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PubMed2003

1-trifluoromethylvinylsilane as a CF2=C(-)-CH2+ synthon: synthesis of functionalized 1,1-difluoro-1-alkenes via isolable 2,2-difluorovinylsilanes.

Various 1,1-difluoro-1-alkenes such as monosubstituted 1,1-difluoro-1-alkenes, 2-bromo-1,1-difluoro-1-alkenes, and 3,3-difluoroallylic alcohols are synthesized in two simple steps from 1-trifluoromethylvinylsilane: (i) its SN2' reaction with nucleophiles to construct 2,2-difluorovinylsilanes and (ii) the subsequent substitution of electrophiles for the vinylic silyl group. The combination of these two processes allows a one-pot synthesis of the functionalized 1,1-difluoro-1-alkenes starting from 1-trifluoromethylvinylsilane, which functions as a CF2=C(-)-CH2+ synthon.

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PubMed2005

Using correlations to compare additions to alkenes: homogeneous hydrogenation by using Wilkinson's catalyst.

Plots of the logarithms of relative rates of homogeneous catalytic hydrogenation of alkenes (log k(rel) values) by using Wilkinson's catalyst versus their ionization potentials (IPs) and versus their lowest unoccupied molecular orbital energy levels (LUMOs) display good-to-excellent correlations. The correlations indicate that the rate-determining step of this reaction is a nucleophilic addition to the alkene double bond, which is dependent upon both electronic effects and steric effects. This conclusion is in agreement with only two of three previously proposed mechanisms for the reaction, effectively ruling out one in which the rate-determining step involves electrophilic addition to the alkene. Characteristics of the analysis using these correlations are compared and contrasted with other additions to alkenes, such as the Wacker oxidation, to probe patterns in transition state characteristics.

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PubMed2004

Electronic state of push-pull alkenes: an experimental dynamic NMR and theoretical ab initio MO study.

The (1)H and (13)C NMR spectra of a number of push-pull alkenes were recorded and the (13)C chemical shifts calculated employing the GIAO perturbation method. Of the various levels of theory tried, MP2 calculations with a triple-zeta-valence basis set were found to be the most effective for providing reliable results. The effect of the solvent was also considered but only by single-point calculations. Generally, the agreement between the experimental and theoretically calculated (13)C chemical shifts was good with only the carbons of the carbonyl, thiocarbonyl, and cyano groups deviating significantly. The substituents on the different sides of the central C=C partial double bond were classified qualitatively with respect to their donor (S,S < S,N < N,N) and acceptor properties (C identical with N < C=O < C=S) and according to the ring size on the donor side (6 < 7 < 5). The geometries of both the ground (GS) and transition states (TS) of the restricted rotation about the central C=C partial double bond were also calculated at the HF and MP2 levels of theory and the free energy differences compared with the barriers to rotation determined experimentally by dynamic NMR spectroscopy. Structural differences between the various push-pull alkenes were reproduced well, but the barriers to rotation were generally overestimated theoretically. Nevertheless, by correlating the barriers to rotation and the length of the central C=C partial double bonds, the push-pull alkenes could be classified with respect to the amount of hydrogen bonding present, the extent of donor-acceptor interactions (the push-pull effect), and the level of steric hindrance within the molecules. Finally, by means of NBO analysis of a set of model push-pull alkenes (acceptors: -C identical with N, -CH=O, and -CH=S; donors: S, O, and NH), the occupation numbers of the bonding pi orbitals of the central C=C partial double bond were shown to quantitatively describe the acceptor powers of the substituents and the corresponding occupation numbers of the antibonding pi orbital the donor powers of the substituents. Thus, for the first time an estimation of both the acceptor and the donor properties of the substituents attached to the push-pull double bond have been separately quantified. Furthermore, both the balance between strong donor/weak acceptor substituents (and vice versa) and the additional influences on the barriers to rotation (hydrogen bonding and steric hindrance in the GSs and TSs) could be differentiated.

