Abstract: Conversion of methane to higher hydrocarbons by its low-temperature activation without forming undesirable carbon oxides is of great scientific and practical importance Methane can be highly activated, yielding high rates of conversion to higher hydrocarbons and aromatics (10 to 45 percent) at low temperatures (400° to 600°C), by its reaction over H-galloaluminosilicate ZSM-5 type (MFI) zeolite in the presence of alkenes or higher alkanes The methane activation results from its hydrogen-transfer reaction with alkenes

Abstract: Kinetic measurements as well as B3LYP/ and MP2/6-31G(d,p) calculations provide evidence that carbonyl oxides formed in the gas-phase ozonolysis of alkylated alkenes are an important source of OH radicals. In the gas-phase ozonolysis of propene, cis-2-butene, trans-2-butene, tetramethylethene, and isoprene, 18, 17, 24, 36, and 19% OH radicals (relative to reacted ozone, error margin ≤4%) are measured using CO as a scavenger for OH. The quantum chemical calculations show that OH radical production depends on syn positioned methyl (alkyl) groups and their interaction with the terminal O atom of a carbonyl oxide. For example, in the gas-phase ozonolysis of ethene only 5% OH radicals are measured while for a carbonyl oxide with syn-positioned methyl (alkyl) group, a much larger amount of OH radicals is formed. This is due to the fact that 1,4 H migration and the formation of an intermediate hydroperoxy alkene, that is prone to undergo OO bond cleavage, is energetically more favorable than isomerization to diox...

Abstract: This review is a summary of the mechanisms of catalytic cracking of small (C3-C6) alkanes. Most of the evidence has arisen from product distributions and kinetics of cracking of these alkanes, interpreted on the basis of solution carbocation chemistry and theoretical chemistry. Cracking of small alkanes catalyzed by solid acids such as the zeolite HZSM-5 proceeds by two mechanisms: (1) The unimolecular (protolytic cracking) mechanism, which proceeds via an alkanium ion formed by protonation of the alkane by the catalyst. This supposed transition state collapses to give either H2 and a carbenium ion or an alkane and a carbenium ion; the carbenium ions give up protons to the catalyst to form alkenes. The cracking products include methane and ethane as well as H2. (2) The classical (bimolecular) cracking mechanism, which involves carbenium ion chain carriers that react with the alkane reactant to abstract hydrides and generate carbenium ions that undergo β-scission. The products include alkanes and alkenes, but not methane, ethane, or H2. Because protolytic cracking gives alkene products, which are much stronger bases than alkanes, the alkenes become the predominant proton acceptors as conversions increase, and thus bimolecular cracking prevails at all but the lowest conversions. Protolytic cracking in the near absence of secondary reactions has been observed only for propane and n-butane at low conversions; secondary reactions appear to be generally significant for other alkanes. Although the product distributions are qualitatively understood, there are still inconsistencies in the literature of quantitative product distributions and kinetics, and more experimental work is needed with standard catalysts such as HZSM-5. Theoretical chemistry is leading to deeper understanding of the transition states, showing that cracking mechanisms involving bare carbocations are oversimplified. Rather, the catalyst surface must be included, and it has been simulated by clusters that are zeolite fragments. Surface alkoxides are more stable than surface carbenium ions, and cracking takes place by concerted bond breaking and formation. Theoretical activation energies for protolytic cracking of alkanes are close to experimental activation energies that have been corrected for the adsorption energy of the reactant, but it appears that more theoretical work (as well as better data) is required for satisfactory agreement of theory and experiment.

Abstract: The epoxidation of alkenes by peroxyl radicals in the gas phase is examined, and it is demonstrated that the activation energies for 36 epoxidation reactions between 17 alkenes and 5 peroxyl radicals correlate well to the charge transfer, or the corresponding energy decrease, in forming the peroxyalkyl adduct, or the difference between the ionization energy of the alkene and the electron affinity of the peroxyl radical. These correlations have been used to estimate five previously unmeasured epoxidation rate constants relevant to propene autoxidation.

Abstract: Electronic effects in osmylation reactions accelerated by pyridine and quinuclidine derivatives were investigated by varying the substituents on the amine ligand as well as on the alkene substrate. Ligand substituent effects were gauged by determination of the equilibrium constants for coordination of the amines to OsO4, evaluation of structural properties and reduction potentials of the amine−OsO4 complexes, and analysis of the kinetics of osmylations in the presence of the amines. Substrate substituent effects were gauged by kinetic Hammett studies using several different amine/alkene combinations. Nonlinear Hammett relationships resulting from alkene substituent effects were observed, and the deviation from a linear free energy relationship was found to depend on the structure, binding capacity, and concentration of the amine. The results were evaluated in terms of the contending “[3 + 2]” and “[2 + 2]” mechanisms currently under consideration. A change in mechanism that depends on the structural and e...

