Formation Of Vinylidene Research Articles

Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers.

1-Hexene transformations in the catalytic systems L2MCl2–XAlBui2 (L = Cp, M = Ti, Zr, Hf; L = Ind, rac-H4C2[THInd]2, M = Zr; X = H, Bu i) and [Cp2ZrH2]2-ClAlR2 activated by MMAO-12, B(C6F5)3, or (Ph3C)[B(C6F5)4] in chlorinated solvents (CH2Cl2, CHCl3, o-Cl2C6H4, ClCH2CH2Cl) were studied. The systems [Cp2ZrH2]2-MMAO-12, [Cp2ZrH2]2-ClAlBui2-MMAO-12, or Cp2ZrCl2-HAlBui2-MMAO-12 (B(C6F5)3) in CH2Cl2 showed the highest activity and selectivity towards the formation of vinylidene head-to-tail alkene dimers. The use of chloroform as a solvent provides further in situ dimer dimerization to give a tetramer yield of up to 89%. A study of the reaction of [Cp2ZrH2]2 or Cp2ZrCl2 with organoaluminum compounds and MMAO-12 by NMR spectroscopy confirmed the formation of Zr,Zr-hydride clusters as key intermediates of the alkene dimerization. The probable structure of the Zr,Zr-hydride clusters and ways of their generation in the catalytic systems were analyzed using a quantum chemical approach (DFT).

Selective Ring-Opening Allene Metathesis: Polymerization or Ruthenium Vinylidene Formation.

Selective ring-opening allene metathesis polymerization (ROAlMP) and ruthenium vinylidene formation from 1,2-cyclononadiene (1) by simple catalyst selection are discussed. Grubbs second-generation catalyst (G2) favors the formation of an alkylidene leading to the ROAlMP of (1). Grubbs first-generation catalyst (G1) favors vinylidene formation and prevents the homopolymerization of (1) even at elevated temperatures. Isolation and characterization of poly(1) by NMR analysis and MALDI-TOF confirms the generation of a well-defined polyallene that exhibits good thermal stability (TD ca. 390 °C) and fluorescent properties. Ring-opening metathesis polymerization (ROMP) of a highly strained norbornene derivative (NBE-iPr) at 80 °C using the ruthenium vinylidene generated from (1) is also investigated. The discovery of ROAlMP allows for the simple access of well-defined polyallenes from commercially available catalysts and will ultimately guide structure-property determinations of polyallenes.

Solvent‐Promoted Regio‐ and Stereoselectivity in Ru‐Catalyzed Hydrocarboxylation of Terminal Alkynes: A DFT Study

AbstractDFT calculations at the M06L‐D3(SMD)/LANL2TZ(f)/6‐311+G(d,p)//M06L/LANL2DZ/6‐31G(d)* level were performed to explore the mechanism, solvent effect, and origin of selectivity in the hydrocarboxylation of terminal alkynes catalyzed by (PCy3)2(CO)RuHCl. This catalytic system offers a unique example in which selectivity in enol‐ester synthesis is controlled in presence of different solvent media under mild reaction conditions. In CH2Cl2 solvent the regioselective gem‐enol‐ester (Pg) was preferred, whereas stereoselective Z‐enol‐ester (PZ) was observed exclusively in THF medium. We have unraveled the complete catalytic mechanism involving the following steps: a) alkyne coordination with subsequent nucleophilic attack and, finally, b) product elimination leading to the Markovnikov product Pg in both solvent media. Herein, the rate‐limiting nucleophilic attack step requires an activation barrier of 20.5 (21.0) kcal mol−1, which is 5.7 (5.6) kcal mol−1 lower than the crucial vinylidene formation step encountered for anti‐Markovnikov addition. Interestingly, a new reaction avenue unfolds under explicit THF medium, in which the inherent coordinating nature of the solvent becomes effective in altering the product regioselectivity. Under such an event, a highly facile ligandassisted proton shuttle (LAPS) mechanism is propagated, leading to vinylidene complex 11A. This intriguing mechanistic scenario facilitates favorably furnishing the PZ product rather than the Pg isomer by 4.3 kcal mol−1.

