Addition Of Carbanions Research Articles

5-Azido-4-dimethylamino-1-methyl-1,2,4-triazolium Hexafluoridophosphate and Derivatives

5-Azido-4-(dimethylamino)-1-methyl-1,2,4-triazolium hexafluoridophosphate was synthesized from the corresponding 5-bromo compound with NaN3. Reaction with bicyclo[2.2.1]hept-2-ene yielded a tricyclic aziridine, addition of an N-heterocyclic carbene resulted in a triazatrimethine cyanine, and reduction with triphenylphosphane gave the 5-amino derivative. The crystal structures of three nitrogen-rich salts were determined. Thermoanalysis of the cationic azide and triazene showed exothermal decomposition. The triazene exhibited negative solvatochromism in polar solvents involving the dipolarity π* and hydrogen-bond donor acidity α of the solvent.

Formation mechanism and conformational structure of 2,3,4-trimethyl-1,5-di(thiophen-2-yl)pentane-1,5-dione: quantum chemical study

The mechanism of formation of 1,5-diketone, a key intermediate in the stereoselective assembly of 2,3,4-trimethyl-7-methylidene-1,5-di(thiophen-2-yl)-6,8-dioxabicyclo[3.2.1]octane, and the conformational structures of the reagents and products were studied by quantum chemical calculations at the MP2/6-311++G**//B3LYP/6-31G* level of theory with solvent effects treated within the polarizable continuum model. The activation barrier for the addition of the 1-oxo-1-(thiophen-2-yl)propan-2-ide ion (1), which is generated from 1-(thiophen-2-yl)propan-1-one, to acetylene is 15.2 kcal mol–1. The reaction affords β,γ-unsaturated ketone. The subsequent addition of carbanion 1 to the C=C double bond of the resulting β,γ-unsaturated ketons yields 1,5-diketone and occurs with insignificant (less than 1 kcal mol–1) activation barrier.

Theoretical study of the mechanism for the sequential N-O and N-N bond cleavage within N2O adducts of N-heterocyclic carbenes by a vanadium(iii) complex.

A theoretical study into the reactions of the N2O adducts of N-heterocyclic carbenes (NHCs) and a V((III)) complex was carried out using DFT calculations. Unlike most transition metal reactions with N2O that simply release N2 following O-atom transfer onto the metal centre, this NHC-based system traps the entire N2O molecule and then cleaves both the N-O and N-N bond in two consecutive reactions. The NHC presence increases the reactivity of N2O by altering the distribution of electron density away from the O-atom towards the two N-atoms. This electronic redistribution enables V-N binding interactions to form a reactive N,O-donor intermediate species. Our results show that bond breaking with concomitant ligand migration occurs via a concerted process for both the N-O and N-N cleavage reactions.

Synthesis and Reactivity of Manganese(II) Complexes Containing N-Heterocyclic Carbene Ligands

A series of manganese(II) complexes containing aryl-substituted N-heterocyclic carbene (NHC) ligands have been synthesized and characterized. Chloride complexes of Mn(II) containing the NHC ligands 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-dimesitylimidazol-2-ylidene (IMes) were prepared in straightforward fashion by direct carbene addition to MnCl2(THF)1.6. These complexes exist as chloride-bridged dimers in solution of formula [Mn2Cl2(μ-Cl)2(NHC)2]. The monomeric complex [MnCl2(IMes)2] has also been prepared and structurally characterized, although NMR studies are consistent with facile dissociation of one of the IMes ligands in solution. [Mn2Cl2(μ-Cl)2(IPr)2] serves as a precursor to dimeric alkyl and aryl compounds of Mn(II) including [Mn2R2(μ-Cl)2(NHC)2] (R = Bn, o-tolyl, and Ph) and the bridging methyl complex [Mn2Me2(μ-Me)2(IPr)2]. Stoichiometric reactions of these hydrocarbyl species with bromocyclohexane demonstrate that they are not chemically competent in C–C coupling react...

A Comparison of the Stability and Reactivity of Diamido- and Diaminocarbene Copper Alkoxide and Hydride Complexes.