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PubMed2006

Determination of absolute configuration using density functional theory calculations of optical rotation and electronic circular dichroism: chiral alkenes.

In principle, the absolute configuration (AC) of a chiral molecule can be deduced from its optical rotation (OR) and/or its electronic circular dichroism (ECD). In practice, this requires reliable methodologies for predicting OR and ECD. The recent application of ab initio time-dependent density functional theory (TDDFT) to the calculation of transparent spectral region OR and ECD has greatly enhanced the reliability with which these phenomena can be predicted. TDDFT calculations of OR and ECD are being increasingly utilized in determining ACs. Nevertheless, such calculations are not perfect, and as a result, ACs determined are not 100% reliable. In this paper, we examine the reliability of the TDDFT methods in the case of chiral alkenes. Sodium d line specific rotations, [alpha]D, are predicted for 26 conformationally rigid alkenes of known AC, ranging in size from 5 to 20 C atoms, and with [alpha]D values in the range of 0-500. The mean absolute deviation of predicted [alpha]D values from experimental values is 28.7. With one exception, beta-pinene, the signs of [alpha]D are correctly predicted. Errors in calculated [alpha]D values are approximately random. Our results define a "zone of indeterminacy" within which calculated [alpha]D values cannot be used to determine ACs with >95% confidence. TDDFT ECD spectra are predicted for eight of the alkenes and compared to experimental spectra. Agreement ranges from modestly good to poor, leading to the conclusion that TDDFT calculations of ECD spectra are not yet of sufficient accuracy to routinely provide highly reliable ACs. TDDFT OR calculations for two conformationally flexible alkenes, 3-tert-butylcyclohexene and trans-4-carene, are also reported. For the former, predicted rotations are incorrect in sign over the range 589-365 nm. It is possible that the AC of this molecule has been incorrectly assigned.

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PubMed1996

Kinetics and Mechanism of the Epoxidation of Alkyl-Substituted Alkenes by Hydrogen Peroxide, Catalyzed by Methylrhenium Trioxide.

Epoxidations of alkyl-substituted alkenes, with hydrogen peroxide as the oxygen source, are catalyzed by CH(3)ReO(3) (MTO). The kinetics of 28 such reactions were studied in 1:1 CH(3)CN-H(2)O at pH 1 and in methanol. To accommodate the different requirements of these reactions, (1)H-NMR, spectrophotometric, and thermometric techniques were used to acquire kinetic data. High concentrations of hydrogen peroxide were used, so that diperoxorhenium complex CH(3)Re(O)(eta(2)-O(2))(2)(H(2)O), B, was the only predominant and reactive form of the catalyst. The reactions between B and the alkenes are about 1 order of magnitude more rapid in the semiaqueous solvent than in methanol. The various trends in reactivity are medium-independent. The rate constants for B with the aliphatic alkenes correlate closely with the number of alkyl groups on the olefinic carbons. The reactions become markedly slower when electron-attracting groups, such as halo, hydroxy, cyano, and carbonyl, are present. The rate constants for catalytic epoxidations with B and those reported for the stoichiometric reactions of dimethyldioxirane show very similar trends in reactivity. These findings suggest a concerted mechanism in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of B. These data, combined with those reported for the epoxidation of styrene (a term intended to include related molecules with ring and/or aliphatic substituents) by B and by the monoperoxo derivative of MTO, suggest that all of the rhenium-catalyzed epoxidations occur by a common mechanism. The geometry of the system at the transition state can be inferred from these data, which suggest a spiro arrangement.

Journal Article↗
PubMed1997

Reactions of Diarylnitrenium Ions with Electron Rich Alkenes: An Experimental and Theoretical Study.