Kinetics and mechanism of the reaction of recombination of vinyl and hydroxyl radicals

The recombination of the vinyl (C2H3) and the hydroxyl (OH) radicals was studied computationally using quantum chemistry and master equation/RRKM. The reaction mechanism includes the initial addition, several isomerization steps, and decomposition via seven different channels. The spectrum of products demonstrates temperature dependence in the 300–3000K range. At low temperatures (below 1600K), CH3+HCO products are dominant but at elevated temperatures vinoxy radical (CH2CHO) and hydrogen atom become more important. The acetyl (CH3CO)+H products and formation of vinylidene (CH2C:) and water products are minor but non-negligible.

Isolation of Ruthenium–Indoline and −Indole Zwitterion Complexes: Insight into the Metal-Induced Cyclization of Aniline-Tethered Alkynes and Strategy to Lower the Activation Barrier of Metal−Vinylidene Formation

Isolation of the first examples of Ru(II)–indoline and −indole zwitterion complexes derived from reactions between [Ru([14]aneS4)Cl2] ([14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane) and aniline-tethered alkynes has been achieved. Mechanistic studies regarding the selectivity between a “vinylidene-involving pathway” and a “direct nucleophilic ring closure mechanism” for these reactions have been conducted experimentally and theoretically. Employment of 1,2-silyl shift as a strategy to lower the activation barrier for the formation of Ru–vinylidene species has also been proved to be successful.

Synthesis of Phosphonium-Substituted Vinylidene Complexes from [HC≡CCH2PPh3]+: Exploring the Competition between Allene and Vinylidene Formation.

Reaction of the alkenyl-substituted carbene complex [Ru(κ2-O2CMe)(OC{Me}O(═CCH═CH2)(PPh3)2]+ with PPh3 results in the formation of the vinylidene-containing species [Ru(κ1-O2CMe)(κ2-O2CMe)(═C═CHCH2PPh3)(PPh3)2]+, among other products. This complex could also be obtained by reaction of [Ru(κ2-O2CR)(PPh3)2] (R = Me, Ph) with [HC≡CCH2PPh3]+ although in this instance competitive formation of allene-containing [Ru(κ1-O2CR)(κ2-O2CR)(η2-H2C═C═CHPPh3)(PPh3)2] occurred. Reaction of [Ru(κ2-O2CMe)2(PPh3)2] with [H2C═C═CHPPh3]+ gives this allene complex in a selective fashion. Similar results were obtained when [RhCl(PiPr3)2]2 was used as the metal precursor. The reaction with [H2C═C═CHPPh3]+ gave [RhCl(η2-H2C═C═CHPPh3)(PiPr3)2]+, whereas treatment with [HC≡CCH2PPh3]+ afforded a mixture of the same complex and [RhCl(η2-HC≡CCH2PPh3)(PiPr3)2]+, which subsequently isomerized to vinylidene-containing [RhCl(═C═CHCH2PPh3)(PiPr3)2]+ via the alkynyl hydride complex [RhCl(H)(C≡CCH2PPh3)(PiPr3)2]+. Consistent with previous findings [HC≡CCH2PPh3]+ isomerizes to [H2C═C═CHPPh3]+ in the absence of a metal and the balance between the vinylidene and allene complexes formed in these reactions appears to relate to the efficiency of the reaction with each substrate.

Formation of Hydroxyindenyl and Vinylidene Ligands by Reaction of Internal Alkynes with Cp*Fe(CO)(NCMe)Ph

The reactivity of Cp*Fe(CO)(NCMe)Ph (1) (Cp* = pentamethylcyclopentadienyl) with internal alkynes has been studied. Reactions of 1 with dihydrocarbyl-substituted alkynes result in the formation of pentamethylcyclopentadienyl hydroxyindenyl sandwich complexes. A divergent reaction pathway is observed in the case of bis(trimethylsilyl)acetylene, where a neutral Fe vinylidene is isolated. The reactions of hydrocarbyl trimethylsilyl-substituted alkynes with complex 1 result in the formation of the pentamethylcyclopentadienyl hydroxyindenyl sandwich complexes. This reactivity suggests the necessity of two trimethylsilyl groups for vinylidene formation.