The mononuclear N-heterocyclic carbene (NHC) copper alkoxide complexes [(6-NHC)CuOtBu] (6-NHC = 6-MesDAC (1), 6-Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert-butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6-NHC)CuMes] (6-NHC = 6-MesDAC (3), 6-Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring-opening reactions, the latter affording [(6-MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6-MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6-MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6-MesDAC-H)Cu(P(p-tolyl)3)] (9) upon trapping by P(p-tolyl)3. Migratory insertion was also observed during attempts to prepare [(6-Mes)CuH], with [(6-Mes-H)Cu(6-Mes)] (10) isolated, following a reaction that was significantly slower than in the 6-MesDAC case. The longer lifetime of [(6-Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi-reduction of alkynes and hydrosilylation of ketones.

ChemInform Abstract: Synthesis of Enantiomerically Pure 2‐Heteroaromatic‐Substituted 1,2‐Amino Alcohols from Chiral tert‐Butanesulfinyl Aldimines.

AbstractNine R/S pairs of heteroaromatic‐substituted amino alcohols are prepared by nucleophilic addition of heteroaromatic carbanions to chiral N‐(tert‐butylsulfinyl)imines.

Formation of the First Derivatives of Endohedral Diterbium Fullerenes via Carbene Addition to a Large Carbon Cage

The first derivatives of diterbium fullerenes were successfully synthesized through the photochemical reaction of a large carbon cage-based diterbium fullerene Tb2C90 with 2-admantane-2,3-[3H]-diazirine. A monoadduct isomer, Tb2C90Ad (Ad = admantylidene), was isolated by multi-stage high-performance liquid chromatography with toluene as eluent. The formation of a series of derivatives of diterbium fullerenes as well as the success in the isolation of Tb2C90Ad was confirmed by high-performance liquid chromatography and matrix-assisted laser adsorption/ionization time-of-flight mass analyses. Vis-NIR absorption showed that the electronic property of Tb2C90 was largely pertained in the derivative, although a slight red-shift was observed for Tb2C90Ad. Cyclic voltammetric determination indicated that the redox potential of Tb2C90Ad was shifted cathodically in comparison with pristine Tb2C90 owing to the electron transfer from Ad to carbon cage.

Electrophilic Addition to Alkenes: The Relation between Reactivity and Enthalpy of Hydrogenation: Regioselectivity is Determined by the Stability of the Two Conceivable Products.

Although electrophilic addition to alkenes has been well studied, some secrets still remain. Halogenations, hydrohalogenations, halohydrin formations, hydrations, epoxidations, other oxidations, carbene additions, and ozonolyses are investigated to elucidate the relation of alkene reactivities with their enthalpies of hydrogenation (ΔHhyd ). For addition of electrophiles to unconjugated hydrocarbon alkenes, ln(k) is a linear function of ΔHhyd , where k is the rate constant. Linear correlation coefficients are about 0.98 or greater. None of the many previously proposed correlations of ln(k) with the properties of alkenes or with linear free-energy relationships match the generality and accuracy of the simple linear relationship found herein. A notable exception is acid-catalyzed hydration in water or in solvents stabilizing relatively stable carbocation intermediates (e.g., tertiary, benzylic, or allylic). (13) C NMR chemical shifts of the two alkene carbons also predict regioselectivity. These effects have not been noted previously and are operative in general, including addition to heteroatom-substituted alkenes.

Mono- and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls.

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)═C═CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,β-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.

Rate and Equilibrium Constants for the Addition of N-Heterocyclic Carbenes into Benzaldehydes: A Remarkable 2-Substituent Effect.

Rate and equilibrium constants for the reaction between N‐aryl triazolium N‐heterocyclic carbene (NHC) precatalysts and substituted benzaldehyde derivatives to form 3‐(hydroxybenzyl)azolium adducts under both catalytic and stoichiometric conditions have been measured. Kinetic analysis and reaction profile fitting of both the forward and reverse reactions, plus onwards reaction to the Breslow intermediate, demonstrate the remarkable effect of the benzaldehyde 2‐substituent in these reactions and provide insight into the chemoselectivity of cross‐benzoin reactions.

