Ylidic Carbon Research Articles

Contribution to the Chemistry of (ClC{PPh3}2)Cl, a Precursor of the Carbodiphosphorane C(PPh3)2

AbstractThe salt (ClC{PPh3}2)Cl (1), formally seen as an addition compound between the carbone C(PPh3)2 and Cl+, reacts with AlBr3 in DCM or 1‐Br‐2‐F‐C6H4 to give the dication (HClC{PPh3}2)2+ with the counterions [AlCl4]– and [AlBr4]–, respectively. If the reaction was carried out in MeNO2 as the solvent the salt (H2C{PPh3}2)Br[AlBr4] was isolated. The dication (HClC{PPh3}2)2+ was also formed from the reaction of 1 with Me3SiOSO2CF3. The protons at the ylidic carbon atoms stem from the solvents. Alternatively, the dication (HClC{PPh3}2)2+ could be obtained more easily upon reacting 1 with HCl in diethyl ether. The compounds were characterized by multi NMR spectroscopy and by X‐ray analyses.

Synthesis, spectroscopic and structural characterization of new linear mononuclear silver(I) complexes containing α-keto phosphorus ylides as ambidentate ligands

The reaction of 2 equiv. phosphorus ylides, (C6H5)3PCHC(O)C6H4R (R = CH3 (L1), Br (L2), Cl (L3), NO2 (L4) and OCH3 (L5)) with 1 equiv. AgNO3 using methanol as solvent leads to formation of linear mononuclear Ag-CH complexes, [{RC6H4CO{(C6H5)3P}CH}2Ag(NO3)], (R = CH3 (C1), Br (C2), Cl (C3), NO2 (C4) and OCH3 (C5)), which contain one NO3 and two phosphorus ylides coordinated via the ylidic carbon atom. The silver complexes were synthesized and investigated by elemental analysis, Fourier transform infrared spectroscopy (FTIR), multinuclear (1H, 31P and 13C) Nuclear magnetic resonance (NMR). In addition, the identities of complexes C1 and C3 were unequivocally determined by single crystal X-diffraction techniques. X-ray single crystal diffraction studies show that both complexes C1 and C3 belong to the triclinic crystal system and P–1 space group. This crystal packing of structures shows a close association between the nitrate anion and the metal centre. The Ag(I) atom is located in a slightly distorted linear environment, surrounded by C atoms of the methine of the ylide. Five Ag(I) complexes, [{RC6H4CO{(C6H5)3P}CH}2Ag(NO3)], (R = CH3 (C1), Br (C2), Cl (C3), NO2 (C4) and OCH3 (C5)), which contain one NO3 and two phosphorus ylides coordinated via the ylidic carbon atom were synthesized. The structures of the complexes C1 and C3 were determined by single crystal X-ray diffraction technique.

Four-coordinate Pd(II) complexes containing non-symmetric phosphorus ylides: Synthesis, characterization, and catalytic behavior towards Suzuki reaction

In this paper it is reported the synthesis of the complexes of the type {[Ph2PCH2PPh2CHC(O)R]PdCl2} (R = 4′-biphenyl (1), OCH2Ph (2), 4-methylphenyl (3), 2-naphtyl (4), 2,4-dichlorophenyl (5), 3-nitrophenyl (6)) derived from the reactions of the dichloro(1,5-cyclooctadiene)palladium(II) with related phosphorus ylides in dichloromethane under mild conditions. These compound have been characterized by, elemental analysis and spectroscopic methods. The structure of complex 1 has been characterized crystallographically. This structure consists of five-membered ring formed by coordination of the ligand through the phosphine group and the ylidic carbon atom to the metal center. The coordination geometry around the Pd atoms in this complex can be defined as slightly distorted square planar. At a supramolecular level C–H⋯Cl and C–H⋯π intermolecular interactions are responsible for the crystal packing. This complex has been found to act as efficient catalysts for the Suzuki cross-coupling reaction. Various aryl bromides were coupled with aryl boronic acids in DMF, under air, in the presence of 0.2 mol% of the catalyst to afford corresponding cross-coupled products in good to excellent yields.

