Allylic Carbanions Research Articles

Intramolecular Nucleophilic Vinylic Substitution (SNV) by Carbon Nucleophiles: Conformationally Directed Formation of Dienes from N,N'-Diallyl Ureas.

Nucleophilic vinylic substitution (SNV) by carbon nucleophiles allows the formation of vinylic C-C bonds without transition metal catalysts. In this paper, we show that tethering two alkenes together through a urea linkage can lead to the formation of a diene by an intramolecular SNV reaction. The starting materials are fully substituted N,N'-diallyl ureas; the reaction proceeds in the presence of base, and entails a cascade of deprotonations, reprotonations, and an SNV reaction of an allylic carbanion on a rare electrophile: a vinylic urea. As a result, two allylic substituents couple to form a diene, despite the fact that neither is activated towards electrophilic attack. The reaction is tolerant of significant steric bulk, and exhibits regioselectivity with unsymmetrical diallyl ureas: β-substituted allyl groups invariably behave as nucleophiles, while electrophilic behavior may be enforced by the use of an E-vinylic urea substituent that cannot be deprotonated under the reaction conditions.

Visible-Light-Catalyzed Regioselective Arylcarboxylation of Allenes with CO2

AbstractA visible-light-catalyzed arylcarboxylation of allenes with CO2 was developed using [Ir(ppy)2(dtbbpy)]PF6 (ppy = 2-phenylpyridine; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) as a photocatalyst to synthesis β-aryl β,γ-unsaturated carboxylic acids. This multicomponent protocol proceeds in an atom-economical way with exclusive regioselectivity. Preliminary mechanistic experiments suggested that allylic carbanion species are the key intermediates.

Rearrangement Strategy for the Preparation of Polymers With π-Conjugated Structures.

π-Conjugated polymers are usually prepared by polymerization only. In this perspective article, typical synthesis methods of conjugated polymers are briefly summarized, and a novel strategy for preparing conjugated polymers by rearrangement is proposed. During the metalation process, many conjugated structures were generated in polybutadiene by double bond migration. The effects of reaction time, temperature, and catalyst dosage on the product structure were investigated. Moreover, the structure of the products was confirmed by FTIR, 1H NMR, and 2D HSQC NMR spectra. Thus, a possible reaction mechanism was proposed, in which polybutadiene generates allylic carbanions in the presence of n-butyllithium, and then the double bonds migrate through the carbanions rearrangement to generate many conjugated structures in the backbone chain. The method shows promise in facile and low-cost synthesis of conjugated polymers without the need for precious metal catalysts.

КВАНТОВО-ХИМИЧЕСКОЕ ИЗУЧЕНИЕ ВЗАИМОДЕЙСТВИЯ 1,2-ЭТАНДИТИОЛА С 1,3-ДИХЛОРБУТЕНОМ-2 В СИСТЕМЕ ГИДРАЗИНГИДРАТКОН

According to the results of quantum-chemical studies in the framework of the theory
 of functional density of the electron density using the B3LYP / 6-311 ++ G (d, p) method, a theoretical
 mechanism of the reaction of 1,3-dichlorobutene-2 with 1,2-ethanedithiol in the hydrazine hydrateKOH system has been proposed. It has been shown that this interaction proceeds sequentially in several stages, including the nucleophilic substitution of the chlorine atom of the sp3 hybridized carbon
 atom to the sulfur atom with the formation of a monosubstitution product (SN2 mechanism), which under the action of alkali undergoes deprotonation of the methylene group to form an allyl carbanion .
 The resulting carbanion, due to the migration of hydrogen and the removal of chlorine, is converted
 into an allene derivative, which as a result of the intramolecular nucleophilic attack of the free
 thiolate group on the central carbon atom of the allene fragment closes into the final dithian cycle.

Base-promoted cyclization of 3-alkynylquinoxaline-2-carbonitriles with CH-acids: a new method for the phenazine ring synthesis

3-Alkynylquinoxaline-2-carbonitriles were directly transformed into 2,3-disubstituted phenazin-1-amines by treating with a CH-acid (diethylmalonate, ethyl cyanoacetate, malononitrile, 2-tosylacetonitrile, 2-(1-methyl-1H-benzo[d]imidazol-2-yl)acetonitrile, nitromethane) and t-BuOK in THF or DMSO. A new cascade process includes nucleophilic addition of a CH-acid carbanion to the CC bond of 3-alkynylquinoxaline-2-carbonitrile, 6-exo-dig cyclization of the intermediate allyl carbanion with the formation of 2,2-disubstituted 1-imino-1,2-dihydrophenazine and its aromatization via elimination of the C(2)-substituent. Several secondary reactions were also disclosed and discussed.

