Allyl Species Research Articles

Thermal Chemistry of C3 Allyl Groups on Pt(111)

The thermal chemistry of allyl iodide and of allyl bromide adsorbed on Pt(111) was investigated by temperature-programmed desorption (TPD) and reflection−absorption infrared (RAIRS) spectroscopies. On the clean Pt(111) surface, both allyls were found to bond to the surface via the halogen atom in a gauche conformation and with the C−C−C plane close to parallel to the surface. In the presence of surface hydrogen, allyl iodide was found to tilt backward such that the C−C−C plane adopts a more vertical orientation on the surface; in this more upright geometry, both cis and gauche isomers were identified. With both molecules the facile thermally activated cleavage of the carbon−halogen bond results in the formation of C3 allylic species on the surface. A η1 allylic moiety was identified by RAIRS in the high-exposure regime of allyl iodide after annealing to 200 K, and the formation of a second η3 allylic intermediate was inferred from TPD experiments with coadsorbed hydrogen and deuterium. At low surface conc...

Remarkably chemoselective indium-mediated coupling en route to the C21–C40 acyclic portion of the azaspiracids

A densely functionalized acyclic intermediate representing the C21–C40 portion of the novel marine natural product azaspiracid was prepared via a chemoselective indium-mediated coupling between C21–C27 and C28–C40 intermediates. Although the coupling partners contained aldehyde, azide, ketone, enone, and lactone functionalities, the C21–C27 allylic species added selectively to the aldehyde of a C28–C40 intermediate under aqueous indium-mediated conditions. The C21–C27 and C35–C40 fragments were prepared in a divergent fashion from a common syn-1,3-dimethyl synthon 9 .

ChemInform Abstract: Organometallic Chemistry and Surface Science: Mechanistic Models for the Fischer-Tropsch Synthesis

Abstract The Fischer–Tropsch synthesis is an industrially important process for the conversion of synthesis gas (CO/H2) into hydrocarbons and oxygenates. Synthesis gas can be obtained from coal or natural gas. Organometallic model complexes and surface science techniques have been widely used to obtain mechanistic information about this heterogeneous process. A review of the mechanisms for the Fischer–Tropsch synthesis and the evidence for these mechanisms is presented. It is generally accepted that the Fischer–Tropsch reaction may be viewed as a polymerisation of surface methylene (CH2) species, which are formed by the dissociation and hydrogenation of CO on the catalyst surface. The alkyl mechanism proposes that the reaction is initiated by the formation of a surface methyl species, and that chain growth takes place by successive insertions of methylene into the metal-alkyl bond. The alkenyl mechanism proposes that the formation of a surface vinyl species (CHCH2) initiates chain formation, and that chain growth is facilitated by methylene insertion into the metal-vinyl bond to form an allyl species (CH2CHCH2). The allyl species isomerises to form a surface alkenyl (CHCHCH3) which may propagate further. These mechanisms are discussed.

ChemInform Abstract: Palladium-Catalyzed Intramolecular Nucleophilic Addition of Allylic Species, Generated from Allene, to Aryl Aldehydes and Ketones.

2-Haloaryl aldehydes and ketones react with allene (1 atm) in the presence of 2 mol% of a non-phosphine cyclopalladated catalyst to afford cyclopentenols and cyclohexenols via nucleophilic cyclisation of intermediate π-allylpalladium species. A possible mechanism for this unusual reactivity is suggested.

The hydroboration of propargyl bromide. Simple one-Pot three-component routes to (Z)-1-bromoalk-1-en-4-ols and to anti-homoallylic alcohols.