Abstract: A nickel-catalyzed method for cyclizations of electron-deficient alkenes with tethered unsaturation in the presence of organozincs was developed. Considerable flexibility in the structure of each reactive component was observed. Enones, alkylidene malonates, unsaturated β-ketoesters, and nitroalkenes participated as the electron-deficient alkene; alkynes, enones, 1,3-dienes, and aldehydes participated as the tethered unsaturation; and a variety of sp2 and sp3-hybridized organozincs, including those that possess β-hydrogens, participated as the nucleophilic component. Substrate structure, organozinc structure, and ligand structure all played a significant role in determining product selectivities. Of particular synthetic significance was the opportunity to prepare either E or Z tri- or tetrasubstituted alkenes from a common alkyne. A discussion of probable mechanisms is provided.

Abstract: Palladium(0)-catalyzed reactions of N-methanesulfonyl- or N-(arenesulfonyl)-3-alkyl-2-vinylaziridines reveal that 2,3-cis-isomers are more stable than the corresponding 2,3-trans-isomers in accord

Abstract: A significant improvement of metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions of acyclic nitrones with α,β-unsaturated carbonyl compounds is described using succinimide as a new auxiliary for the α,β-unsaturated carbonyl moiety. In the absence of a catalyst, N-crotonoylsuccinimide reacts with C,N-diphenylnitrone to give the endo-isoxazolidine, whereas in the presence of 10 mol % TiCl2(i-PrO)2 the exo-product is obtained. Four different TiCl2−TADDOLate complexes have been tested as catalysts for the 1,3-dipolar cycloaddition reactions, and the most successful catalyst was applied (5 mol %) in a series of reactions between two different alkenoylsuccinimides and three different nitrones. The crude products containing an N-acylsuccinimide moiety were converted directly into the corresponding carboxamides upon treatment with hydrazine. The 1,3-dipolar cycloaddition reactions proceed with a high degree of exo-selectivity, often >90% de, which is an improvement compared with previous experiments pe...

Abstract: The interaction of hydrocarbons with metal surfaces usually results in the formation of strongly held residues either by dehydrogenation or by polymerization or by fragmentation into monocarbon species. The extent and structure of this deposit, which may be characterized in particular by AES, SIMS and LRS, depends on the nature of the metal or the composition of the alloy used: large ensembles of the active metal atoms favour disruption of hydrocarbon molecules, which is also helped by higher temperatures and low H2 pressures. Excessive multiple bond formation with the surface is inhibited by chemisorbed H atoms and by alloying the active metal either with an inert component that interferes with the sites at which disruption occurs, or with an element that is more active in hydrogenolysis of carbon-metal bonds. Monocarbon species can homologate; species such as ethylidyne (≡C—CH3) may participate in alkene hydrogenation in the steady state; and excessively dehydrogenated forms of alkanes act as poisons for their hydrogenolysis. Partial deactivation of Pt catalysts by these species alters product selectivities for alkane reactions in useful and informative ways.

Abstract: The oligomerization of propene on H-mordenite is the predominant process proceeding very rapidly already at room temperature. This process can be conveniently studied by fast FTIR spectroscopy. However due to the rapid blockage of the pore entrances caused by the growing chains, it is not possible to obtain a full sequence of spectra documenting the steps of the oligomerization process involving not only the catalytic centres at the pore entrances but also those located in inner positions. In this paper we demonstrate that, by changing the experimental conditions, i.e. by using a temperature-programmed FTIR spectroscopy experiment, operating in the temperature range 100–300 K, the formation of the hydrogen-bonded precursor (at low temperature) and the successive oligomerization involving all the protonic centres can be followed without difficulty. It is demonstrated that under these conditions and as found in the H-ZSM-5/propene system, the oligomerization proceeds through three steeps: (i) formation of a short-lived hydrogen-bonded precursor by interaction of the alkene with both the external and internal acidic Bronsted sites; (ii) protonation of the hydrogen-bonded precursor and (iii) propagation of the chain via insertion of monomers. Although a comparison between H-ZSM5 and H-mordenite Bronsted acidity (made on the basis of the downward shifts of ν(OH) and ν(CC) stretching frequencies in the hydrogen-bonded precursor) shows virtually identical acidity, the protonation of propene proceeds faster on H-mordenite. A plausible explanation for this effect is discussed. While the main oligomerization process discussed so far leads to saturated chains, the parallel formation of minor amounts of unsaturated species can elude IR detection. It is demonstrated that UV–VIS spectroscopy is a very sensitive tool for the detection of unsaturated cationic species formed via a side reaction involving hydrogen transfer.

