Showing papers on "Double bond published in 2006"

Characterising the Electronic Structure of Ionic Liquids: An Examination of the 1‐Butyl‐3‐Methylimidazolium Chloride Ion Pair

Abstract: In this paper we analyse the electronic properties of gas-phase 1-butyl-3-methylimidazolium Cl ion pairs, [C(4)C(1)im]Cl, in order to deepen our understanding of ionic liquids in general. Examination of charge densities, natural bond orbitals (NBO), and delocalised molecular orbitals computed at the B3LYP and MP2/6-31(++)G(d,p) levels have enabled us to explain a number of experimental phenomena: the relative acidity of different sites on the imidazolium ring, variations in hydrogen-bond donor and acceptor abilities, the apparent contradiction of the hydrogen-bond-donor parameters for different types of solute, the low probability of finding a Cl(-) anion at the rear of the imidazolium ring and the expansion of the imidazolium ring in the presence of a strong hydrogen-bond acceptor. The unreactive but coordinating environment and large electrochemical window have also been accounted for, as has the strong electron-donating character of the carbon atoms to the rear of the ring in associated imidazolylidenes. The electronic structure of the [C(4)C(1)im](+) cation is best described by a C(4)==C(5) double bond at the rear, and a delocalised three-centre 4 e(-) component across the front (N(1)-C(2)-N(3)) of the imidazolium ring; delocalisation between these regions is also significant. Hydrogen-bond formation is driven by Coulombic stabilisation, which compensates for an associated destabilisation of the electronic part of the system. Interactions are dominated by a large positive charge at C(2) and the build up of pi-electron density above and below the ring, particularly that associated with the double bond between C(4) and C(5). The NBO partial charges have been computed and compared with those used in a number of classical simulations.

Contribution of First- versus Second-Generation Products to Secondary Organic Aerosols Formed in the Oxidation of Biogenic Hydrocarbons

Abstract: Biogenic hydrocarbons emitted by vegetation are important contributors to secondary organic aerosol (SOA), but the aerosol formation mechanisms are incompletely understood. In this study, the formation of aerosols and gas-phase products from the ozonolysis and photooxidation of a series of biogenic hydrocarbons (isoprene, 8 monoterpenes, 4 sesquiterpenes, and 3 oxygenated terpenes) are examined. By comparing aerosol growth (measured by Differential Mobility Analyzers, DMAs) and gas-phase concentrations (monitored by a Proton Transfer Reaction Mass Spectrometer, PTR-MS), we study the general mechanisms of SOA formation. Aerosol growth data are presented in terms of a "growth curve", a plot of aerosol mass formed versus the amount of hydrocarbon reacted. From the shapes of the growth curves, it is found that all the hydrocarbons studied can be classified into two groups based entirely on the number of double bonds of the hydrocarbon, regardless of the reaction systems (ozonolysis or photooxidation) and the types of hydrocarbons studied: compounds with only one double bond and compounds with more than one double bond. For compounds with only one double bond, the first oxidation step is rate-limiting, and aerosols are formed mainly from low volatility first-generation oxidation products; whereas for compounds with more than one double bond, the second oxidation step may also be rate-limiting and second-generation products contribute substantially to SOA growth. This behavior is characterized by a vertical section in the growth curve, in which continued aerosol growth is observed even after all the parent hydrocarbon is consumed.