Formation of Vinylidene in Polypropylene/Ethylene Vinyl Acetate (PP/EVA) Blends During Degradation

The effect of natural exposure and artificial weathering on selected properties of PP/EVA blends was investigated. The vinyl acetate concentration decreased with exposure time due to acetylation. The most pronounced effect of radiation is associated with oxidative degradation. The cross-linking of PP/EVA blends was attributed to the formation of vinylidene group. Weathering decreased the tensile strength, elongation at break, and Young's modulus of the naturally and artificially exposed blends. This reduction was the result of blends’ molecular weight reduction due to the degradation induced by weathering.

Ruthenium‐Catalyzed Three‐Component Coupling via Hydrative Conjugate Addition of Alkynes to Alkenes: One‐Pot Synthesis of 1,4‐Dicarbonyl Compounds

A catalytic three-way rendezvous: Terminal alkynes undergo metal vinylidene formation, anti-Markovnikov hydration to give a metal acyl complex, and conjugate addition to produce synthetically useful 1,4-dicarbonyl compounds under ruthenium catalysis. This one-pot three-component coupling reaction is a useful platform for further exploration in alkyne functionalization.

η1‐Vinylidene Formation from Internal Alkynones by C–C Bond Migration

AbstractThe [μ2‐N2{CpRu(PPh3)2}2]2+ (12+) dication is a source of the formal 16‐electron [CpRu(PPh3)2]+ fragment, and it reacts with internal alkynes such as 1,3‐diphenylpropynone, 4‐phenyl‐3‐butyn‐2‐one, and 3‐cyclopropyl‐1‐phenylpropynone to yield η1‐vinylidene complexes 2a+–2c+ by C–C bond activation. In the case of 1,3‐diphenylpropynone, the η1‐ketone complex [CpRu(PPh3)2{PhC(η1‐O)C≡CPh}]+ (3a+) was isolated as its PF6– salt, and it is apparently an intermediate in the formation of the vinylidene complex. Complexes 2a+–2c+ were characterized as their BArF4– [ArF = 3,5‐bis(trifluoromethyl)phenyl] salts by NMR and IR spectroscopic methods, ESI‐MS, and EA. The single crystal X‐ray diffraction structure of 2aBArF4 is presented. These reactions represent the first examples where an internal alkyne undergoes C–C bond migration to form an η1‐vinylidene complex. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Hydrogen Migration and Vinylidene Pathway for Formation of Methane in the 193 nm Photodissociation of Propene: CH3CHCH2 and CD3CDCD2

Photodissociation channels and the final product yields from the 193 nm photolysis of propene-h6 (CH(2)=CHCH(3)) and propene-d6 (CD(2)=CDCD(3)) have been investigated, employing gas chromatography, mass spectroscopy, and flame ionization (GC/MS/FID) detection methods. The yields of methane as well as butadiene relative to ethane show considerable variations when propene-h6 or propene-d6 are photolyzed. This suggests significant variances in the relative importance of primary photolytic processes and/or secondary radical reactions, occurring subsequent to the photolysis. Theoretical calculations suggest the potential occurrence of an intramolecular dissociation through a mechanism involving vinylidene formation, accompanied by an ethylenic H-migration through the pi-orbitals. This process affects the final yields of methane-h4 versus methane-d4 with respect to other products. The product yields from previous studies of the 193 nm photolysis of methyl vinyl ketone-h6 and -d6 (CH(2)=CHCOCH(3), CD(2)=CDCOCD(3)), alternative precursors for generating methyl and vinyl radicals, are compared with the current results for propene.

Kinetic modeling of slurry propylene polymerization using rac‐ET(Ind)2ZrCl2/MAO

AbstractThe slurry homopolymerization of propylene catalyzed by the isospecific metallocene rac‐Et(Ind)2ZrCl2/MAO was investigated using a semi‐batch reactor. A full factorial design with three temperatures (50, 65 and 75°C) and four monomer partial pressures (1.5, 2.5, 3.2, and 3.8 atm) was performed. Analysis by 1H NMR revealed the formation of vinylidene, cis‐2‐butenyl and 4‐butenyl end‐groups. A kinetic model based on a coordination‐insertion mechanism was developed to predict instantaneous reaction rate, molecular weights and polymer chain ends. The kinetic rate constants were estimated using a systematic optimization strategy. The model predicts that the insertion of the first propylene molecule is rate limiting with respect to propagation. Molecular weight decreases with temperature due to the high activation energy of the main chain transfer reaction (101 kJ/mol) relative to propagation (52 kJ/mol). The percentage of butenyl end‐groups with respect to vinylidene decreases with temperature, thus predicting a lower activation energy for the β‐hydride elimination after secondary insertion (27 kJ/mol). © 2006 American Institute of Chemical Engineers AIChE J, 2006

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three ( iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene ( m/ z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl 2 group favours the diradical dechlorination mechanism. The single m/ z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.