The hydroboration of propargyl bromide with dialkylboranes takes place regioselectively to give 3-bromoprop-1-en-1-yl dialkylboranes 13 which, upon quaternization with bromide ion, undergo a series of transformations into a number of allylic boron species. By a suitable choice of the experimental conditions it is possible to trap the reaction intermediates with aldehydes and to steer the process toward either the synthesis of (Z)-1-bromoalk-1-en-4-ols 6 or anti-homoallylic alcohols 8. Two one-pot three-component processes were developed based on a sequence of four reactions; preparation of dialkylborane and hydroboration of propargyl bromide are the first steps. Then, quaternization with TEBABr may be carried out either in the presence of the aldehyde when (Z)-1-bromoalk-1-en-4-ols 6 are requested, or in the absence of the aldehyde in order to allow the formation of gamma-substituted allyl borane 18 which, successively, adds to the aldehyde affording anti-homoallylic alcohols 8.

Photochemistry of (η5-C5H5)(η5-C4H4N)Fe and (η5-C5H5)(η1-N-C4H4N)Fe(CO)2 in Low-Temperature Matrixes and Room-Temperature Solution. Evidence for a Photoinduced Haptotropic Shift of the π-Coordinated Pyrrolyl Ligand

Azaferrocene, (η5-C5H5)(η5-C4H4N)Fe, undergoes a η5 → η1 haptotropic shift of the pyrrolyl ligand upon long-wavelength photolysis (λexc > 495 nm) both in alkane solvents at room temperature and in frozen matrixes at 12 K. Room-temperature photolysis (λexc > 495 nm) in CO-saturated cyclohexane solution generated (η5-C5H5)(η1-N-C4H4N)Fe(CO)2. Irradiation with λexc = 532 nm also produced an allyl monocarbonyl species, exo-(η5-C5H5)(η3-C-C4H4N)Fe(CO), identified by IR spectroscopy. In CO-doped matrixes at 12 K both (η5-C5H5)(η1-N-C4H4N)Fe(CO) and (η5-C5H5)(η1-N-C4H4N)Fe(CO)2 are formed following broad-band irradiation (λexc > 495 nm) of (η5-C5H5)(η5-C4H4N)Fe, in a ratio dependent on the concentration of CO in the matrix. Initial irradiation with λexc = 538 nm followed by broad-band photolysis (λexc > 495 nm) in CO-doped matrixes formed additional monocarbonyl species, exo-(η5-C5H5)(η3-C-C4H4N)Fe(CO), and a species absorbing at 1962 cm-1, which is either the appropriate endo-isomer or aza-allyl species. Laser ...

Study of MoO 3 (0 1 0) surface clusters by ab initio HF approaches

The clusters Mo 2O 11H 10, Mo 3O 16H 14 and Mo 7O 32H 22 of MoO 3 (0 1 0) surface were studied by using ab initio HF approaches. The geometry of each cluster was fully optimized at RHF or UHF/MINI/ECP-SBK level of theory. The bonding properties, electronic structures and orbital populations of structurally different oxygens were obtained. It was shown that there exist some differences in the bonding properties between Mo atoms and the different oxygens. The symmetrically bridging oxygens exhibit more ionic feature while the terminal oxygens are more covalent. Moreover, information on explaining the active sites for the insertion of oxygen into allylic species in the second step of the oxidation processes of propylene to acrolein could be inferred from the frontier orbital populations of MoO 3 surface cluster.

Investigation of the Adsorption and Reactions of Thiophene on Sulfided Cu, Mo, and Rh Catalysts

The surface chemistry of thiophene (C4H4S) on sulfided Cu/Al2O3, Mo/Al2O3, and Rh/Al2O3 catalysts has been investigated under well-defined conditions using infrared (IR) spectroscopy and temperature programmed desorption (TPD). The results of these experiments have been found to correlate nicely with CO chemisorption measurements of site densities and thiophene hydrodesulfurization (HDS) activities measured in a flow reactor system. In agreement with the literature, the catalysts were found to have dramatically different thiophene HDS activities increasing in the following order: sulfided Cu < sulfided Mo < sulfided Rh. The trend of HDS activities is mirrored in similar trends for the site densities and turnover frequencies for the sulfide catalysts. IR spectra of adsorbed thiophene on the sulfided Cu, Mo, and Rh catalysts at low temperatures show that the amount of thiophene adsorbed on cus metal sites of the sulfide catalysts correlates with the CO chemisorption estimates of site densities. Thiophene reactivity on the catalysts differs significantly, with reaction in thiophene/H2 mixtures observed on the sulfided Mo and Rh catalysts annealed at 300 and 500 K, respectively, and not at all on a sulfided Cu catalyst annealed up to 700 K. Strongly adsorbed species produced on the sulfided Cu, Mo, and Rh catalysts during the annealing sequence were subjected to TPD experiments that yielded C4 hydrocarbons and H2S, with the predominant C4 product being butenes. IR and TPD measurements of 1,3-butadiene and 1-butene on sulfided Rh/Al2O3 catalysts, both in the pure gas and in gas/H2 mixtures, provide evidence for the tentative assignment of strongly adsorbed species produced by cleavage of C−S bonds in thiophene to be a σ-bonded allyl species with C4H7 stoichiometry.