Phenolic Reactions during Winemaking and Aging

Abstract: The reactivity of polyphenols is due to the position of the hydroxyl groups on their aromatic nuclei. Ortho-hydroxyl groups promote oxidation while meta-hydroxyl groups induce electrophilic aromatic substitution. Both hydroxylation patterns are encountered in flavonoid structures, on the B and A rings, respectively. In addition to oxidation and electrophilic aromatic substitution, flavonoids undergo nucleophilic addition on the central C ring when it is positively charged. Reactions of the A and C rings are pH-dependent. The A ring of flavonoids undergoes a polycondensation reaction mediated by an aldehyde. The products are anthocyanin and flavanol polymers and copolymers constituted of both. Flavanol polymers are not stable and rearrange into vinyl flavanols and xanthylium pigments. Vinyl flavanols can react with the positively charged C ring of anthocyanins, yielding pyranoanthocyanins, which can also be formed from components that have a reactive double bond, such as carbonyl and ethylene bonds. The positively charged C ring primarily undergoes direct reactions. Since the positive charge on the C ring of anthocyanins and flavanols is pH-dependent, their dehydration and interflavan bond cleavage reactions are also pH-dependent. This leads to flavanol–anthocyanin (F-A + ) adducts at lower pH values and anthocyanin–flavanol (A + -F) adducts above pH 3.8. Temperature seems to favor formation of the latter.

Atmospheric chemistry of N-methyl perfluorobutane sulfonamidoethanol, C4F9SO2N(CH3)CH2CH2OH: kinetics and mechanism of reaction with OH.

Abstract: Relative rate methods were used to measure the gas-phase reaction of N-methyl perfluorobutane sulfonamidoethanol (NMeFBSE) with OH radicals, giving k(OH + NMeFBSE) = (5.8 ± 0.8) × 10-12 cm3 molecule-1 s-1 in 750 Torr of air diluent at 296 K. The atmospheric lifetime of NMeFBSE is determined by reaction with OH radicals and is approximately 2 days. Degradation products were identified by in situ FTIR spectroscopy and offline GC−MS and LC−MS/MS analysis. The primary carbonyl product C4F9SO2N(CH3)CH2CHO, N-methyl perfluorobutane sulfonamide (C4F9SO2NH(CH3)), perfluorobutanoic acid (C3F7C(O)OH), perfluoropropanoic acid (C2F5C(O)OH), trifluoroacetic acid (CF3C(O)OH), carbonyl fluoride (COF2), and perfluorobutane sulfonic acid (C4F9SO3H) were identified as products. A mechanism involving the addition of OH to the sulfone double bond was proposed to explain the production of perfluorobutane sulfonic acid and perfluorinated carboxylic acids in yields of 1 and 10%, respectively. The gas-phase N-dealkylation produc...

Global Reaction Route Mapping on Potential Energy Surfaces of Formaldehyde, Formic Acid, and Their Metal-Substituted Analogues

Abstract: Global reaction route mapping of equilibrium structures, transition structures, and their connections on potential energy surface (PES) has been done for MCHO (M = H, Li, Na, Al, Cu) and HCO2M (M = H, Li). A one-after-another technique based on the scaled hypersphere search method has been successfully applied to exploring unknown chemical structures, transition structures, and reaction pathways for organometallic systems. Upon metal substitution, considerable changes of stable structures, reaction pathways, and relative heights of transition structures have been discovered, though some features are similar among the analogues. Al and Cu atoms were found to behave as very strong scissors to cut the CO double bond in MCHO. Energy profiles of the CO insertion into Li-H and Li-CH3 bonds were found to be very similar, especially around the structures where the Li atom is not directly connected with the methyl group, which indicates little effects of alkyl substitution on the reaction route topology.

Remarkable aspects of unsaturation in trinuclear metal carbonyl clusters: the triiron species Fe3(CO)n (n = 12, 11, 10, 9).

Abstract: The trinuclear iron carbonyls Fe3(CO)n (n = 12, 11, 10, 9) have been studied by density functional theory using the B3LYP and BP86 functionals. The experimentally known C2v isomer of Fe3(CO)12, namely Fe3(CO)10(μ-CO)2, is found to be the global minimum below the unbridged D3h isomer analogous to the known structures for Ru3(CO)12 and Os3(CO)12. The lowest-energy isomer found for Fe3(CO)11 is Fe3(CO)9(μ3-CO)2 with iron−iron distances in the Fe3 triangle, suggesting the one double bond (2.460 A by B3LYP and 2.450 A by BP86) and two single bonds (2.623 A by B3LYP and 2.604 A by BP86) required to give each Fe atom the favored 18-electron configuration. Two different higher-energy dibridged structures Fe3(CO)9(μ2-CO)2 are also found for Fe3(CO)11. The lowest-energy isomer found for Fe3(CO)10 is Fe3(CO)9(μ3-CO) with equivalent iron−iron distances in the Fe3 ring (2.47 A by B3LYP or BP86). The lowest-energy isomer found for Fe3(CO)9 is Fe3(CO)6(μ-CO)3 with distances in the Fe3 triangle possibly suggesting one si...