An unusual regiochemistry of reactions of a cyclohexenylphosphonate bearing a β-ethoxycarbonyl group with aldehydes

Reactions of lithiated ethyl 6-(dimethoxyphosphoryl)cyclohex-1-enecarboxylate with aliphatic, aromatic, and unsaturated aldehydes were studied and determined to proceed with α or δ regioselectivity. Such an unusual regioselectivity results from the contribution of two allylic carbanions: one, stabilized by the phosphonate moiety and the other stabilized by the carboethoxy group. The course of the reaction depends mainly on the structure of the aldehyde and the reaction conditions. The products of Horner–Wadsworth–Emmons reaction, including an analogue of some retinol metabolites, were formed under kinetic conditions whereas the δ-adducts were obtained as thermodynamic products.

ChemInform Abstract: A Facile Synthesis of 1,5-Dienes by a Regioselective Allylation of the Allylic Carbanions Generated from Allyl Phenyl Selenides.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Synthesis of (2S,3R,4E,8E)-9-Methyl-4,8-sphingadienine via a Novel SN2′ Type Reaction Mediated by a Thioether Carbanion.

A stereoselective synthesis of sphingadienine 3 as a new analogue of sphingosine has been achieved by a novel copper-catalyzed S=N2' type homoallylic coupling reaction between an allylic mesylate and a thioether-stabilized allylic carbanion

Crystal Structure and Putative Mechanism of 3-Methylitaconate-Δ-isomerase from Eubacterium barkeri

3-Methylitaconate-Δ-isomerase (Mii) participates in the nicotinate fermentation pathway of the anaerobic soil bacterium Eubacterium barkeri (order Clostridiales) by catalyzing the reversible conversion of (R)-3-methylitaconate (2-methylene-3-methylsuccinate) to 2,3-dimethylmaleate. The enzyme is also able to catalyze the isomerization of itaconate (methylenesuccinate) to citraconate (methylmaleate) with ca 10-fold higher Km but >1000-fold lower kcat. The gene mii from E. barkeri was cloned and expressed in Escherichia coli. The protein produced with a C-terminal Strep-tag exhibited the same specific activity as the wild-type enzyme. The crystal structure of Mii from E. barkeri has been solved at a resolution of 2.70 Å. The asymmetric unit of the P212121 unit cell with parameters a=53.1 Å, b=142.3 Å, and c=228.4 Å contains four molecules of Mii. The enzyme belongs to a group of isomerases with a common structural feature, the so-called diaminopimelate epimerase fold. The monomer of 380 amino acid residues has two topologically similar domains exhibiting an α/β-fold. The active site is situated in a cleft between these domains. The four Mii molecules are arranged as a tetramer with 222 symmetry for the N-terminal domains. The C-terminal domains have different relative positions with respect to the N-terminal domains resulting in a closed conformation for molecule A and two distinct open conformations for molecules B and D. The C-terminal domain of molecule C is disordered. The Mii active site contains the putative catalytic residues Lys62 and Cys96, for which mechanistic roles are proposed based on a docking experiment of the Mii substrate complex. The active sites of Mii and the closely related PrpF, most likely a methylaconitate Δ-isomerase, have been compared. The overall architecture including the active-site Lys62, Cys96, His300, and Ser17 (Mii numbering) is similar. This positioning of (R)-3-methylitaconate allows Cys96 (as thiolate) to deprotonate C-3 and (as thiol) to donate a proton to the methylene carbon atom of the resulting allylic carbanion. Interestingly, the active site of isopentenyl diphosphate isomerase type I also contains a cysteine that cooperates with glutamate rather than lysine. It has been proposed that the initial step in this enzyme is a protonation generating a tertiary carbocation intermediate.

