Attack Of Carbanion Research Articles

Cover Picture

This cover picture shows a novel type of 10C‐carbolong complexes constructed by a formal [7C+3C] method, from the reactions of a metallapentalyne with benzyl carbanions. The newly formed metallacyclopentadiene ring in the 10C‐carbolong complexes was generated through a nucleophilic attack of carbanions to the M≡C bond followed by C–H activation. This work provides a new convenient route to construct metallacyclopentadienes and 10C‐carbolong complexes. More details are discussed in the article by Xia et al. on page 235—242.image

Construction of a Metallacyclopentadiene Ring through the Attack of Carbanions to M≡C Bond Followed by C—H Activation†

Comprehensive SummaryMetallacyclopentadienes are important metallacycles and regarded as intermediates in many reactions, therefore, new methods to achieve them are anticipated. In this study, a formal [3+2] method, through the reactions of an osmapentalyne with benzyl carbanions, was developed. The reactions underwent a nucleophilic attack of carbanions to the Os≡C bond, followed by C—H activation to form the five‐membered osmacyclopentadiene ring. Most of the reactions were carried out at room temperature, the substituents on the aromatic rings of benzyl carbanions are diverse, and the resulting products contain an Os—H bond, representing a novel type of 10C‐carbolong complexes. This work provides a new convenient route to construct metallacyclopentadienes, which is expected to further promote the development of such a type of substances.

Kinetic studies of reactions of 4‐nitrobenzofuroxan with carbanions derived from benzyltriflones in methanol

AbstractThis paper reports the rate measurements for the reactions of carbanions derived from benzyltriflones, 2, with 4‐nitrobenzofuroxan, 4, in methanol, to give anionic σ‐adducts. 1H NMR studies in DMSO‐d6 indicate that the products formed by the reaction of 2 and 4 in the presence of triethylamine are consistent with the products formed by the elimination of trifluoromethylsulfinic acid from σ‐adducts, initially formed by a carbanion attack at the 5 position of 4. The low value of β, which is the slope of the linear plot of log k5 versus pKa, provides evidence for the high steric requirements of the benzyltriflone anions. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 546–554, 2012

1H NMR and kinetics studies of the reaction of 4-methyl, 4-bromo and 3-trifluoromethyl benzyltriflones with aromatic nitro-compounds

Rate measurements are reported for the reactions in methanol of carbanions derived from benzyltriflones, 2a–c, with 4-nitrobenzofurazan derivatives, 4a and 4b, to give anionic σ-adducts. 1H NMR studies in DMSO-d6 of the reaction of benzyltriflones, 2, and 4-nitrobenzofurazan, 4a, in the presence of triethylamine are consistent with products formed by the elimination of trifluoromethyl sulfinic acid from σ-adducts initially formed by carbanion attack at the 5-position of 4a. Evidence for the high steric requirements of the benzyltriflone anions come from the low value of β; the slope of the linear plot of values of logk5 versus pKa.

Sulfonyl transfer mechanism in C–S coupling of phenylmagnesium bromide with phenyl arenesulfonates

The kinetics of C–S coupling of phenylmagnesium bromide with phenyl arenesulfonates has been studied in THF:toluene (7:10) at 90 °C. Kinetic data and Hammett relationship are consistent with an asynchronous S Na mechanism in which rate determining thiophilic attack of carbanion takes place much ahead of phenoxy group departure.

Mechanism and characterization of alkylene‐ and p‐ethylenebenzylene‐spaced polycarbosilanes ceramic precursor synthesized by electropolymerization

AbstractAn electropolymerization of haloalkylhalosilanes (ClRSiCH3Cl2) that possess two types of electroactive sites, that is, the CCl and SiCl bond is described. The one‐pot synthesis method is shown to yield branched polycarbosilanes having a regular carbon block‐spaced silicon backbone structure. A series of branched polycarbosilanes, [RSiCH3]n with R being CH2, C2H4, C3H6, and CH2C6H4C2H4, have been successfully electropolymerized with Mn up to 42,600 Dalton. Experimental and simulation cyclic voltammetry of these monomers and the computational examination of their LUMOs are applied to study the electropolymerization mechanism. The results suggest that polymerization proceeds by iterating steps involving (1) electroreduction of a CCl bond to a carbanion, which is catalyzed by silylanion radical [Cl SiCl(CH3)RCl] and/or Ni(0)/TDA‐1; and (2) nucleophilic attack of carbanions to SiCl bonds of a second monomer or oligomer to extend the polymer chain. The investigation reveals that the R spacer has a considerable impact on the polymerizability of the corresponding monomer. Such interfacial polymerization resembles a template polymerization, leading to unique microstructures that were preserved even after converted to silicon carbide ceramics at high temperatures. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7677–7689, 2008

Kinetics and mechanism of the C-S coupling reactions of aryl Grignard reagents with aryl arenesulfonates

Abstract The kinetics of the C-S coupling of arylmagnesium bromides with phenyl tosylate has been studied in THF: toluene at 90°C. The reaction is first order in Grignard reagent and first order in phenyl tosylate. Kinetic data, Hammett relationship and activation parameters are consistent with a nucleophilic addition mechanism involving rate determining attack of carbanion to sulfonyl group followed by a fast phenoxide group leaving.