Optimization of the chemoenzymatic epoxidation of soybean oil

Abstract: The lipase Candida antarctica (Novozyme 435) immobilized on acrylic resin was used as an unconventional catalyst for in situ epoxidation of soybean oil. The reactions were carried out in toluene. The peracid used for converting TG double bonds to oxirane groups was formed by reaction of FFA and hydrogen peroxide. The reaction conditions were optimized by varying the lipase concentration, solvent concentration, molar ratio of hydrogen peroxide to double bond, oleic acid concentration, and reaction temperature. The kinetic study showed that 100% conversion of double bonds to epoxides can be obtained after 4 h. The addition of free acids was not required for the reaction to proceed to conversions exceeding 80%, presumably owing to generation of FFA by hydrolysis of soybean oil. The enzyme catalyst was found to deteriorate after repeated runs.

Equilibrium Structures for Butadiene and Ethylene: Compelling Evidence for Π-Electron Delocalization in Butadiene

Abstract: Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 A, and the formal C-C bond length is 1.454 A. For ethylene, the C=C bond length is 1.3305 A. These values appear to be good to 0.001 A. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 A and decreasing the length of the formal single bond by 0.016 A. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.

Intramolecular Pd-catalyzed carboetherification and carboamination. Influence of catalyst structure on reaction mechanism and product stereochemistry.

Abstract: The intramolecular Pd-catalyzed carboetherification of alkenes affords 2-indan-1-yltetrahydrofuran products in moderate to good yield with good to excellent levels of diastereoselectivity. The stereochemical outcome of these reactions is dependent on the structure of the Pd catalyst. Use of PCy(3) or P[(4-MeO)C(6)H(4)](3) as the ligand for Pd leads to syn-addition of the arene and the oxygen atom across the double bond, whereas use of (+/-)-BINAP or DPP-benzene affords products that result from anti-addition. The catalyst-induced change in stereochemistry is likely due to a change in reaction mechanism. Evidence is presented that suggests the syn-addition products derive from an unprecedented transannular alkene insertion of an 11-membered Pd(Ar)(OR) complex. In contrast, the anti-addition products appear to arise from Wacker-type anti-oxypalladation. Studies on analogous Pd-catalyzed intramolecular carboamination reactions, which afford 2-indan-1-ylpyrrolidines that result from syn-addition, are also described.

Amorphous silica-like carbon dioxide.

Abstract: High pressure modifies the interatomic and intermolecular interactions in condensed matter, profoundly altering the physical and chemical properties of materials. This is dramatically demonstrated in a newly discovered form of carbon dioxide, dubbed a-carbonia. This nonmolecular amorphous carbon dioxide is a high pressure modification of the CO2 molecular solid. It is a glassy material, homologous to amorphous silica (SiO2) and germania (GeO2). The discovery could initiate new research areas in the solid-state chemistry of light elements. Among the group IV elements, only carbon forms stable double bonds with oxygen at ambient conditions. At variance with silica and germania, the non-molecular single-bonded crystalline form of carbon dioxide, phase V, only exists at high pressure1,2,3,4,5,6,7,8,9. The amorphous forms of silica (a-SiO2) and germania (a-GeO2) are well known at ambient conditions; however, the amorphous, non-molecular form of CO2 has so far been described only as a result of first-principles simulations9. Here we report the synthesis of an amorphous, silica-like form of carbon dioxide, a-CO2, which we call ‘a-carbonia’. The compression of the molecular phase III of CO2 between 40 and 48 GPa at room temperature initiated the transformation to the non-molecular amorphous phase. Infrared spectra measured at temperatures up to 680 K show the progressive formation of C–O single bonds and the simultaneous disappearance of all molecular signatures. Furthermore, state-of-the-art Raman and synchrotron X-ray diffraction measurements on temperature-quenched samples confirm the amorphous character of the material. Comparison with vibrational and diffraction data for a-SiO2 and a-GeO2, as well as with the structure factor calculated for the a-CO2 sample obtained by first-principles molecular dynamics9, shows that a-CO2 is structurally homologous to the other group IV dioxide glasses. We therefore conclude that the class of archetypal network-forming disordered systems, including a-SiO2, a-GeO2 and water, must be extended to include a-CO2.

