 and 4 play important roles in determining the stereoselectivities. Moreover, the analyses of distortion/interaction, natural bond orbital (NBO), and molecular orbital (MO) have been performed to better understand this title reaction. Furthermore, the pathway corresponding to the RR configurational product is the most favorable path, which is consistent with the experimental result. This work should be helpful for understanding the detailed reaction mechanism and the origin of stereoselectivities of the title reaction and thus could provide valuable insights into rational design of more efficient catalysts for this type of reactions.

ChemInform Abstract: Enantioselective Insertion of a Carbenoid Carbon into a C—C Bond to Expand Cyclobutanols to Cyclopentanols.

When a carbenoid species generated from a tosylhydrazone is reacted with a cyclobutanol in the presence of a chiral rhodium catalyst, a C–C single bond of the cyclobutanol is cleaved, and the carbenoid carbon is inserted therein to furnish a ring-expanded cyclopentanol in an enantioselective manner.

Synthesis and the antimicrobial activity of metalcarbene compounds of the triazole series

Мetalcarbene compounds of a series of [1,2,4] and [1,2,3]triazoles have been synthesized to study their antimicrobial activity. Monocarbene complexes of palladium (9a) and copper(I) (9b,12,15), biscarbene complexes of palladium (4a) and copper(I) (4b,14), a carbenoid complex of cobalt (5) have been synthesized by the direct interaction of stable carbenes with transition metal salts or by analogous in situ reactions. The intermediate sterically hindered carbenes – [1,2,4]triazol-5-ylidenes 3a-c have been isolated in the individual state. Being intermediate in the synthesis of mesoionic complexes 9a,b 1-adamantyl-4-phenyl-[1,2,3]triazole has been synthesized by [3+2]-cycloaddition of 1-adamantylazide to phenylacetylene in the presence of the biscarbene complex of copper(I) iodide 14. Characteristic signals of the carbenoid carbon atoms are detected in the 13С NMR spectra of complexes in the range of 165-203 ppm. A high antimicrobial activity has been found for complexes 5.12; for compound 12 it corresponds to the minimal bacteriostatic concentration (MBsC) 15.6 mkG/mL, the minimal bactericidal concentration (MBcC) 31.2 mkG/mL (М. luteum), and MBsC 31.2 mkG/mL, MBcC 62.5 mkG/mL (S. aureus). The highest activity in the series studied is observed for the carbenoid complex of cobalt (5) on the test-culture of M. luteum (MBsC 15.6 mkG/mL and MBcC 31.2 mkG/mL), and on the test-culture of C. tenuis (the minimal fungistatic concentration is 7.8 mkG/mL and the minimal fungicidal concentration is 31.2 mkG/mL).

Synthesis and the antimicrobial activity of precarbene and metalcarbene compounds of the imidazole series

Precarbene and metalcarbene compounds of a series of imidazole have been synthesized to study their antimicrobial activity. Calix[4]arene imidazolium salts 3,4a,b have been obtained from the corresponding chloromethyl derivatives of calix[4]arenes and N-substituted imidazoles in dimethylformamide or tetrahydrofuran, and salt 5 – from p-xylylenediimidazoles and 1-bromoadamantane in o-dichlorobenzene. Monocarbene complexes of palladium 8a-c, copper(I) 8d and biscarbene complexes of nickel 9a and cobalt 9b have been synthesized by the direct interaction of stable carbenes with transition metal salts or by the analogous reactions in situ in tetrahydrofuran. The NMR spectra data of the compounds synthesized are given. The most characteristic signals of the carbenoid carbon atoms are detected in the 13С NMR spectra of complexes 8a-d, 9a in the range of 165-178 ppm. A high antimicrobial activity has been found for carbenoid salts 4a,b, 5 on the test-culture of M. Luteum. It corresponds to the minimal bacteriostatic concentration (MBsC) of 15.6 mkg/mL and the minimal bactericidal concentration (MBcC) of 62.5 mkg/mL for compound 2. The higher activity has been found for carbene complexes of nickel 9a and cobalt 9b on the test-culture of M. luteum (MBsC is 7.8 mkg/mL and MBcC is 15.6 mkg/mL), and the highest 9b on the test-cultures of M. luteum and C. tenuis (the minimal fungistatic concentration is 1.9 mkg/mL and the minimal fungicidal concentration is 3.9 mkg/mL).