Structure, Bonding, and Dynamics of Several Palladium η3-Allyl Carbene Complexes

A series of Pd-allyl carbene complexes, [PdX(η3-C3H5)(IPr)], 1 (X = a, Cl−, b, Br−, c, I−, d, N3−, e, NCO−, f, SCN−, g, CN−, h, OAc−, i, OTf−, j, 4-Me-pyridine), have been studied by one- and two-dimensional NMR techniques. 13C, 1H, and phase-sensitive NOE NMR studies on these relatively simple complexes reveal that (a) the trans influence of the carbene carbon in 1 seems to be smaller than that found for PPh3 and other P-donor ligands, (b) the selective η3−η1 opening of the allyl is under electronic control, and (c) the rates of η3−η1 allyl isomerization depend on the X ligand. The solid-state structure of 1c is reported, as well as selected 15N chemical shift data for the coordinated carbene ligand.

Structure, Bonding, and Dynamics of Several Palladium η3-Allyl Carbene Complexes

Structure, Bonding, and Dynamics of Several Palladium η³-Allyl Carbene Complexes

Regio- and Diastereochemical Aspects of the Additions of Li or Zn Derivatives of Methoxypropene to Oxazolidines Derived from Phenylglycinol

Diastereoselective additions of methoxypropene-derived lithium and zinc reagents to oxazolidines are investigated. The lithium allylic carbanion reacts with oxazolidines to afford mainly the β-amino alcohols with an enol ether function (y-adduct) and the zinc derivative leads to amino alcohols with an allyl ether function (a-adduct).

Facile Migratory Insertion of a N-Heterocyclic Carbene into a Ruthenium−Carbon Double Bond: A New Type of Reaction of a NHC Ligand

The reaction of [RuCp(IPri)(CH3CN)2]PF6 (IPri = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with HCCR (R = COOMe, COOEt, COMe) yields the allyl carbene complexes [RuCp(=C(R)-eta3-CHC(R)CH-IPri)]PF6. This conversion involves selective head-to-tail coupling of two alkynes and an unusual migratory insertion of the N-heterocyclic carbene into the ruthenium-carbon double bond of a ruthenacyclopentatriene intermediate.

Convenient synthesis of dithiafulvene carbene complexes as potential precursors of extended TTF

The condensation of α-carbanion of Fischer-type carbene complex with 2-thiomethyl-1,3-dithiolium salts 1 gave new heterocyclic organometallic carbenes 2 potentially precursors of extended TTF. Reaction of allylic carbanion 4 derived from (methoxypropenyl)pentacarbonyl carbene complex afforded a mixture of monoheterocyclic condensation products 7 and diheterocyclic carbene complexes 8 produced from successive α and γ attacks of the heterocyclic cations.

The stereochemistry of addition of allyl sulfone carbanions to aldehydes. Formation of dihydrofurans

The reaction of the mono-anion of the bromoallyl sulfone 1 with aldehydes 2 was examined with the aim of obtaining selectively substituted tetrahydrofurans. At -100oC syn- and anti- open chain adducts 3 and 4 were isolated together with a low yield of 4-methylene-2,3-disubstituted tetrahydrofuran 5. In the presence of HMPA or at higher temperature the reaction led to formation of 2,5-dihydrofurans 6. The stereochemical results are consistent with initial addition of 1 to the aldehyde involving Li ion chelation.

Selective C-C bond formation between alkynes mediated by the [RuCp(PR)(3)](+) fragment leading to allyl, butadienyl, and allenyl carbene complexes--an experimental and theoretical study.