Secondary Organic Aerosol Formation from Limonene Ozonolysis: Homogeneous and Heterogeneous Influences as a Function of NOx

Abstract: Limonene has a high emission rate both from biogenic sources and from household solvents. Here we examine the limonene + ozone reaction as a source for secondary organic aerosol (SOA). Our data show that limonene has very high potential to form SOA and that NOx levels, O3 levels, and UV radiation all influence SOA formation. High SOA formation is observed under conditions where both double bonds in limonene are oxidized, but those conditions depend strongly on NOx. At low NOx, heterogeneous oxidation of the terminal double bond follows the initial limonene ozonolysis (at the endocyclic double bond) almost immediately, making the initial reaction rate limiting. This requires a high uptake coefficient between ozone and the first-generation, unsaturated organic particles. However, at high NOx, this heterogeneous processing is inhibited and gas-phase oxidation of the terminal double bond dominates. Although this chemistry is slower, it also yields products with low volatility. UV light suppresses production o...

Ozonolysis of phospholipid double bonds during electrospray ionization: a new tool for structure determination.

Abstract: Ozonolysis of double bonds is observed during the negative ion electrospray ionization of unsaturated phospholipids under conditions that produce a corona discharge. Ionic products of the ozonolysis are detected and characterized by mass spectrometry, and the mass-to-charge ratio of these species can be used to unambiguously assign the double bond position within the unsaturated fatty acid radyls. The reaction products are consistent with the gas-phase ozonolysis of desolvated phospholipid ions in an atmosphere rich in volatilized solvent. Reactions may be carried out in a conventional electrospray ionization mass spectrometer and provide a new method for the structural characterization of phospholipids.

Conjecture: Imines as Unidirectional Photodriven Molecular Motors—Motional and Constitutional Dynamic Devices

Abstract: Compounds containing the C==N group, such as imines and their derivatives, may undergo syn-anti isomerization by two different routes: 1) photochemically, by out-of-plane rotation around the carbon-nitrogen double bond through a "perpendicular" form, and 2) thermally, by in-plane nitrogen inversion through a "linear" transition state. When the two interconversions occur in sequence, a full, closed process is accomplished, restoring the initial state of the system along two different steps. In a chiral imine-type compound, for example, with an asymmetric center next to the C==N function, photoinduced rotation may be expected to occur in one sense in preference to the opposite one. Thus, photoisomerization followed by thermal isomerization in a chiral imine compound generates unidirectional molecular motion. Generally, imine-type compounds represent unidirectional molecular photomotors converting light energy into mechanical motion. As they are also able to undergo exchange of the carbonyl and amine partners, they present constitutional dynamics. Thus, imine-type compounds are double dynamic, motional, and constitutional devices.