Photolysis of N-(diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate (1a) and N-[bis(4-methylphenyl)amino]-2,4,6-trimethylpyridinium salt (1b) gives products attributable to diarylnitrenium ion (Ar(2)N(+), 2). The major products of these reactions include products from nucleophilic addition of various pi-nucleophiles (e.g. electron rich alkenes) to the ortho and para positions of one of the phenyl rings. Nanosecond and EPR spectroscopy show that radicals also form. These radicals are thought to give rise to the diarylamines isolated as minor products from the photolysis of 1aand 1b. In addition to the para addition products and Ph(2)NH, N-phenylindoles and N-phenylindolinones are isolated when silyl enol ethers and silyl ketene acetals are used as trapping agents, respectively. The indoles and indolinones are generated from initial addition of the nucleophile to the ortho position on 2 followed by cyclization of the resulting intermediate. A product resulting from N addition of the nucleophile to 2 is isolated only when silyl ketene acetals are used. A number of electronic sturcture calculations at different levels of molecular orbital and density functional theory were carried out on Ph(2)N(+). There do not seem to be effects associated with either the charge distribution or the LUMO that would strongly influence ortho/para/N selectivity in nucleophilic trapping. Laser flash photolysis on 1a provides absolute rate constants for the nucleophilic addition of various alkenes to Ph(2)N(+). These fall in the range of 10(9)-10(10) M(-)(1) s(-)(1) and correlate with the oxidation potential of the alkene. From these data it is clear that the more easily oxidized the alkene the faster it will react with Ph(2)N(+).

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PubMed1999

The Activation of Carbon-Chlorine Bonds in Per- and Polyfluoroalkyl Chlorides: DMSO-Induced Hydroperfluoroalkylation of Alkenes and Alkynes with Sodium Dithionite.

In DMSO, the addition reactions of perfluoroalkyl chlorides, R(F)Cl, to alkenes or alkynes can occur smoothly in the presence of 1.5 equiv of Na(2)S(2)O(4) and NaHCO(3) at 75-80 degrees C for 4-10 h to give the corresponding adducts (RCH(2)CH(2)R(F) or RCH=CHR(F)). Ethyl chlorofluoro- (1f), chlorodifluoro- (1g) acetates, even nonfluorinated compounds, such as ethyl dichloro- (1h), chloro- (1i) acetates, and chloroform (1j) can undergo the similar reaction. Treatment of omega-iodo (or chloro-) perfluoroalkyl chlorides [X(CF(2))(n)()Cl, n = 2, 4, X= I or Cl] with >3 equiv of alkenes and Na(2)S(2)O(4) gives directly the symmetrically disubstituted alkanes (RCH(2)CH(2))(2)(CF(2))(n)(). The symmetrically and unsymmetrically disubstituted adducts RCH(2)CH(2)(CF(2))(n)()CH(2)CH(2)R' from omega- iodoperfluoroalkyl chlorides can be also obtained stepwise, i.e., through the further addition reactions of monoadducts, RCH(2)CH(2)(CF(2))(n)()Cl to olefins. However, for alpha,omega-dichloroperfluoroalkanes, the similarly stepwise reactions with an alkene is not clean, both bis-adducts and the corresponding omega-hydrides, RCH(2)CH(2)(CF(2))(n)()H as byproducts are also formed. In the absence of alkenes or alkynes, per- and polyfluoroalkyl chlorides can be converted to their sulfinate salts and sulfonyl chlorides.

Journal Article↗
PubMed2006

Butatrienes as extended alkenes: barriers to internal rotation and substitution effects on the stabilities of the ground states and transition states.

The barriers to internal rotation of methylated, ethynylated, and vinylated butatrienes and alkenes were calculated at the CASPT2/6-31G(d)//B3LYP/6-31G(d) level. Calculated butatriene rotational barriers are lower than those of analogous alkenes, but there is a larger variance in rotational barrier for alkenes than for butatrienes. The barriers to rotation were analyzed by isodesmic equations designed to estimate the substituent effects in the ground (GS) and transition (TS) states individually. The GSs of both series are stabilized to roughly the same extent. In contrast, the TSs of butatrienes are more stabilized overall than those of alkenes. Much of the stabilization in the TS of butatrienes comes from the internal triple bond and not from the substituent. Estimation of the substituent stabilization alone reveals the TSs of ethylenes to be more stabilized by substitution than butatrienes.