New platinum complexes with carbodiphosphorane as pincer ligand via ortho phenyl metallation

The platinum complex [Pt(η 3-C 8H 11)(C 6H 4PPh 2CPPh 3)] ( 2) is a halve side o-phenyl insertion product resulting from the reaction of C(PPh 3) 2 and [I 2Pt(cod)] and is unstable in halogenated hydrocarbons. In a slow reaction 2 abstracts HCl from CH 2Cl 2, CHCl 3 or CDCl 3 (DCl) and converts the η 3-C 8H 11 ligand into cod which is replaced by Cl −. Simultaneously, a second insertion of the Pt atom into an o-phenyl CH bond occurs under movement of the o-phenyl proton to the ylidic carbon atom resulting in the C, C, C-pincer complex [ClPt{CH(PC 6H 4Ph 2) 2}] ( 3). Surprisingly, crystals of the unusual carbonyl complex [(CO)Pt{C(PC 6H 4Ph 2) 2}] ( 5) were obtained upon attempts to replace Cl − in 3 by PPh 3 in CHCl 3 solution. Instead, an intermediate was identified by 31P NMR spectroscopy in which the coordinated CH group is replaced by PPh 3; however, no crystals could be obtained. From the resulting oily material after some month crystals of 5 separated. We suggest, that hydrolysis of an intermediate CCl 2 ligand (from CHCl 3) to CO via a leaking stopcock has taken place. The compounds are characterized by X-ray analyses, 31P NMR, and IR spectroscopy.

Synthesis, characterization and structural studies of new palladium(II) complexes including non-symmetric phosphorus ylides

The reaction of the non-symmetric phosphorus ylides, Ph 2P(CH 2) n PPh 2C(H)C(O)PhR [Y 1–Y 4: n = 1, R = Cl, Br, NO 2, OCH 3 and Y 5–Y 8: n = 2, R = Cl, Br, NO, OCH 3] with dichloro(1,5-cyclooctadiene)palladium(II) in dichloromethane under mild conditions afford the monomeric P–C chelated complexes, [(Y)PdCl 2] (Y = Y 1–Y 8). These complexes were fully characterized by elemental analysis and spectroscopic techniques such as IR, 1H, 31P, and 13C NMR. In addition, the identity of complexes [(Y 5)PdCl 2] ( 1b) and [(Y 8)PdCl 2] ( 4b) was unequivocally determined by single crystal X-diffraction techniques, both structures consisting of six-membered rings formed by coordination of the ligands through the phosphine group and the ylidic carbon atom to the metal center. The coordination geometry around the Pd atoms in both these complexes be defined as slightly distorted square planar. Furthermore, their electrochemical behavior was also investigated by cyclic voltammeters, thus the cyclic voltammetry of complex [(Y 1)PdCl 2], in dichloromethane solution with Pt electrode, shows that the redox reaction of the pair Pd(II)/Pd(0) is irreversible with the cathodic peak potential at −1.08 V versus Ag wire.

Towards the stability limit of cyclic diphosphonium bis-ylides

A typology of molecules containing two phosphonium ylide groups (bis-ylides) is proposed, versus the bridge length (fused, ω- and α-bisylides) and their relative topographical orientations (head-to-head, tail-to-tail or head-to-tail). The formal electrostatic constraint occurring in the head-to-head series is systematically addressed for cyclic representatives based on the o-bis(diphenylphosphonio)benzene framework. After a survey of previously reported results in the fused and β-bis-ylide series, emphasis is given to the cyclic α-bis-ylides. The non-substituted, non-stabilized version escaped isolation through spontaneous fragmentation to bis(diphenylphosphino)benzene and acetylene. Inspired by this result, the reverse process was employed for the generation of stabilized representatives with ethoxycarbonyl and benzoyl substituents at the ylidic carbon atoms. The stability and stereochemistry of the obtained head-to-head α-bis-ylides was investigated by NMR techniques and reproduced and analyzed by DFT calculations. The role of electrostatics in determining structural and reactivity features of cyclically constrained species is here illustrated.

Four-coordinate and pseudo five-coordinate Hg(II) complexes of a new bidentate phosphorus ylide: X-ray crystal structure and spectral characterization