Macrocyclic inhibitors of hepatitis c virus

Abstract: Inhibitors of HCV replication of formula (I) and the N-oxides, salts, or stereoisomers thereof, wherein each dashed line represents an optional double bond; X is N, CH and where X bears a double bond it is C; R1 is -OR6, -NH-SO2R7; R2 is hydrogen, and where X is C or CH, R2 may also be C1-6alkyl; R3 is hydrogen, C1-6alkyl, C1-6alkoxyC1-6alkyl, or C3-7cycloalkyl; n is 3, 4, 5, or 6; R4 and R5 taken together with the nitrogen atom to which they are attached form a bicyclic ring system selected from formula (II) wherein said ring system may optionally be substituted with 1-3 substituents; R6 is hydrogen; aryl; Het; C3-7cycloalkyl optionally substituted with C1-6alkyl; or C1-6alkyl optionally substituted with C3-7cycloalkyl, aryl or with Het; R7 is aryl; Het; C3-7cycloalkyl optionally substituted with C1-6alkyl; or C1-6alkyl optionally substituted with C3-7cycloalkyl, aryl or with Het; aryl is phenyl or naphthyl, each of which may be optionally substituted with 1-3 substituents; Het is a 5 or 6 membered saturated, partially unsaturated or completely unsaturated heterocyclic ring containing 1 to 4 heteroatoms each independently selected from N, O or S, and being optionally substituted with 1-3 substituents pharmaceutical compositions containing compounds (I) and processes for preparing compounds (I). Bioavailable combinations of the inhibitors of HCV of formula (I) with ritonavir are also provided.

What Factors Influence the Ratio of C¿H Hydroxylation versus C¿C Epoxidation by a Nonheme Cytochrome P450 Biomimetic?

Abstract: Density functional calculations on a nonheme biomimetic (Fe=O(TMCS+) have been performed and its catalytic properties versus propene investigated Our studies show that this catalyst is able to chemoselectively hydroxylate C=H bonds even in the presence of C=C double bonds This phenomenon has been analyzed and found to occur due to Pauli repusions between protons on the TMCS ligand with protons attached to the approaching substrate The geometries of the rate determining transition states indicate that the steric hindrance is larger in the epoxidation transition states than in the hydroxylation ones with much shorter distances; hence the hydroxylation pathway is favored over the epoxidation Although, the reactant experiences close lying triplet and quintet spin states, the dominant reaction mechanism takes place on the quintet spin state surface; ie, Fe=O(TMCS)+ reacts via single-state reactivity Our calculations show that this spin state selectivity is the result of geometric orientation of the transition state structures, whereby the triplet ones are destabilized by electrostatic repulsions between the substrate and the ligand while the quintet spin transition states are aligned along the ideal axis The reactivity patterns and geometries are compared with oxoiron species of dioxygenase and monoxygenase enzymes Thus, Fe=O(TMCS)+ shows some similarities with P450 enzyme reactivity: it chemoselectively hydroxylates C=H bonds even in the presence of a C=C double bond and therefore is an acceptable P450 biomimetic However, the absolute barriers of substrate oxidation by Fe=O(TMCS)+ are higher than the ones obtained with heme enzymes, but the chemoselectivity is lesser affected by external perturbations such as hydrogen bonding of a methanol molecule toward the thiolate sulfur or a dielectric constant This is the first oxoiron complex whereby we calculated a chemoselective hydroxylation over epoxidation in the gas phase

Ruthenium‐Catalyzed Cycloaddition of 1,6‐Diynes and Nitriles under Mild Conditions: Role of the Coordinating Group of Nitriles

Abstract: In the presence of a catalytic amount of [Cp*RuCl(cod)] (Cp*=pentamethylcyclopentadienyl, cod=1,5-cyclooctadiene), 1,6-diynes were allowed to react chemo- and regioselectively with nitriles bearing a coordinating group, such as dicyanides or alpha-halonitriles, at ambient temperature to afford bicyclic pyridines Careful screening of nitrile components revealed that a C[triple chemical bond]C triple bond or heteroatom substituents, such as methoxy and methylthio groups, proved to act as the coordinating groups, whereas C==C or C==O double bonds and amino groups failed to promote cycloaddition This suggests that coordinating groups with multiple pi-bonds or lone pairs are essential for the nitrile components

Direct estimate of the strength of conjugation and hyperconjugation by the energy decomposition analysis method.