Zirconocene Allyl Complexes: Dynamics in Solution, Reaction with Aluminum Alkyls, B(C6F5)3-Induced Propene Insertion, and Density-Functional Calculations on Possible Formation and Reaction Pathways

The complex (C5H5)2Zr(Me)(methallyl) (1) was prepared and studied as a model for Zr-bound allyl species which are likely to arise in zirconocene-based polymerization systems. 1 undergoes allyl−alky...

Electron paramagnetic resonance study of interaction between MoO3 and propene

Interaction of MoO3 with propene at the temperature range 420–520 K results in the appearance of several EPR signals. Four of them have been assigned to different Mo(V) centres, one to localised conduction electrons. The analysis of the signals' parameters and the sequence of their appearance allows the conclusion that the allyl species formed in the first stage of interaction are adsorbed in a certain distance from the simultaneously created Mo(V). This is an important finding, modifying the existing hypotheses on the reaction mechanism. In the next stage of reduction the precursors of shear structures are formed due to the removal of lattice oxygen by the desorbing oxidation products.

Iron antimony oxide selective oxidation catalysts – an inelastic neutron scattering study

The inelastic neutron scattering spectra of allyl iodide (3-iodopropene, CH2=CHCH2I) and allylpalladium chloride, and allyl iodide dosed onto activated iron(III) oxide and iron antimonate catalysts at room temperature have been determined to characterise the adsorbed allyl species. The spectra are energy loss vibrational spectra in the range 16–4000 cm−1. Allyl iodide is not decomposed on the surface and interacts through the localised C = C bond, more strongly with iron antimonate than with iron(III) oxide.

Palladium catalyzed intramolecular nucleophilic addition of allylic species, generated from allene, to aryl aldehydes and ketones

2-Haloaryl aldehydes and ketones react with allene (1 atm) in the presence of 2 mol% of a non-phosphine cyclopalladated catalyst to afford cyclopentenols and cyclohexenols via nucleophilic cyclisation of intermediate π-allylpalladium species. A possible mechanism for this unusual reactivity is suggested.

Homogeneous organolanthanide catalysts for the selective polymerization of styrene without aluminium cocatalysts

Anionic or neutral allylic samarium or neodymium species catalyse the polymerization of styrene (catalyst/styrene ratio = 1:1000) without addition of a cocatalyst. Random syndiotactic-rich material is obtained from tetra-allyl-lanthanides, whereas neutral trisallyl-lanthanides or anionic ansa-bis(cyclopentadienyl)bisallyl-lanthanides afford isotactic-rich polystyrene.

Propane ammoxidation on an Al–Sb–V–W-oxide catalyst. A mechanistic study using the TAP-2 reactor system