Reactions of Diamidonaphthalene-Bridged Diiridium Tetrahydrides with Alkynes: Hydrogenation, Vinylidene Formation, and Catalytic C−C Coupling

The diamidonaphthalene-bridged diiridium tetrahydride [Ir2{μ-1,8-(NH)2C10H6}(μ-H)H3(NCMe)(PiPr3)2] (1) reacts with an excess of internal alkynes such as diphenylacetylene, 2-butyne, or 1-phenyl-1-propyne to give the diiridium(I) alkene compounds [Ir2{μ-1,8-(NH)2C10H6}{η2-Z-CH(R)CHR‘}2(PiPr3)2] (R, R‘ = Ph, 4; R, R‘ = Me, 5; and R = Ph, R‘ = Me, 6), respectively. The bis-Z-alkenyl reaction intermediate [Ir2{μ-1,8-(NH)2C10H6}(μ-H)H{Z-C(Ph)CHPh}2(NCMe)(PiPr3)2] (3) has been isolated and characterized in the case of diphenylacetylene as substrate. The reactions of diphenylacetylene and 2-butyne with the carbonyl analogue of 1, [Ir2{μ-1,8-(NH)2C10H6}(μ-H)H3(CO)(PiPr3)2] (2), afford the alkene-carbonyl compounds [Ir2{μ-1,8-(NH)2C10H6}{η2-Z-CH(R)CHR}(CO)(PiPr3)2] (R = Ph, 7; R = Me, 8). Similar reactions using silyl-substituted alkyne substrates seem to involve alkyne to vinylidene rearrangements. Thus, reaction of 1 with excess trimethylsilyl-1-propyne affords the product [Ir2{μ-1,8-(NH)2C10H6){η2-CH2C(Me)SiMe3}2(PiPr3)2] (10) through the bis-gem-alkenyl intermediate [Ir2{μ-1,8-(NH)2C10H6}(μ-H)H{CHC(Me)SiMe3}2(NCMe)(PiPr3)2] (9). A similar reaction with bis(trimethylsilyl)acetylene forms the bis-vinylidene complex [Ir2{μ-1,8-(NH)2C10H6){CC(SiMe3)2}2(PiPr3)2] (11) together with 2 equiv of H2CC(SiMe3)2. The corresponding reactions of these silyl-substituted alkynes with complex 2 give the mixed-valence compound [Ir2{μ-1,8-(NH)2C10H6}H{CHC(Me)SiMe3}(CO)(PiPr3)2] (12) and the vinylidene-carbonyl derivative [Ir2{μ-1,8-(NH)2C10H6){CC(SiMe3)2}(CO)(PiPr3)2] (13) as final products, respectively. The complexes 4−6 and 11 are effective catalysts for 1-alkyne couplings to give mixtures of Z-butenyne dimers and hexadienyne trimers.

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.

The reaction of ferrocenyl acetylene with [RuCp(PR 3)(CH 3CN) 2]PF 6 (R=Me, Ph, Cy). Formation of the first allenyl carbene complexes

The reaction of [RuCp(PR 3)(CH 3CN) 2]PF 6, where R=Ph and Cy, with ferrocenyl acetylene (HCCFc) affords the first η 2-allenyl carbene complexes [CpRu(C(Fc)-η 2-CHCCH(Fc))(PR 3)]PF 6. The new type of CC bond formation is affected by the strong electronic coupling of the ferrocenyl moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the RuC double bond. The vinylidene intermediate appears to occur only in the presence of bulky substituents of the phosphine co-ligand. For small R=Me, head-to-tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual η 3-allyl carbene complex [CpRu(C(Fc)-η 3-CHC(Fc)CHPMe 3)]PF 6. The X-ray structure of an η 2-allenyl carbene complex is presented.