The reaction of Ph 2PCH 2PPh 2 (dppm) with BrCH 2C(O)C 6H 4NO 2 in chloroform produces the new phosphonium salt [Ph 2PCH 2PPh 2CH 2C(O)C 6H 4NO 2]Br ( 1). Further, by reacting the phosphonium salt with appropriate base the bidentate phosphorus ylide, Ph 2PCH 2PPh 2 C(H)C(O)C 6H 4NO 2 ( 2) was obtained. The reaction of ligand 2 with mercury(II) halides in dry methanol led to the formation of the P, C-coordinated mononuclear complexes [HgX 2(Ph 2PCH 2PPh 2C(H)C(O)C 6H 4NO 2)] [X = Cl ( 3), Br ( 4), I ( 5)]. Characterization of the obtained compounds was performed by elemental analysis, IR, 1H, 31P, and 13C NMR. The X-ray crystal structure of 5 as well as the complex derived from crystallization of 4 in DMSO, [HgBr 2(Ph 2PCH 2PPh 2C(H)C(O)C 6H 4NO 2)(DMSO)] ( 6), is reported. In both complexes 5 and 6 the title ylide is coordinated through the ylidic carbon and the phosphine atom. However, in compound 6 the Hg–C bond length is considerably weakened due to coordination of DMSO molecule to the metal ion and locating of ylidic carbon atom in the axial position of the resulting trigonal bipyramidal complex. Theoretical studies on ligand and all complexes at DFT (B3LYP) level of theory are also reported. The results show that the addition of DMSO molecule to the compound 4 and formation of compound 6 is energetically favored.

Efficient access to peptidyl ketones and peptidyl diketones via C‐alkylations and C‐acylations of polymer‐supported phosphorus ylides followed by hydrolytic and/or oxidative cleavage

Novel syntheses of peptidyl ketones and peptidyl diketones on polymer support are described. Peptidyl phosphoranylidene acetates were prepared via C-acylation of polymer-supported phosphorus ylides. Selective alkylation of the ylide carbon with various alkyl halides, such as methyl iodide and benzyl bromide was established. Peptidyl diketones were obtained by oxidative cleavage. Peptidyl ketones were furnished by hydrolysis of the peptidyl phosphorus ylides under either basic or acidic conditions.

Comparison of Conformations of Diesters of Stabilized Phosphonium Ylides in Solution and in the Crystal

Computed bond lengths and angles of methyl ethyl and dimethyl triphenyl phosphonium ylidic diesters, 1b, c , respectively, are similar to those in the crystal, as for the diethyl ester, 1a , where both acyl oxygens are anti to phosphorus. The 1 H and 13 C NMR spectra of the methyl ethyl diester, 1b , where one acyl oxygen is syn and the other anti to phosphorus, are as expected in terms of the conformation in the crystal, but the dimethyl ester, 1c , in the crystal is an equimolar mixture of conformers. For a given ylidic diester the different conformers have similar energies from B3LYP//6-31G(d) computations, interconversions of conformers should not be slow at ambient temperatures and 1 H and 13 C NMR signals in solution are sharp. Estimation of Natural Atomic Charges indicates significant cationoid character on phosphorus and the acyl carbons, and anionoid character on the ylidic carbon and the ester oxygens depending on orientations towards phosphorus.

Carbenes Stabilized by Ylides: Pushing the Limits

Being neighborly: The concept of placing two reactive intermediates next to each other to gain stability is borne out in amino(ylide)carbenes (AYCs). Different kinds of ylides stabilize an adjacent singlet carbene by donation of electron density from the ylidic carbon atom into the empty p orbital to give AYCs of various structural formats (see picture) that are distinguished by their exceptionally strong σ-donor qualities.

Cyclic (Amino)[bis(ylide)]carbene as an Anionic Bidentate Ligand for Transition-Metal Complexes

The 3-(triphenylphosphonio)-N-(2,6-diisopropylphenyl)pyrrole reacts with 2 equiv of methyllithium to afford a lithium adduct in which a cyclic (amino)[bis(ylide)]carbene, a novel type of NHC, acts as a 1,4-bidentate ligand via the carbene center and the exocyclic ylidic carbon. This species readily undergoes transmetalation reactions, which allows for the synthesis of a variety of transition-metal complexes.

Synthesis and coordination of the bifunctionalized ylides Ph 2P(CH 2) n(Ph) 2P [dbnd]CHCOOMe ( n = 1, 2) and ketenylidene Ph 2P(CH 2) 2(Ph) 2P [dbnd]C [dbnd]C [dbnd]O to Pd and Pt complexes