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PubMed2005

Ab initio and microcalorimetric investigations of alkene adsorption on phosphotungstic acid.

The adsorption of ethene, propene, 1-butene, trans-2-butene, and isobutene on phosphotungstic acid has been characterized by density functional theory (DFT) calculations and microcalorimetric experiments. The DFT-calculated chemisorption energies to form the corresponding alkoxides for ethene, propene, 1-butene, trans-2-butene, and isobutene were -86.8, -90.3, -102.6, -79.9, and -91.4 kJ mol(-1), respectively (for their most-favorable binding modes). The relative chemisorption energies to form the alkoxides are dictated by the strength of interaction of the acidic proton with the carbon atom of the double bond that becomes protonated. The activation barrier for chemisorption was greatest for alkenes with primary (1 degrees) carbenium-like transition states followed by secondary (2 degrees) and tertiary (3 degrees) transition states. The adsorption enthalpy established from microcalorimetric experiments with propene and isobutene was approximately -100 kJ mol(-1), which is close to the DFT-calculated values. Chemisorption of ethene on phosphotungstic acid during microcalorimetric experiments was minimal, presumably because of the large activation barrier associated with a 1 degrees carbenium-like transition state. The results from this study are compared with those in the literature for the adsorption of alkenes on zeolites, which have a similar adsorption mechanism. Our results suggest that alkene adsorption is stronger on phosphotungstic acid than on zeolites, as supported by the more exothermic chemisorption energies. Additionally, activation barriers for alkene adsorption are lower over phosphotungstic acid than over zeolites.

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PubMed2004

Alkyl-eta2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl-eta5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations.

Group 5 metal complexes [M(eta5-C5H5)[eta5-C5H4SiMe2(CH2-eta]2-CH=CH2)]X] (M = Nb, X = Me, CH2Ph, CH2SiMe3; M = Ta, X = Me, CH2Ph) and [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2-eta2-CH=CH2)]X] (X = Cl, Me, CH2Ph, CH2SiMe3) containing a chelating alkene ligand tethered to a cyclopentadienyl ring have been synthesized in high yields by reduction with Na/Hg (X = Cl) and alkylation with reductive elimination (X = alkyl) of the corresponding metal(iv) dichlorides [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]Cl2] (Cp = C5H5, M = Nb, Ta, Cp = C5Me5, M = Ta). These chloro- and alkyl-alkene coordinated complexes react with CO and isocyanides [CNtBu, CN(2,6-Me2C6H3)] to give the ligand-substituted metal(III) compounds [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]XL] (X = Cl, Me, CH2Ph, CH2SiMe3). Reaction of the chloro-alkene tantalum complex with LiNHtBu results in formation of the imido hydride derivative [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2CH=CH2)]H(NtBu)]. NMR studies for all of the new compounds and DFT calculations for the alkene-coordinated metal complexes are compared with those known for related group 4 metal cations.

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PubMed2006

Synthesis, stereochemistry, and photochemical and thermal behaviour of bis-tert-butyl substituted overcrowded alkenes.

In order to study the structural limits in the design of molecular motors, a tert-butyl substituted analogue was prepared. The unexpected photochemical and thermal isomerisation processes and the stereochemistry of new overcrowded alkene are described. The bis tert-butyl substituted alkenes were synthesised in a five-step sequence with an overall yield of 7.5%. Structural assignments of the isomers based on experimental data were supported by calculations of all four isomers of the alkene. X-Ray crystal analysis showed a strongly twisted alkene (torsion angle 39 degrees ) for a less stable photochemically generated cis-isomer.

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PubMed2006

Metallocene-catalyzed alkene polymerization and the observation of Zr-allyls.