The reaction of alkynes with [RuCp(PR(3))(CH(3)CN)(2)]PF(6) (R=Me, Ph, Cy) affords, depending on the structure of the alkyne and the substituent of the phosphine ligand, allyl carbene or butadienyl carbene complexes. These reactions involve the migration of the phosphine ligand or a facile 1,2 hydrogen shift. Both reactions proceed via a metallacyclopentatriene complex. If no alpha C[bond]H bonds are accessible, allyl carbenes are formed, while in the presence of alpha C[bond]H bonds butadienyl carbenes are typically obtained. With diphenylacetylene, on the other hand, a cyclobutadiene complex is formed. A different reaction pathway is encountered with HC[triple bond]CSiMe(3), ethynylferrocene (HC[triple bond]CFc), and ethynylruthenocene (HC[triple bond]CRc). Whereas the reaction of [RuCp(PR(3))(CH(3)CN)(2)]PF(6) (R=Ph and Cy) with HC[triple bond]CSiMe(3) affords a vinylidene complex, with HC[triple bond]CFc and HC[triple bond]CRc this reaction does not stop at the vinylidene stage but subsequent cycloaddition yields allenyl carbene complexes. This latter C[bond]C bond formation is effected by strong electronic coupling of the metallocene moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the Ru[double bond]C bond. The vinylidene intermediate appears only in the presence of bulky substituents of the phosphine coligand. For the small R=Me, head-to-tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual allyl carbene complexes. X-ray structures of representative complexes are presented. A reasonable mechanism for the formation of both allyl and allenyl carbenes has been established by means of DFT calculations. During the formation of allyl and allenyl carbenes, metallacyclopentatriene and vinylidene complexes, respectively, are crucial intermediates.

Reactions of RuCp and RuCp* Allyl Carbene Complexes: Products Derived from Activation of Phenyl, Cyclohexyl, and Methyl C−H Bonds in PPh3, PCy3, and Cp* Ligands

Ruthenium allyl carbene complexes of the type [RuCp(C(R)-(η3-CHC(R)CHPR‘3))]PF6 (R, R‘ = aryl, alkyl substituents) are characterized by two reaction modes. (i) They behave as masked coordinatively unsaturated complexes and react readily with the donor ligands PR3 and P(OR)3 to give η3-butadienyl complexes. This is not a simple nucleophilic addition at the metal center but involves changes in both the bonding mode and the stereochemistry of the allyl carbene C4-chain. The incoming nucleophile induces C−H bond activation effecting formally a 1,4 hydrogen shift. (ii) They are capable of activating C−H bonds of aryl and alkyl groups in the bulky tertiary phosphine ligands PPh3 and PCy3 to give novel η4-butadiene complexes. In these, one R of PR3 is σ-bonded to the metal center. A divergent rearrangement results if the allyl carbene complex derives from parent acetylene (instead of a terminal alkyne). In this case the C4-unit features a η3-allyl moiety and the metal center is η2-coordinated to an arene ring of...

ChemInform Abstract: Stereochemistry. Part 92. Stereoselective Michael Additions of Phosphorylated Allyl Carbanions — Synthesis of Functionalized Cyclopentylphosphonates and Phosphane Oxides.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reactivity of 1,1-diphenylalkyl and triphenylmethyl compounds as initiators of anionic polymerization of nonpolar monomers.Some considerations connected with the use of low basicity initiators

The kinetics of initiation of styrene and butadiene polymerizations by bis(triphenylmethyl)barium (Tr 2Ba) and 1,1-diphenylalkylbarium and -sodium were studied. It was found that the initiation efficiency of Tr 2Ba is very low in THF but greatly increases when THF is replaced by benzene. The dependencies of initiator efficiency and the MWD of the polymers formed on k i/ k p and M 0/ I 0 were investigated theoretically. According to the results obtained, at certain conditions the initiators studied can be considered as suitable for controlled polymerization despite the fact that the basicity of 1,1-diphenylalkyl and, especially, triphenylmethyl carbanions is much lower than that of benzyl and allyl carbanions. The general aspects related to initiation with low basicity carbanions are discussed. In particular, attention was paid to the fact that, besides triphenylmethyl anion, some other low basicity carbanions such as poly( N, N-dimethylacrylamide) and poly(4-vinylpyridine) living chain ends are also capable of initiating polymerization of nonpolar monomers.

Reactivity of a metallacyclopentatriene intermediate: metal-to-ligand-to-metal re-migration of a phosphine ligand versus a 1,2 hydrogen shift.

The reaction of nona-2,7-diyne, deca-2,8-diyne and hex-1-yne with [RuCp(PR3)(MeCN)2]PF6 (R = Cy, Ph, Me) affords, depending on the structure of the alkyne and the substituent of the phosphine ligand, ruthenium metallacyclopentatriene, allyl carbene and/or butadienyl carbene complexes involving either metal-to-ligand-to-metal migration of the phosphine ligand with concomitant C-H activation or a facile 1,2 hydrogen shift.