In this paper it is reported the synthesis of the phosphonium salts [Ph 2P(CH 2) n (Ph) 2PCH 2COOMe]Br ( n = 1 ( 1), 2 ( 2)) and [Ph 2P(CH 2COOMe)(CH 2) n (Ph) 2PCH 2COOMe]Br 2 ( n = 3 ( 3)) derived from the reactions of the diphosphines dppm, dppe and dppp with methyl bromoacetate. By reaction of the monophosphonium salt of dppm and dppe with the strong base Na[N(SiMe 3) 2] the corresponding carbonyl stabilized ylides Ph 2P(CH 2) n (Ph) 2P CHCOOMe ( n = 1 ( 4), 2 ( 5)) were obtained. The Ph 2P(CH 2) 2(Ph) 2P CHCOOMe ( 5) ylide was reacted with Pd(II) and Pt(II) substrates. From these reactions were isolated exclusively complexes in which the ylide was chelated to the metal through the free phosphine group and the ylidic carbon atom. A further reaction of the Ph 2P(CH 2) 2(Ph) 2P CHCOOMe ( 5) ylide with 1.5 equiv. of Na[N(SiMe 3) 2] gives the bifunctionalized ketenylidene Ph 2P(CH 2) 2(Ph) 2P C C O ( 6) system. This cumulenic ylide reacts with Pt(II) complexes to form a chelated derivative in which IR and NMR spectra suggest the breaking of the C C bond of the –C C O group.

Computational Investigations on the General Reaction Profile and Diastereoselectivity in Sulfur Ylide Promoted Aziridination

Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me(2)S(+)CH(-)R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO(2)Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches.

Structural features of the phosphorus heterocumulene ylides (sp-ylides)

The features in phosphorus heterocumulene yilides Ph3PCCX [X = NPh, NC(O)Ph, C(CN)CO · (OMe), O, S] with the ylide carbon geometry close to sp-hybridization are studied using ab initio methods. Estimation of structural parameters of a new member in the Ph3PCα=Cβ=NC(O)Ph series on the B3LYP/6-31G(d,p) level and together with experimental data for related ylide (X = NPh) gave the following values: r(PCα) 1.665 A, r(Cα=Cβ) 1.237 A, ω(PCαCβ) 145.2°. Nonvalent interactions of spλ orbital of the ylide carbon atom bearing extra negative charge with coplanar π-electrons of the nearest Cβ=N bond are found close to those of nitrogen lone pair with the Cα=Cβ bond and leading to up to 10° nonlinearity of Cα-Cβ=N triad of the phosphorus iminoketene ylides. Chemical nonequivalence of the PCipso bonds in the triphenylphosphonium fragment, relation of \(^1 J_{PC^\alpha } \) spin-spin coupling and antisymmetric bonding vibration νas(C=C=X) to the structure of the carbanion, and inductive hyperconjugative influence of atom (or group) X on the ylide carbon geometry are discussed.

A study of influence of temperature and N, N-acyl protected keto ylides structure on their predominant transformations

The effect of temperature and keto ylides structure on preference of their intramolecular cyclization leading to N-containing heterocyclic compounds or linear products formation has been investigated at the B3LYP/6-31G(d,p) level of theory. It has been determined that the thermodynamic advantage of the cyclization reactions of ylides increases with temperature, while Gibbs free energies of linear products formation reactions depend insignificantly on temperature. The Wittig and the Corey–Chaykovsky reactions are least probable in the case of the sulfonium and ammonium ylides considered. However, for phosphonium ylides the Wittig reaction must be considerably preferable in comparison with other routes, while behavior of the arsonium ylides is predicted to be more complex. Research of S-ylides transformations shows that formation of methylthio-substituted heterocycles with five-, six- or seven- membered rings is possible from a thermodynamic standpoint, while conversion of the corresponding ylide to a four-membered heterocycle is disadvantageous. Presence of a methyl substituent and its position in the ylide carbon chain depends ambiguously on the behavior of sulfur keto ylides.

Quantum chemical study of the effect of N,N-acyl group nature on the structure and Gibbs free energy of the sulfur ketoylides and products of their transformations

With the purpose to study the effect of the nature of the imido-group of N,N-acyl-protected β-methyl-substituted keto-stabilized sulfonium ylides on the formation of methylthiopyrrolisinediones by cyclization, the structure features of these ylides are investigated by ab initio methods. The imido-group nature is shown to insignificantly affect in most cases the structure and the relative free energy of conformers of the ketoylides in question and the products of a cyclization reaction with their participation. The energy difference between the anti-peri-planar and syn-clinal orientation of nitrogen atoms in the imido-group and ylide carbon atom is negligible. The latter orientation is necessary for the reaction of intramolecular cyclization to proceed because in this case carbon atoms between which the bond forms are drawn together. The structure of cyclic pentanomial N,N-acyl group slightly affects the calculated values of Gibbs free energy of sulfur ketoylide reactions, and a temperature increase promotes the intramolecualr cyclization reactions. Wittig and Corey-Chaykovsky reactions typical of ylides are the least thermodynamically favorable for the sulfonium ketoylides under study.