Single-site polymerization catalysts enable exquisite control over alkene polymerization reactions to produce new materials with unique properties. Knowledge of catalyst speciation and fundamental kinetics are essential for full mechanistic understanding of zirconocene-catalyzed alkene polymerization. Currently the effect of activators on fundamental polymerization steps is not understood. Progress in understanding activator effects requires determination of fundamental kinetics for zirconocene catalysts with noncoordinating anions such as [B(C6F5)4]-. Kinetic NMR studies at low temperature demonstrate a very fast propagation rate for 1-hexene polymerization catalyzed by [(SBI)Zr(CH2SiMe3)][B(C6F5)4] [where SBI is rac-Me2Si(indenyl)2] with complete consumption of 1-hexene before the first NMR spectrum. Surprisingly, the first NMR spectrum reveals, aside from uninitiated catalyst, Zr-allyls as the sole catalyst-containing species. These Zr-allyls, which exist in two diastereomeric forms, have been characterized by physical and chemical methods. The mechanism of Zr-allyl formation was probed with a trapping experiment, leading us to favor a mechanism in which Zr-polymeryl undergoes beta-H transfer to metal without dissociation of coordinated alkene followed by sigma-bond metathesis to form H2 and Zr-allyl. Zr-allyl species undergo slow reactions with alkene but react rapidly with H2 to form hydrogenation products.

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PubMed1983

Haloperoxidases: Enzymatic Synthesis of alpha,beta-Halohydrins from Gaseous Alkenes.

The enzymatic synthesis of alpha,beta-halohydrins from gaseous alkenes is described. The enzymatic reaction required an alkene, a halide ion, dilute hydrogen peroxide, and a haloperoxidase enzyme. A wide range of gaseous alkenes were suitable for this reaction, including those containing isolated, conjugated, and cumulative carbon-carbon double bonds. Chlorohydrins, bromohydrins, and iodohydrins could be formed. The combining of this enzymatic synthesis with a previously described enzymatic synthesis of epoxides from alpha,beta-halohydrins provides an alternate pathway, other than the well-known enzymatic direct epoxidation pathway, from alkene to an epoxide.

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PubMed2007

Evidence for in situ ethanolamine phospholipid adducts with hydroxy-alkenals.

Hydroxy-alkenals, such as 4-hydroxy-2(E)-nonenal (4-HNE; from n-6 fatty acids), are degradation products of fatty acid hydroperoxides, including those generated by free radical attack of membrane polyunsaturated fatty acyl moieties. The cytotoxic effects of hydroxy-alkenals are well known and are mainly attributable to their interaction with different molecules to form covalent adducts. Indeed, ethanolamine phospholipids (PEs) can be covalently modified in a cellular system by hydroxy-alkenals, such as 4-HNE, 4-hydroxy-2(E)-hexenal (4-HHE; from n-3 fatty acids), and 4-hydroxy-dodecadienal (4-HDDE; from the 12-lipoxygenase product of arachidonic acid), to form mainly Michael adducts. In this study, we describe the formation of PE Michael adducts in human blood platelets in response to oxidative stress and in retinas of streptozotocin-induced diabetic rats. We have successfully characterized and evaluated, for the first time, PEs coupled with 4-HHE, 4-HNE, and 4-HDDE by gas chromatography-mass spectrometry measurement of their ethanolamine moieties. We also report that aggregation of isolated human blood platelets enriched with PE-4-hydroxy-alkenal Michael adducts was altered. These data suggest that these adducts could be used as specific markers of membrane disorders occurring in pathophysiological states with associated oxidative stress and might affect cell function.

Aldehydes↗
PubMed2001

The Type 2 Intramolecular Diels-Alder Reaction: Synthesis and Chemistry of Bridgehead Alkenes.