Abstract: The intrinsic strength of pi interactions in conjugated and hyperconjugated molecules has been calculated using density functional theory by energy decomposition analysis (EDA) of the interaction energy between the conjugating fragments. The results of the EDA of the trans-polyenes H2C=CH-(HC=CH)n-CH=CH2 (n = 1-3) show that the strength of pi conjugation for each C=C moiety is higher than in trans-1,3-butadiene. The absolute values for the conjugation between Si=Si pi bonds are around two-thirds of the conjugation between C=C bonds but the relative contributions of DeltaE pi to DeltaE orb in the all-silicon systems are higher than in the carbon compounds. The pi conjugation between C=C and C=O or C=NH bonds in H2C=CH--C(H)=O and H2C=CH-C(H)=NH is comparable to the strength of the conjugation between C=C bonds. The pi conjugation in H2C=CH-C(R)=O decreases when R = Me, OH, and NH2 while it increases when R = halogen. The hyperconjugation in ethane is around a quarter as strong as the pi conjugation in ethyne. Very strong hyperconjugation is found in the central C-C bonds in cubylcubane and tetrahedranyltetrahedrane. The hyperconjugation in substituted ethanes X3C-CY3 (X,Y = Me, SiH3, F, Cl) is stronger than in the parent compound particularly when X,Y = SiH3 and Cl. The hyperconjugation in donor-acceptor-substituted ethanes may be very strong; the largest DeltaE pi value was calculated for (SiH3)3C-CCl3 in which the hyperconjugation is stronger than the conjugation in ethene. The breakdown of the hyperconjugation in X3C-CY3 shows that donation of the donor-substituted moiety to the acceptor group is as expected the most important contribution but the reverse interaction is not negligible. The relative strengths of the pi interactions between two C=C double bonds, one C=C double bond and CH3 or CMe3 substituents, and between two CH3 or CMe3 groups, which are separated by one C-C single bond, are in a ratio of 4:2:1. Very strong hyperconjugation is found in HC[triple bond]C-C(SiH3)3 and HC[triple bond]C-CCl3. The extra stabilization of alkenes and alkynes with central multiple bonds over their terminal isomers coming from hyperconjugation is bigger than the total energy difference between the isomeric species. The hyperconjugation in Me-C(R)=O is half as strong as the conjugation in H2C=CH-C(R)=O and shows the same trend for different substituents R. Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of sigma interactions and electrostatic forces may compensate for the hyperconjugative effect.

A Metal-Free Approach to the Synthesis of Indoline Derivatives by a Phenyliodine(III) Bis(trifluoroacetate)-Mediated Amidohydroxylation Reaction

Abstract: A novel approach to the synthesis of indoline derivatives is presented. The key cyclization step features the phenyliodine(III) bis(trifluoroacetate)- (PIFA-) mediated formation of a N-acylnitrenium ion and its succeeding intramolecular trapping by the olefin fragment. In addition, difunctionalization of the alkene moiety is achieved since the in situ generation of an additional hydroxy group at the terminal position of the original double bond accompanies the intramolecular C-N bond formation.

A class of nonplanar conjugated compounds with aggregation-induced emission: structural and optical properties of 2,5-diphenyl-1,4-distyrylbenzene derivatives with all cis double bonds.

Abstract: We have studied the structural and optical properties of four 2,5-diphenyl-1,4-distyrylbenzene derivatives with all cis double bonds. These compounds belong to a class of nonplanar conjugated compounds possessing a typical Aggregation-Induced Emission (AIE) property that has no emission in solution but intense emission in crystal. The four molecules are packed in different stacking modes with different intermolecular interactions, resulting in different crystalline state photoluminescence (PL) efficiency. The torsional molecular configuration increases the intermolecular distances effectively in the crystalline state, which decreases the difference of the optical properties from the frozen isolated molecules to the crystalline state. The Stokes shifts of these compounds are very large and the PL spectra have only one broad emission band with poor structure, due to the relatively large configuration difference between the ground state and the first singlet excited state, and the abundant vibration energy l...