The reaction mechanism of propane ammoxidation was studied on an Al–Sb–V–W-oxide catalyst using a TAP-2 reactor system. Analyses of the responses from both high-speed pulse transients with reactants and TPD experiments were performed. Since the ammoxidation process with three reactants proceeds from propane to acrylonitrile over propylene as an intermediate, the experiments comprised oxidation of propylene, oxidation of ammonia, ammoxidation of propylene, oxidation of propane, and ammoxidation of propane. The results show that propane is irreversibly adsorbed at the surface forming propylene, which desorbs. Propylene then readsorbs forming an intermediate allyl species, which reacts with lattice oxygen to give acrolein. Acrolein is unstable and some of it reacts further to produce either carbon oxides, or, acrylonitrile. Formation of the nitrile occurs by adsorbed acrolein reacting with an NH x species. The latter species is short-lived and reacts competitively to form N 2, N 2O and NO. Lattice oxygen plays an important role in the pathway to acrylonitrile. However, weakly adsorbed oxygen species are also present at the catalyst surface, and these species participate in degradation routes producing waste products. Consideration of the mechanistic scheme which is derived from the experimental results shows the possibility to achieve improvement of the ammoxidation process by using either a recirculating solids reactor, or a high propane/oxygen ratio in the feed.

Secondary Deuterium Kinetic Isotope Effects in Irreversible Additions of Hydride and Carbon Nucleophiles to Aldehydes: A Spectrum of Transition States from Complete Bond Formation to Single Electron Transfer

The competitive kinetics of hydride and organometallic additions to benzaldehyde-H and -D were determined at −78 °C using LiAlH4, LiBEt3H, NaBH4, LiBH4, LiAl(O-tert-butoxy)3H, NaB(OMe)3H, NaB(OAc)3H (at 20° C). methyl, phenyl, and allyl Grignard, and methyl-, phenyl-, n-butyl-, tert-butyl-, and allyllithium. The additions of hydride were found to have an inverse secondary deuterium kinetic isotope effects in all cases, but the magnitude of the effect varied inversely with the apparent reactivity of the hydride. In the additions of methyl Grignard reagent and of methyllithium and phenyllithium, inverse secondary deuterium isotope effects were observed; little if any isotope effect was observed with phenyl Grignard or n-butyl- and tert-butyllithium. With allyl Grignard and allyllithium, a normal secondary deuterium kinetic isotope effect was observed. The results indicate that rate-determining single-electron transfer occurs with allyl reagents, but direct nucleophilic reaction occurs with all of the other reagents, with the extent of bond formation dependent on the reactivity of the reagent. In the addition of methyllithium to cyclohexanecarboxaldehyde, a less inverse secondary deuterium kinetic isotope effect was observed than that observed in the addition of methyllithium to benzaldehyde, and allyllithium addition to cyclohexanecarboxaldehyde had a kinetic isotope effect near unity. The data with organometallic additions, which are not incompatible with observations of carbonyl carbon isotope effects, suggest that electrochemically determined redox potentials which indicate endoergonic electron transfer with energies less than ca. 13 kcal/mol allow electron-transfer mechanisms to compete well with direct polar additions to aldehydes, provided that the reagent is highly stabilized, like allyl species. Methyl- and phenyllithium and methyl and phenyl Grignard reagents are estimated to undergo electron transfer with endoergonicities greater than 30 kcal/mol with benzaldehyde, so these react by direct polar additions. A working hypothesis is that butyllithium reagents undergo polar additions, despite redox potentials which indicate less than 13 kcal/mol endoergonic electron transfer, because of the great exoergonicity associated with the two-electron addition, which is responsible for a low barrier for polar reactions.

A Cp 2TiCl 2-Me 3Al (1 : 4) reagent system: An efficient reagent for generation of allylic titanocene derivatives from vinyl halides, vinyl ethers and carboxylic esters

The reagent prepared in advance by stirring a mixture of a Cp 2TiCl 2-Me 3Al (a 1 : 4 ratio) reagent system in toluene for 3 days is found to be effective in generating allylic titanocene on treatment with vinyl halides, vinyl ethers and carboxylic esters in THF. The process for the generation of an allylic titanocene species from these starting materials was suggested to proceed through a formation of titanacyclobutane and the subsequent elimination of the halogen or alkoxyl group. The generated allylic titanocene intermediate was proved to be an allylic Ti(IV) species and showed the typical reactivity to carbonyl compounds.