Carbanion reactivity; studies of σ‐adduct formation from benzyltriflone anions and 4‐nitrobenzofurazan derivatives

Abstract1H NMR studies in DMSO‐d6 of the reaction of benzyltriflones, 2, with 4,6‐dinitrobenzofuroxan, 4, indicate the formation, with or without the presence of added base, of anionic σ‐adducts. Spectra obtained from solutions of 2 and 4‐nitrobenzofurazan, 6a, in the presence of triethylamine are consistent with products formed by the elimination of trifluoromethylsulfinic acid from σ‐adducts initially formed by carbanion attack at the 5‐position of 6a. Kinetic studies of the latter reaction in methanol allow the determination of rate constants for nucleophilic attack by the carbanions at the 5‐position. The low value of β for these reactions together with the failure to observe reaction with 1,3,5‐trinitrobenzene suggest that the benzyltriflone anions have unusually large steric requirements. Copyright © 2007 John Wiley & Sons, Ltd.

On the mechanism of the N,N‐dimethyl amination of Grignard reagents: a kinetic study

AbstractA direct kinetic study is reported for the electrophilic amination of substituted phenylmagnesium bromides with N,N‐dimethyl O‐(mesitylenesulfonyl)hydroxylamine in THF. Rate data, Hammett relationship, and activation entropy are consistent with a SN2 displacement involving the attack of carbanions to sp3N in the amination reagent (AR). Copyright © 2007 John Wiley & Sons, Ltd.

Carbanion reactivity, kinetic and equilibrium studies of σ-adduct formation and elimination in the reactions of 4-nitrobenzofurazan derivatives with nitroalkane anions

1H NMR studies are reported of the reactions in [2H(6)]-DMSO of 4-nitrobenzofurazan, 2a, and its 7-chloro- and 7-methoxy-derivatives, 2b and 2c respectively, with anions derived from nitromethane, 3, nitroethane, 4, and 2-nitropropane, 5. The initial reactions result in sigma-adduct formation by carbanion attack at the 5-position of 2a-c and in the case of reaction of 2a with 5 the adduct at the 7-position is also observed. These reactions may be followed by base catalysed elimination of nitrous acid to yield anionic alkene derivatives. Kinetic and equilibrium measurements of these reactions were made spectrophotometrically in methanol. The carbon nucleophilicities of the carbanions decrease in the order 3> 4> 5, as also found in their reactions with benzhydrylium cations, and are much lower than the nucleophilicities of some cyano-substituted carbanions. Comparison with corresponding sigma-adduct forming reactions of 1,3,5-trinitrobenzene, TNB, show that here 2 and TNB have similar electrophilicity, although the value of the intrinsic rate coefficient k(o) = 0.05, for reaction of 2 is rather lower than that, k(o) = 0.20, for the TNB reactions. Literature data suggest that for reaction with a variety of nucleophiles 2 and TNB show similar electrophilicities. Measurements of the rates of elimination of nitrous acid from some 5-adducts in methanol catalysed by methoxide ions are reported. Values of rate constants may be influenced both by steric requirements at the reaction centre and by the electronic effects of the 7-substituent.

Dual thiophilic or carbophilic reactivity of N, N-dialkyl perfluorocarboxylic thioamides with organometallic reagents

N, N-dialkyl amides of perfluorothiocarboxylic acids react with organomagnesium and organolithium reagents via thiophilic or carbophilic attack of carbanion on the C S bond. The chemoselectivity depends on the nature of the organometallic species. Lithium reagents react at sulfur, with a subsequent β-elimination of fluoride, giving an N, S-ketene acetal. Simple organomagnesium reagents do not react, whereas allylmagnesium halide reacts at carbon, giving an adduct which can be trapped by methyl iodide and converted to the corresponding N, S-acetal. The latter can be transformed into a perfluoroalkyl dienamine via oxidation.