Exploring the ground-state singlet potential energy surface for the H 2C CH2→H 2CC+H 2 reaction with one and three parameter hybrid density functional theory, gaussian, and complete basis set ab initio methods

Several pathways for the hydrogen molecule elimination from ethylene with the target being to obtain vinylidene were explored at the B3LYP/6-31G(d,p) theory level. It was determined that the most likely pathway in the formation of vinylidene is through 1,1-hydrogen molecule elimination from acetylene. Therefore, an extensive computational study on the 1,1-hydrogen molecule elimination from ethylene was performed with highly accurate ab initio and hybrid density functional theory methods. It was demonstrated that B1LYP produces energies that are slightly closer to the CBSQ energies when compared to traditional B3LYP energies. A new and what is believed to be a more accurate activation barrier for the reaction is suggested.

Benzene formation from acetylene on Pd(1 1 1): a high-resolution electron energy loss spectroscopy study

High-resolution electron energy loss spectroscopy (HREELS) is employed to detect the formation of benzene following acetylene adsorption on the Pd(1 1 1) surface. At low exposures (0.25 L) the resulting spectra at 253 K are due to ethylidyne formation, but at higher exposures the presence of low coverages of benzene are indicated by the appearance of the strong benzene ν 4 (720 cm −1) band. A spectral subtraction method for HREELS is introduced which reveals an essentially complete benzene spectrum. The subtraction method proves favorable in removing spectral features which arise from the competing channel of vinylidene formation. The benzene coverage as a function of acetylene exposure is in close agreement with previous thermal desorption results and consistent with scanning probe microscopy studies.

Reactivity of [Re(η 2-H 2)(CO) 2P 3] + cations with alkynes: preparation of vinylidene and propadienylidene complexes

Treatment of [Re(η 2-H 2)(CO) 2P 3] + cations with phenylacetylene leads to the displacement of H 2 and the formation of vinylidene [Re{CC(H)Ph}(CO) 2P 3] + ( 1– 3) [P=P(OEt) 3, PPh(OEt) 2 or PPh 2OEt] derivatives. Infrared and NMR data support equilibrium in solution [Re(CO) 2P 3] ++PhCCH⇄[Re{CC(H)Ph}(CO) 2P 3] + involving the unsaturated complex, free alkyne and vinylidene derivative. 1,4-Diethynylbenzene also tautomerises to the Re(I) centre, affording the [Re{CC(H)(1,4-C 6H 4CCH)}(CO) 2P 3]BF 4 [P=P(OEt) 3 or PPh(OEt) 2] vinylidene derivatives. Vinylidene complexes 1– 3 are deprotonated easily by NEt 3 to give acetylides Re(CCR)(CO) 2P 3 ( 4– 6) (R=Ph or 1,4-C 6H 4CCH), which may in turn be reprotonated with HBF 4·Et 2O to reform vinylidenes 1– 3. Acetylide complexes 4– 6 were also prepared by reacting unsaturated cations [Re(CO) 2P 3] + with lithium acetylide. Binuclear complexes {Re(CO) 2P 3} 2(μ-1,4-CCC 6H 4CC) ( 7, 8) [P=PPh(OEt) 2 or PPh 2OEt] were obtained by sequential treatment of [Re(CO) 2P 3] + cations, first with 1,4-HCCC 6H 4CCH and then with an excess of NEt 3. Propadienylidene complexes [Re(CCCPh 2)(CO) 2P 3]BF 4 ( 9, 10) [P=PPh(OEt) 2 or PPh 2OEt] were prepared by allowing [Re(η 2-H 2)(CO) 2P 3] + cations or unsaturated species [Re(CO) 2P 3] + to react with HCCC(Ph 2)OH at room temperature. The characterisation of all new complexes by IR and 1H-, 31P{ 1H}- and 13C{ 1H}-NMR spectra is also discussed.

Alkyl migration aptitudes in the vinylidene-acetylene rearrangement and isotope effect in the vinylidene formation process from a deuterium-labeled cyclopropene.

The secret of the mechanism of vinylidene rearrangements has been unlocked by the use of specifically labeled cyclopropenes under mild thermal conditions (see the Equations). (13) C labeling gives the surprising 1,2-alkyl migratory aptitude sequence Et>iPr>Me. Deuterium labeling yields the first measurement of the primary kinetic isotope effect in the ring opening of a cyclopropene to form a vinylidene.