Palladium(II) allyl complexes with nitrogen–sulfur bidentate ligands. Substituent effects in the mechanism of allyl amination

The reactivity of palladium(II) allyl complexes containing the nitrogen–sulfur bidentate ligand N–SR (N–SR=2-(phenylthiomethyl)pyridine, 2-(phenylthiomethyl)-6-methylpyridine, 2-( tert-butylthiomethyl)pyridine) was studied in CHCl 3 in the presence of the activated olefin fumaronitrile (fn). The stepwise mechanism involves a fast pre-equilibrium in which the N–SR ligand is displaced by the amine with formation of an inert bis-amino allyl species and concomitant rate-determining bimolecular attack of the amine on the coordinated allyl moiety to give the allylamine and the olefin-stabilized Pd(0) complexes [Pd( η 2-fn)(N–SR)]. The influence of the substituents at the allyl fragment and at the nitrogen–sulfur ligand is rationalized together with the fluxional behavior in solution.

Novel palladium(II) allyl complexes with nitrogen-sulfur donor bidentate ligands. Mechanism of allyl amination of [Pd( η3-allyl)-(N-SR)]ClO 4 (allyl = C 3H 5; N-SR = C 5H 4N-2-CH 2SR, R = C 6H 5, C 2H 5) in the presence of activated olefins. X-ray structure determination and fluxional behavior

The reactions of [Pd(η 3-C 3H 5)(C 5H 4-N-2-CH 2SR) (R = C 6H 5, C 2H 5) with amines in the presence of fumaronitrile (in), involving the formation of allylamines, were studied kinetically in CHCl 4, by UV-Vis techniques. The proposed stepwise mechanism involves a fast pre-equilibrium in which the mixed N-SR ligand is displaced by the amine, giving an inert bis-amino allyl species. The concomitant rate-determining bimolecular attack by the amine yields the allylamine and the corresponding Pd(O) complexes [Pdη 3-fn) (C 3H 54N-2-CH 2SR)]. The solid state structure of complex was determined by X-ray crystallography. A preliminary 1H-NMR study of the fluxional manifestations was carried out. The apparent rotation of the allyl moiety is operative at the lowest temperature increasing the temperature promotes the breaking and rearangement of the Pd-S bond and, finally, the η ′-η ′-η ′ rearrangement of the allyl fragment.

Strained rings to assess the “ endo preference” of 2-alkyl branched 2-alkenylpotassium species

Upon deprotonation using trimethylsilylmethylpotassium or butyllithium in the presence of potassium tert-butoxide, two allylic organopotassium species ( 2) are generated from the ( E)-isomeric caryophyllene and the ( Z)-isomeric isocaryophyllene. Torsional equilibration occurs slowly at −50 °C (in tetrahydrofuran) and rapidly at 0 °C (in hexane) to afford endo- 2/exo- 2 mixtures of about 95:5 (as evidenced by the isomeric composition of the trapping products 3). The relative thermodynamic stabilities of the two stereoisomeric organometallic intermediates can be derived by mere superposition of ring-strain differences and the “ endo preference” of open-chain 2-alkyl-2-alkenylpotassiums.

Plasma Copolymerization of Allyl Alcohol/1,7-Octadiene: Surface Characterization and Attachment of Human Keratinocytes

Plasma copolymers (PCPs) of allyl alcohol/1,7-octadiene were prepared and characterized using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and contact angle measurements. The use of a hydrocarbon diluent in the monomer feed allowed the deposition of films with controlled concentrations of hydroxyl groups. ToF-SIMS data have shown these PCPs to be rich in unsaturation. Dimeric allyl alcohol species were observed in the ToF-SIMS spectra, which reduced in intensity with the proportion of hydrocarbon diluent in the monomer feed. Contact angle measurements have shown the nondispersive component of surface energy to increase with hydroxyl group concentration, while the dispersive component remained approximately constant. Human keratinocytes were cultured on these PCP surfaces and collagen I. The level of keratinocyte attachment over 24 h was measured. Keratinocyte attachment increased with hydroxyl group concentration (the nondispersive component of surface...