Ring Closure of Alkoxycarbonyl(tetracarbonyl)pyruvoyliron Complexes into Metallalactones Induced by Nucleophilic Attack of Carbanions

AbstractThe reaction of carbanions with the pyruvoyl‐substituted iron complex [(CO)4Fe(CO2CH3){C(O)C(O)CH3}] (1) affords the anionic trifunctionalized metallalactones [(CO)3Fe{C(O)C(CH3)(CRR′R″)OC4(O)(Fe–C4)}(CO2CH3)]– (3), whose formation results from the addition of the nucleophile to the β carbonyl of the pyruvoyl moiety, followed by attack of the oxygen of this β carbonyl on a terminal carbonyl ligand. These anionic lactones react, at low temperature, with HCl to give rise to the neutral lactones [(CO)4Fe{C(O)C(CH3)(CRR′R″)OC4(O)(Fe–C4)}] (2), which were previously obtained by addition of NuH nucleophiles to 1. Complex 3(3), whose lactonic ring formation has been performed using the diethyl malonate anion (R = R′ = CO2C2H5; R″ = H), and the dimethyl‐substituted neutral lactone 2(1) (R = R′ = R″ = H) have been characterized by X‐ray diffraction studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis of 1,3‐Difunctionalized Cyclopentenes and 1,3,5‐Trifunctionalized Cyclohexanes by Silicon‐Induced Domino Reactions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of 1,3‐Difunctionalized Cyclopentenes and 1,3,5‐Trifunctionalized Cyclohexanes by Silicon‐Induced Domino Reactions

AbstractA novel domino process based on a 1,4‐C⇄O shift of a silyl group (4 ⇄ 3) and a Michael‐induced ring‐closure reaction (3 ⇄ 2) is investigated. Specifically, attack of carbanions 5 on vinyloxiranes 6 usually occurs on the oxirane unit to give the desired silyl shift. When starting from vinyloxiranes 6a and 6b, however, the reaction stops at this stage to give silyl ethers 7. Sulfur (6c) or silicon activation (6d−f) of the C=C unit is required to yield cyclopentenes 1a−d. Analogously, carbanion 5a and allyloxiranes 15 give cyclization products 19−22, particularly if the ring‐closure step is supported by silicon substitution. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Preparation of Substituted Methyl o-Nitrophenyl Sulfides

The nucleophilic substitution of substituted o-nitrochlorobenzenes with substituted methanethiolates, catalysed with triethylamine or pyridine, has been used to prepare a series of appropriately substituted methyl-o-nitrophenylsulfides. The prepared compounds were identified by their 1H- and 13C-NMR spectra. The base catalysed ring closure of methyl 2-(methoxycarbonylmethylsulfanyl)-3,5-dinitrobenzenecarboxylate only results in an attack of carbanion on the ester group, not on a nitro group as with the other compounds prepared. The cyclisation product is methyl 3-hydroxy-5,7-dinitro-benzo[b]thiophene-2-carboxylate (11).

Nucleophilic Attack of Carbanions on Cationic Platinum(II) Nitrile Complexes Affording Stable η1-Imine Enol and η1-Enamine Coordinated Fragments

Cationic nitrile platinum(II) complexes of formula [PtMe(RCN)(bipy)]+ undergo nucleophilic addition of carbanions −CH(COX)(COY) (X, Y= Me, OMe), −CH(CO2Me)CN and −CH(CN)2 to afford neutral products. According to the nature of the nucleophile, the organic ligand fragment displays either an imino enol or an enamine isomeric structure. The Pt−N bond can be cleaved upon reaction with acids with formation of the corresponding β-enaminodione.

Facile Formation and Dissociation Behaviour of C–C Bond Resulted from the Nucleophilic Attack of Carbanions on a Carbonyl Carbon in [Pt(hfac)2

Abstract [Pt(hfac)2] (hfac: hexafluoroacetylacetonate) reacts with MeNH2 in CH2Cl2/MeOH to give an –NHMe adduct complex on one of the carbonyl carbons, (MeNH3)[Pt(hfac)(hfac–NHMe)] 1 which is a tetrahedral intermediate of a Schiff base complex, [Pt(CF3COCHC(Nme)CF3)2] 2. Complex 1 activates H2O, MeOH, MeNO2 or acetone in solution to form the corresponding conjugate base adducts. The C–C bond in –CH2NO2 adduct 6, easily cleaves and generates nitromethane in solution.