Homogeneous versus heterogeneous catalytic reactions to eliminate organics from waste water using H2O2

Abstract: Homogeneous (Cu2+ ions) and heterogeneous (Cu2+-pillared clay) Fenton-like catalysts have been compared in the conversion of p-coumaric acid. The performances of the two classes of catalysts are similar for an analogous amount of copper, but there are some relevant differences in terms of (i) the presence of an induction time, (ii) the turnover frequency, (iii) the efficiency in the use of H2O2, (iv) the initial attack of p-coumaric acid (hydroxylation on the aromatic ring or oxidative attack on the double bond of the lateral chain), and (v) the effect of dissolved oxygen on the removal of total organic carbon (TOC). These differences were interpreted in terms of reaction network of generation of radical oxygen species and of organics conversion. The possible formation of a surface peroxo adduct coordinated to a copper binulcear site was also evidenced for the solid heterogeneous catalyst.

Aluminium(III) trifluoromethanesulfonate as an efficient catalyst for the intramolecular hydroalkoxylation of unactivated olefins: experimental and theoretical approaches.

Abstract: The Al(OTf)(3)-catalyzed cycloisomerization of unactivated unsaturated alcohols was studied from experimental and theoretical points of view. A series of cyclic ethers was obtained in excellent yields and regioselectivities. This catalyst system provides one of the most straightforward routes to cyclic ethers with Markovnikov-type regioselectivity under mild conditions. Theoretical and NMR studies were carried out in order to better determine the mechanism of this reaction. The NMR studies were in agreement with preferential complexation of Al(OTf)(3) to the oxygen atom of the unsaturated alcohol, but did not exclude complexation to the double bond of the alcohol. Theoretical calculations indicated strong acidification of the hydroxyl proton when Al(OTf)(3) was complexed to the alcohol oxygen atom. A plausible catalytic cycle for the Al(OTf)(3)-catalyzed intramolecular hydroalkoxylation of unactivated olefins is proposed.

Cobalt-mediated cyclic and linear 2:1 cooligomerization of alkynes with alkenes: a DFT study.

Abstract: The mechanism of the cobalt-mediated [2 + 2 + 2] cycloaddition of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) has been studied by means of DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct “collapse” pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co−C σ-bond rather than inter- or intramolecular [4 + 2] cycloaddition. The resulting seven-membered metallacycle 7 is a key intermediate which leads to either CpCo-complexed cyclohexadiene 5 or hexatriene 13. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization observed occasionally in these reactions. With aromatic double bonds, a C−H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed investigations of high- and low-spin potential energy su...

Acetonitrile covalent adduct chemical ionization mass spectrometry for double bond localization in non-methylene-interrupted polyene fatty acid methyl esters.

Abstract: Covalent adduct chemical ionization (CACI) using a product of acetonitrile self-reaction, (1-methyleneimino)-1-ethenylium (MIE; CH2CN+CH2), has been investigated as a method for localizing double bonds in a series of 16 non-methylene-interrupted fatty acid methyl esters (NMI-FAME) of polyenes with three and more double bonds. As with polyunsaturated homoallylic (methylene-interrupted) FAME and conjugated dienes, MIE (m/z 54) reacts across double bonds to yield molecular ions 54 mass units above the parent analyte. [M + 54]+ ions of several 20- and 22-carbon FAME that include one double bond in the C2−C3 position separated by two to five methylene units from a three, four, or five C homoallylic system dissociated according to rules for the homoallylic system, with an additional fragment corresponding to cleavage between the lone double bond and the carboxyl group and defining the position of the lone double bond. Triene FAME with both methylene and ethylene interruption yielded characteristic fragments dis...