Enantioselective insertion of a carbenoid carbon into a C-C bond to expand cyclobutanols to cyclopentanols.

When a carbenoid species generated from a tosylhydrazone is reacted with a cyclobutanol in the presence of a chiral rhodium catalyst, a C-C single bond of the cyclobutanol is cleaved, and the carbenoid carbon is inserted therein to furnish a ring-expanded cyclopentanol in an enantioselective manner.

Synthesis and stability of Li/Cl carbenoids based on bis(iminophosphoryl)methanes.

The synthesis and stability of two bis(iminophosphoryl) substituted lithium chloride carbenoids with different N-substituents (tolyl and adamantyl) were examined. Their preparation was accomplished via the mild oxidation of the corresponding dilithio bis(iminophosphoryl) methandiides, BIPM(Tol) and BIPM(Ad), which are easily accessible by direct double deprotonation. In the case of the adamantyl substituted system the alternative preparation method via lithiation of the chlorinated precursor was found to be more selective. Here, the chlorinated precursor turned out to be highly CH acidic. Intramolecular deprotonation by the imino moiety results in the formation of the NH tautomer with a carbanionic centre as the most stable isomer. The prepared carbenoids are stable at room temperature in the solid state. Yet, in solution the stability was found to depend on the substituent bound at the imine nitrogen atom. As such, the tolyl substituted system was fairly stable in ether and hydrocarbon solvents, thus allowing crystallization. In contrast, the adamantyl derivative exhibits limited stability in ether solvents at room temperature. Decomposition resulted in LiCl elimination and transfer of the imino moiety to the carbenoid carbon atom.

Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes.

Quantum chemical characterization of magnesium carbenoids

Magnesium carbenoids, in which the magnesium and chlorine atoms are connected to the same carbon atom, are both nucleophilic and electrophilic. To elucidate their unconventional reactivity, density functional theory calculations were performed on magnesium carbenoids using the 6-311++G(d,p) basis set at the B3LYP level. Molecular orbital calculations were also performed on model compounds to determine their electronic structures. The C–Cl bond lengths of magnesium carbenoids were longer than those of the corresponding organic chlorides by 0.07–0.11 Å, and the bond angles around the carbenoid carbon atom deviated from those of standard sp3- and sp2-hybridized carbon atoms. Natural bond orbital analysis of the model compounds indicated that their unconventional reactivity and unusual structures were due to the increased s-character of the carbenoid carbon atom in the C–Mg bond and the increased p-character of the carbenoid carbon atom in the C–Cl bond.

Coordination and reactivity study of titanium phenoxo complexes containing a bulky bidentate imino-N-heterocyclic carbene ligand

The synthesis and structural characterisation of early transition metal complexes containing the aryl-substituted acyclic imino-N-heterocyclic carbene ligand are reported. X-ray crystallographic studies of titanium phenoxo complexes confirmed that the NHC ligand coordinates through the carbenoid carbon and the imine nitrogen atoms to form an octahedral complex. NMR spectroscopic analyses revealed restricted rotation about the aryl bonds. Attempts to prepare the corresponding titanium imido complex repeatedly led to decomposition products, presumably due to the enhanced reactivity of the imine group resulting in its cleavage from the C^Imine ligand scaffold. The catalytic activities of the phenoxo complexes towards ethylene polymerisation were assessed.

A new approach to the synthesis of 1-oxaspiro[4.n]alkanes and tetrahydrofurans by the 1,5-CH insertion reaction of magnesium carbenoids

1-Alkoxy-1-[2-chloro-2-(p-tolylsulfinyl)ethyl]cycloalkanes were prepared from various cyclic ketones in good overall yields. Treatment of these cycloalkanes bearing a sulfinyl group with i-PrMgCl resulted in the formation of 1-oxaspiro[4.n]alkanes in high to quantitative yields via the 1,5-CH insertion reaction of generated magnesium carbenoid intermediates. When this procedure was commenced with acyclic ketones, multi-substituted tetrahydrofurans were obtained in up to a 96% yield. This procedure provides a new and good way for the synthesis of 1-oxaspiro[4.n]alkanes and tetrahydrofurans with the formation of a carbon–carbon bond between a carbenoid carbon and a non-activated carbon in high yields. The oxygen atom in the magnesium carbenoid intermediates was proved to act very important roles in the 1,5-CH insertion reaction.