Synthesis of alkoxysilyl-terminated polyisoprenes by means of 'living' anionic polymerization, 2. Synthesis of trialkoxysilyl-terminated 1,4-polyisoprenes by reaction of polyisoprenyllithium with various functional trialkoxysilanes selected as end-capping reagents

In order to prepare trimethoxysilyl (or triethoxysilyl)-terminated 1,4-polyisoprene with good yields, the deactivation of polyisoprenyllithium (Mn = 800) was investigated by using various functional trialkoxysilanes (RO)3Si—G (G = 3-chloropropyl, methoxy or ethoxy) as silane end-capping agents. The studies were carried out in presence of THF by varying the molar ratio r = [1,4-polyisoprenyllithium chains]/[alkoxysilane]. The resulting products were characterized (qualitatively and quantitatively) by means of 1H and 29Si NMR spectroscopies (one or two dimensions), and also by supercritical fluid chromatography. The corresponding rate of substitution depends on r and on the nature of the starting alkoxysilane reagent. It was shown that alkoxy groups at the silicon center are easily substituted by the polyisoprenyl carbanions. Whatever the alkoxysilane used, two families of products are formed: (i) polymer derivatives resulting from nucleophilic substitution reactions of alkoxy groups from alkoxysilane reagent by ω-carbanionic polyisoprene chains, and (ii) substituted side-products issued from substitutions involving n-BuLi. The presence of these side products is explained by the fact that the butyllithium initially introduced to initiate the isoprene anionic polymerization is not totally consumed. The comparison of the results with those previously obtained in hexane with n-BuLi used as model molecule of ω-carbanionic polyisoprene shows some differences concerning the order of reactivity of the various reactive substituted species formed during the substitution reaction, as well as the selectivity of the substitution reaction in relation with the alkoxysilane reagent. These differences were attributed to the chelating properties of THF. On the other hand, results are the same concerning the attempts realized with 3-chloropropyltrimethoxysilane which have also shown that nucleophilic attack of polyisoprenyl carbanion occurs exclusively on the silicon atom and not on the carbon bearing the chlorine.

Synthesis of alkoxysilyl-terminated polyisoprenes by means of "living" anionic polymerization, 1. Modeling of the termination step by studying the reaction of butyllithium with various alkoxysilane reagents

In order to optimize the preparation of trimethoxysilyl- or triethoxysilyl-terminated 1,4-polyisoprene and to facilitate their characterization, the termination reaction of the anionic polymerization of isoprene was investigated by means of a model reaction. Butyllithium (n-Buli) was used as a model molecule of the “living” ω-carbanionic polymer chain, and its reaction with various alkoxysilyl reagents (3-chloropropyltrimethoxysilane, tetramethoxysilane and tetraethoxysilane) was investigated. Studies were carried out by varying the molar ratio r = [n-BuLi]/[alkoxysilane] (0.5 ≤ r ≤ 3). The corresponding rate of substitution depends on r and on the nature of the coupling reagent. It was shown that alkoxy groups at the silicon center are easily n-Bu-substituted. Whatever, r, higher n-Bu-substituted derivatives are always simultaneously formed with the n-Bu-monosubstituted compound. With tetraalkoxysilane reagents, the formation of butyltrialkoxysilane is always favoured, compared to that of the di- and trisubstituted homologs. This was interpreted in terms of reactivity differences existing between the reacting alkoxysilanes present in the mixture. Attempts realized with 3-chloropropyltrimethoxysilane in order to obtain selective formation of alkyltrimethoxysilane derivative showed that chlorine substitution was impossible because nucleophilic attack of butyl carbanion occurs exclusively on the silicon atom.

Cationic and Neutral [4]-Cumulenes CCCCC with Five Cumulated Carbons and Three to Four Ferrocenyl Termini

Starting from 1,1-diferrocenyl-1-methoxypropyne, the first air-stable C5-cumulenium salt (Fc)2CCCCC(Fc)+BF4- (Fc = ferrocenyl) can be synthesized by a metalation, iodination, cross-coupling, and dehydration sequence. Attempted nucleophilic attack of ferrocenyl carbanion affords only radical decomposition products and none of the desired tetraferrocenyl-C5-cumulene (Fc)2CCCCC(Fc)2. In contrast, reaction with phenyllithium yields the nucleophilic addition product (Fc)2CCC(R)C⋮CFc (R = C6H5) with 100% regioselectivity. Starting from diferrocenyl ketone, the first air-stable protonated C5-cumulenium salt (Fc)2CCHCCC(Fc)2+BF4- is prepared by stepwise substitution of propyne with diferrocenyl(methoxy)methyl groups and subsequent 2-fold dehydration. Addition of methoxide affords (Fc)2CCC(R)CΗC(Fc)2 (R = OCH3); attempted conversion to tetraferrocenyl-C5-cumulene (Fc)2CCCCC(Fc)2 by thermal elimination of methanol in vacuo fails but yields an unusual cyclobutene dimer with eight ferrocenyl substituents. Finally, de...