A Promiscuous De Novo Retro-Aldolase Catalyzes Asymmetric Michael Additions via Schiff Base Intermediates.

Recent advances in computational design have enabled the development of primitive enzymes for a range of mechanistically distinct reactions. Here we show that the rudimentary active sites of these catalysts can give rise to useful chemical promiscuity. Specifically, RA95.5-8, designed and evolved as a retro-aldolase, also promotes asymmetric Michael additions of carbanions to unsaturated ketones with high rates and selectivities. The reactions proceed by amine catalysis, as indicated by mutagenesis and X-ray data. The inherent flexibility and tunability of this catalyst should make it a versatile platform for further optimization and/or mechanistic diversification by directed evolution.

A Promiscuous De Novo Retro‐Aldolase Catalyzes Asymmetric Michael Additions via Schiff Base Intermediates

AbstractRecent advances in computational design have enabled the development of primitive enzymes for a range of mechanistically distinct reactions. Here we show that the rudimentary active sites of these catalysts can give rise to useful chemical promiscuity. Specifically, RA95.5‐8, designed and evolved as a retro‐aldolase, also promotes asymmetric Michael additions of carbanions to unsaturated ketones with high rates and selectivities. The reactions proceed by amine catalysis, as indicated by mutagenesis and X‐ray data. The inherent flexibility and tunability of this catalyst should make it a versatile platform for further optimization and/or mechanistic diversification by directed evolution.

Approach to Loschmidt's tetracyclic benzene isomer and related tetra‐ and tricyclic isomers by intramolecular carbene additions

Potential carbene precursors have been considered for three tetracyclic benzene isomers: carbene 13.. for tetracyclo[3.1.0.01,3.03,5]hexane, 11; carbene 19.. for tetracyclo[3.1.0.01,4.02,3]hexane, 10, considered by Loschmidt as a structure for benzene; and carbene 20.. for Ladenburg's prismane, 4. The bicyclic (CH)6 isomer, 20.., avoids intramolecular cycloaddition; it is converted instead to Dewar benzene, 2; the system 13.. – 11.. is a “foiled” carbene system; finally, we were unable to characterize the putative precursor, 19.., for Loschmidt's hydrocarbon; an attempt to generate it by adding an electron to the corresponding carbene cation, 19.+, resulted in conversion to anti‐10. The trajectory of this conversion indicates that “19..” lies at or below the inflection point of the trajectory connecting a putative transition state with anti‐10. The free energies of all six species lie in the range 120–165 kcal/mol, well above any species that may be involved in the topomerization of benzene. Copyright © 2014 John Wiley & Sons, Ltd.

Formation of the first derivatives of praseodymium-containing metallofullerenes via regioselective carbene addition to Pr@C2v (9)-C82

The first derivatives of praseodymium-containing metallofullerenes are formed through regioselective cycloaddition of adamantylidene carbene to Pr@C2v(9)-C82, and two isomers out of the 35 possible monoadducts are successfully isolated. Optical adsorption and electrical chemistry characterizations indicate that the electronic property of Pr@C2v(9)-C82 is largely pertained in these derivatives, since their π-electron systems are not altered markedly.

Catalytic Copper-Mediated Ring Opening and Functionalization of Benzoxazoles

A novel reaction involving benzoxazole, ethyl diazoacetate, and water has been discovered, with TpBr3Cu(NCMe) (TpBr3 = hydrotris(3,4,5-tribromopyrazolyl)borate) as the catalyst. The fused azoles are converted into highly functionalized substituted benzenes bearing aldehyde, amine carboxylate and hydroxyl groups. The protocol has been applied for a series of benzoxazoles with several diazo compounds. Experimental data and theoretical calculations have led to a mechanistic proposal that includes carbene addition, ylide formation, and water addition to the latter, all those steps being catalyzed by the copper center.

ChemInform Abstract: Diastereoselective Formation of Aziridines from Diazocarbonyl Compounds and N‐(O‐Pivaloylated D‐Galactosyl)benzylideneamines and Ring‐Opening Reactions with P‐Toluenethiol.