Density Functional Theory Investigations on Sulfur Ylide Promoted Cyclopropanation Reactions: Insights on Mechanism and Diastereoselection Issues

The mechanism and diastereoselectivity of synthetically useful sulfur ylide promoted cyclopropanation reactions have been studied using the density functional theory method. Addition of different substituted ylides (Me2S+CH-R) to enone ((E)-pent-3-en-2-one, MeHC=CH-COMe) has been investigated. The nature of the substituent on the ylidic carbon brings about subtle changes in the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode, leading to 1,2-trans and 1,2-cis cyclopropanes, respectively, via syn and anti betaine intermediates. The simplest and highly reactive model ylide (R=H) prefers a transoid addition mode. Diastereoselectivity is controlled by the barrier for cisoid-transoid rotation in the case of stabilized ylides, whereas the initial electrophilic addition is found to be the diastereoselectivity-determining step for semistabilized ylides. High selectivity toward trans cyclopropanes with stabilized ylides are predicted on the basis of the relative activation energies of diastereomeric torsional transition states. The energy differences between these transition states could be rationalized with the help of weak intramolecular as well as other stereoelectronic interactions.

Substituent-Dependent Anomeric Effects as a Source of Conformational Preference in Pyridinium Methylides

A systematic density functional theory level investigation of differently substituted pyridinium methylides was carried out to determine the role of C(ylidic) lone-pair-associated hyperconjugative and negative hyperconjugative interactions in deciding conformational preferences. Deviation from the coplanar orientation of the carbanionic center with the pyridine ring and its substituent dependence has been found to correlate well with the relative opportunities for conjugative and negative hyperconjugative interactions of a ylidic moiety with different substituent groups present at the ylidic carbon. The contribution of individual n-->pi* conjugative, n-->sigma* negative hyperconjugative, and sigma-->pi* hyperconjugative interactions in a particular conformation of pyridinium dichlorophosphinomethylides was assessed from donor-acceptor stabilization energies, as obtained from natural bond orbital (NBO) analysis. The relative extent of conjugative and negative hyperconjugative interactions with the substituents present at the ylidic carbon plays an important role in permitting the delocalization of ylidic charge into the pyridine ring, thereby controlling the relative orientation of the latter with the carbanionic plane.

Reaction of the Carbodiphosphorane Ph3PCPPh3with Platinum(II) and -(0) Compounds: Platinum Induced Activation of C−H Bonds

The complex [(cod)PtI2] (cod = 1,5-cyclooctadiene) reacts with 3 equiv of the hexaphenylcarbodiphosphorane Ph3PCPPh3 (1) in THF solution to give the novel Pt(II) complex [(η3-C8H11)Pt(C6H4PPh2CPPh3)] (2) along with the salt [HC(PPh3)2]I. In addition to the coordination of the ylidic carbon atom at the Pt atom, 2 contains two further Pt−C σ bonds originating from H to Pt exchange in the ortho position of one phenyl group of the carbodiphosphorane ligand and in the former cod ligand. The resulting C8H11 moiety is coordinated to the Pt atom in an η3 manner via a double bond and a σ bond and contains a further uncoordinated double bond. From a 1:1 reaction mixture in toluene/CH2Cl2 the majority of the crystals consist of the salt [HC(PPh3)2]I·2CH2Cl2 (3) with small amounts of platinum compounds as byproducts. The Pt(0) complex [(PPh3)2Pt(CH2CH2)] does not react with 1 but decomposes at elevated temperatures to give the known dinuclear complex [Pt2(PPh3)2(μ-PPh2)2] (4). The complexes 2 and 4 and the salt 3 cou...

Aspects of three-electron two-centre, four-electron three-centre and six-electron five-centre bonding in cycloimmonium ylides

The weights for several types of local valence bond structures which are involved in two-electron two-centre, three-electron two-centre and four-electron three-centre bonding units for the π-electrons of pyridinium dicyanomethylide are calculated via a Poly-Electron Population Analysis of a Hartree–Fock MO wavefunction. It is shown that three-electron two-centre and four-electron three-centre behavior for the ylide bond is better developed towards the substituents of the ylide carbon atom, rather than the pyridinium group; this gives rise to a polarized increased-valence bonding, which is the principal specificity of cycloimmonium ylides. Qualitative consideration is given to six-electron five-centre bonding units for the π-electrons.