Anti-Bredt alkenes, bicyclic molecules that contain a bridgehead double bond, were for many years regarded as chemical curiosities. The type 2 intramolecular Diels-Alder (IMDA) reaction provides a one-step entry into this fascinating class of molecules. The reaction has made available numerous anti-Bredt alkenes for structural and chemical studies. X-ray crystallography has revealed the magnitude of the deformations associated with the bridgehead double bond, and rate studies of reactions of bridgehead alkenes have allowed quantification of the kinetic consequences of the torsional distortions. More recently, the type 2 intramolecular Diels-Alder reaction and the resulting anti-Bredt alkenes have found application in organic synthesis. The constraints resulting from the connectivity in the Diels-Alder precursor creates a strong regio- and stereochemical bias in the cycloaddition step. The end result of this bias is the stereoselective synthesis of highly substituted six-membered rings. The reaction also achieves a facile synthesis of seven- and eight-membered rings in a single step from acyclic precursors. The utility of this reaction has been verified in recent applications of the type 2 IMDA reaction as a key step in the total synthesis of complex natural products.

Journal Article↗
PubMed2000

Identification of cuticular hydrocarbons and the alkene precursor to the pheromone in hemolymph of the female gypsy moth, Lymantria dispar.

Hydrocarbons were extracted from the surface of the cuticle and from the hemolymph of adult female gypsy moths. GC and GC/MS analysis indicated that the cuticular hydrocarbons with chain lengths >21 carbons were the same as those found in the hemolymph. These consisted of mostly saturated straight chain hydrocarbons with heptacosane the major component. Methyl branched hydrocarbons were also identified including a series of tetramethylalkanes with chain lengths of 30, 32, and 34 carbons. In addition to those found on the cuticle surface, the hemolymph contained the alkene pheromone precursor, 2-methyl-Z7-octadecene and two saturated analogues, 2-methyl-octadecane and 2-methyl-hexadecane. No evidence was obtained for the presence of the pheromone 2-methyl-7, 8-epoxy-octadecane in the hemolymph. Pheromone gland extracts indicated that small amounts (<1 ng) of the alkene precursor were also present in the gland. Relatively larger amounts of the alkene precursor were found in the hemolymph at the time when pheromone titers were higher on the gland. The presence of the hydrocarbon pheromone precursor in the hemolymph is discussed in relation to possible biosynthetic pathways for producing the gypsy moth pheromone.

Alkanes↗
PubMed2003

What is the nature of the first-formed intermediates in the electrophilic halogenation of alkenes, alkynes, and allenes?

The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f). Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K(f) value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of the bromonium ion. The effect of various polar groups situated in equatorial homoallyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series.

Alkadienes↗
PubMed1998

Long-chain alkenes of the haptophytes Isochrysis galbana and Emiliania huxleyi.

The major alkenes of the haptophytes Isochrysis galbana (strain CCAP 927/14) and Emiliania huxleyi (strains CCAP 920/2 and VAN 556) have been identified by nuclear magnetic resonance spectroscopy and by mass spectrometric analysis of their dimethyl disulfide adducts. The dominant alkene in I. galbana is (22Z)-1 ,22-hentriacontadiene, with 1,24-hentriacontadiene and 1,24-tritriacontadiene present in much lower abundance; (22Z)-1,22-hentriacontadiene also occurs in E. huxleyi (strain CCAP 920/2), together with (2Z,22Z)-2,22-hentriacontadliene (the major hydrocarbon) and (3Z,22Z)-3,22-hentriacontadiene. Minor abundances of 2,24-hentriacontadiene and 2,24-tritriacontadiene are also present in this strain. In contrast, the dominant alkene in E. huxleyi (strain VAN 556) is (15 E,22E)-1,16,23-heptatriacontatriene with the related alkatriene 1,15,22-octatriacontatriene also present and (22Z)-1,22-hentriacontadiene occurring as a minor component. From structural relationships (15E,22E)-1,15,22-heptatriacontatriene is proposed to derive from the same biosynthetic pathway as that of the characteristic C37 alkenones which occur in both E. huxleyi and I. galbana. The C31 and C33 dienes likely derive from chain extension and decarboxylation of (Z)-9-octadecenoic acid or (Z)-7-hexadecenoic acid, using a pathway analogous to that elucidated previously in the chlorophyte Botryococcus braunii. Therefore, long-chain dienes and trienes, which can co-occur in haptophytes, may have distinct biosynthetic pathways.

Alkenes↗