N-Galactosyl aziridines were synthesized via BF3·OEt2 promoted addition of carbenes generated from diazocarbonyl compounds with O-pivaloylated β-D-galactosylimines in good yields and high diastereoselectivity. The ring-opening reactions with p-toluenethiol of the aziridines provided enantiometrically pure β-S-substituted phenylalanine derivatives in a highly regioselective manner.

The role of functional moieties on carbon nanotube surfaces in solar energy conversion.

An easily dispersible multiwalled carbon nanotube (MWCNT) derivative is prepared, and provides a platform for the synthesis of the phenyl butyric acid methyl ester (PCBM) analog. The carbene addition reaction of MWCNTs makes derivatives that are less soluble in organic solvents; by exploiting this differential solubility, PCBM analogs can be separated from the unreacted functionalized MWCNTs. Our experimental evidences indicate that it is the unique properties of the butyric acid methyl ester moiety that makes the acceptor material perform better in organic photovoltaics (OPVs). Studying the combination of the butyric acid methyl ester moiety and the deagglomerated functionalized MWCNT structures provides us an insight into nanoscale charge transfer and transportation inside the donor-acceptor domain. It is demonstrated that a strong structure-property relationship exists for the functionalized MWCNTs, which enables us to correlate the functionality on the carbon nanostructures with performance in OPVs.

Catalytic Functionalization of Indoles by Copper‐Mediated Carbene Transfer

AbstractThe complex [TpBr3Cu(NCMe)] (TpBr3=hydrotris(3,4,5‐tribromo)pyrazolylborate) efficiently catalyzes the CH functionalization of indole derivatives at C3 by carbene transfer from different diazoesters in a high‐yield transformation involving low catalyst loadings and short reaction times. This system has shown that the previously proposed dichotomy of carbene addition (to the double bond) vs carbene insertion (to the CH bond) corresponds to two consecutive reaction steps: the cyclopropane intermediates, observed in the reaction mixtures, are the precursors of the final CH functionalization derivatives in a ring‐opening process involving acid catalysis. Those in situ generated cyclopropanes undergo nucleophilic ring opening with Me2CuLi to afford both C2 and C3 functionalized indoles.

English

DFT calculations have been performed on the interactions of carbon nanotubes and carbenes, in order to perform a comparative study of the sidewall functionalization of the two classes of nanotubes, i.e. armchair and zigzag. The systematic investigation of cycloaddition of both zigzag and armchair carbon nanotubes with :CX2 (where X = H, F, Cl, Br, I) has been undertaken. The models selected for zigzag and armchair CNT are (10,0) and (6,6), respectively, since they have comparative radii (~8 A). The bond lengths and binding energies of products are analysed in great detail. The band gaps of these non periodic structures are investigated and correlated with the electronic behavior of 'armchair' and 'zigzag' nanotubes. Our results also confirm the previous studies that armchair nanotubes are thermodynamically more stable than zigzag nanotubes. On comparison of armchair carbon nanotubes with zigzag nanotubes, it is found that trends towards carbenic reactivity are the same in both the cases. The dependence of addition of carbenes on the diameter of the carbon nanotubes is also investigated. The models used for nanotubes in this study range from (3,3) to (10,10). Their reactions with :CH2 have been studied with first principles DFT calculations. An attempt to rationalize the diameter effects is made on the basis of the structures of the nanotubes. It seems that diameter dependence of the reactivity is because of different pyramidalization and π-orbital misalignment angles in different carbon nanotubes. This leads to higher local strain for tubes of smaller diameter that can be partially relaxed by sidewall addition.

Cascade Olefin Isomerization/Intramolecular Diels–Alder Reaction Catalyzed by N‐Heterocyclic Carbenes

The addition of an N-heterocyclic carbene to the carbonyl group of an α,β,γ,δ-unsaturated enol ester affords a hemiacetal azolium intermediate that enables a cascade olefin isomerization/Diels-Alder reaction, for which mechanistic studies implicate Lewis base catalysis. Preliminary studies into the utility of the products have been undertaken with reductive and oxidative cleavage, giving materials for potential use in complex-target synthesis.