Abstract: Nocardia coraltina B-276 possesses a constitutive multi-component alkene monooxygenase which catalyses the epoxidation of terminal and sub-terminal alkenes The epoxygenase component of this system has been purified with an overall yield of 35% The electron paramagnetic resonance spectrum of the oxidised protein has a weak signal at g= 43, which we ascribe to rhombic iron and a free radical signal at gave= 201 Upon partial reduction with dithionite using methyl viologen as a mediator, a signal at gave= 19 appeared Upon further reduction with excess dithionite a signal at g= 15 appeared with the concomitant disappearance of the gave= 19 signal These results indicate that the epoxygenase contains a bridged dinuclear iron centre similar to that found in a variety of proteins involved in oxygen transport and activation as well as desaturation of fatty acids Analysis of the products of the reaction indicates that AM0 is capable of stereospecific epoxidation of alkenes producing the R-enantiomer in high yield, a reaction catalysed by very few oxygenase enzymes Whole cells gave lower enantiomeric excess values for the epoxide and a stereospecific epoxidase enzyme has been proposed to account for this difference Although alkene monooxygenase was not inhibited by ethyne, a potent inhibitor of soluble methane monooxygenase with which alkene monooxygenase shares many common features, it was weakly inhibited by propyne with an apparent K, value of 340 μM The mechanistic implications of these physico-chemical features of the enzyme are discussed

Abstract: The first synthesis of the title compound is described. The synthesis features an acetal-vinylsilane cyclization to stereoselectively form the cis-2,7-disubstituted oxepene ring and introduce Δ4 unsaturation. Starting with (2R,3S)-2,3-epoxypentan-1-ol (16), mixed acetal 10 is formed in five steps and 72% overall yield. Treatment of 10 with excess BCl3 in CH2Cl2 at −78 → 0 °C promotes cyclization to afford Δ4-oxepene 39 in 90% yield after deprotection of the silyl ether. Elaboration of the (E)-enyne functionality of the six-carbon side chain completes the synthesis of (+)-isolaurepinnacin.

Abstract: Carbonyl products have been identified and their formation yields measured in the gas phase reaction of ozone with unsaturated oxygenates in experiments carried out at ambient T, p = 1 atm. of purified humid air (RH = 50%) and with sufficient cyclohexane added to scavenge the hydroxyl radical. The compounds studied are the esters methyl acrylate, vinyl acetate and cis-3-hexenyl acetate, the carbonyl crotonaldehyde, the hydroxy-substituted diene linalool, the ether ethylvinyl ether and the keto-ether trans-4-methoxy-3-buten-2-one. The alkene 1-pentene was included for comparison. The nature and formation yields of the carbonyl products from this study and those measured in earlier work under the same conditions are compared to those of alkenes and are supportive of a reaction mechanism that is similar to that for the reaction of ozone with alkenes, i.e. O3 + R1R2C=CR3X → α(R1COR2 + R3XCOO) + (1 − α)(R3COX + R1R2COO), where Ri are the alkyl substituents, X is the oxygen-containing substituent (–CHO for aldehydes; –C(O)R for ketones; –C(O)OR and –OC(O)R for esters; –OH and hydroxyalkyl for alcohols; and –OR for ethers), R1COR2 is the primary carbonyl, R3COX is the other primary product and R1R2COO and R3XCOO are the carbonyl oxide biradicals. The biradicals lead to carbonyls in reactions that are also analogous to those involved in carbonyl formation from biradicals in the ozone-alkene reaction. These features make it possible to predict the nature and formation yields of the major carbonyl products of the reaction of ozone with unsaturated oxygenates that may be components of biogenic emissions.

Abstract: The yttrium hydride dimer (Cp*2YH)2 (2) reacted rapidly with 3,3-dimethyl-1,4-pentadiene in methylcyclohexane-d14 at −78 °C to form the d0 yttrium(III) pentenyl chelate complex Cp*2Y[η1,η2-CH2CH2C(CH3)2CHCH2] (4). Low-temperature 1H, 13C, and 1H NOESY NMR spectroscopy of 4 established bonding of the tethered alkene to the d0 metal center. Quantitative analysis of the NOESY time course using the Conformer Population Analysis method demonstrates that the dominant conformers in solution are interconverting pairs of chelated complexes, one in a twist-boat conformation and the other a chair conformer. No significant contribution by a free alkene conformation is required to explain the spectroscopic data. Addition of THF to pentenyl chelate 4 at −78 °C displaced the alkene and formed the yttrium(III) pentenyl THF adduct Cp*2Y[η1-CH2CH2C(CH3)2CHCH2](THF) (5), in which there is no interaction between the pendant alkene and the d0 metal center. Yttrium hydride dimer 2 also reacted with either 1,4-pentadiene or met...

Abstract: The invention relates to the use of pharmacologically valuable pyridyl alkane, pyridyl alkene and/or pyridyl alkine acid amides according to general formula (I) in the treatment of tumors or for immunosuppression.