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PubMed2004

Electronic effects steer the mechanism of asymmetric hydrogenations of unfunctionalized aryl-substituted alkenes.

Density functional theory (PBE and B3LYP) was used to study asymmetric hydrogenations of alkenes catalyzed by an iridium imidazolylidine oxazoline complex. The calculation predicts that the alkene preferentially coordinates to the site trans to the carbene. The coordinated alkene then reacts first with the H2 ligand, then with the hydride to form alkane. Finally, the alkane is released by equilibrating with extrinsic H2 and alkene. Enantioface selectivities for hydrogenations of trisubstituted alkenes seem to be driven primarily by steric interactions with the adamantyl part of the ligand; only the smallest substituents can adopt a site close to it. Application of this theoretical model leads to correct predictions regarding the experimentally observed sense and magnitude of the enantioselectivities.

Journal Article↗
PubMed2004

Alkylation of alkenes: ethylaluminum sesquichloride-mediated hydro-alkyl additions with alkyl chloroformates and di-tert-butylpyrocarbonate.

A general method for the hydro-alkyl addition to the nonactivated C=C double bond of alkenes using alkyl chloroformates (primary, secondary), 12, and di-tert-butylpyrocarbonate, 52, mediated by ethylaluminum sesquichloride (Et(3)Al(2)Cl(3)) has been developed. Reaction of 12 and 52, respectively, with Et(3)Al(2)Cl(3) gives an alkyl cation which is added to the alkene; hydride transfer to the adduct carbenium ion or, if applicable, 1,2-H shift followed by hydride transfer from Et(3)Al(2)Cl(3) to the rearranged adduct carbenium ion gives the saturated addition product. The reaction has been applied to 1-alkenes, 2-methyl-1-alkenes, internal double bonds, and to three cyclic alkenes. Special interest has been focused on alkylations of unsaturated fatty compounds, such as oleic acid (2), which are important renewable feedstocks. 2-Methylalkanes, 3-methylalkanes, 2,4-dimethylalkanes, 2,3-dimethylalkanes, 2,2,4-trimethylalkanes, cyclohexylalkanes, and carboxylic acids and esters with the respective branched alkyl chain have been synthesized with good to moderate yields.

Journal Article↗
PubMed2006

Nonchelated alkene and alkyne complexes of d(0) zirconocene pentafluorophenyl cations.

This paper describes the generation and properties of nonchelated d(0) zirconocene-aryl-alkene and alkyne adducts that are stabilized by the presence of beta-SiMe(3) substituents on the substrates and the weak nucleophilicity of the -C(6)F(5) ligand. The cationic complexes [(C(5)H(4)R)(2)Zr(C(6)F(5))][B(C(6)F(5))(4)] (4a: R = H, 4b: R = Me) were generated by methide abstraction from (C(5)H(4)R)(2)Zr(C(6)F(5))Me by Ph(3)C(+). NMR studies show that 4a,b contain an o-CF...Zr dative interaction and probably coordinate a PhCl molecule in PhCl solution. Addition of allyltrimethylsilane (ATMS) to 4a,b in C(6)D(5)Cl solution at low temperature produces an equilibrium mixture of (C(5)H(4)R)(2)Zr(C(6)F(5))(H(2)C=CHCH(2)SiMe(3))(+) (7a,b), 4a,b, and free ATMS. Similarly, addition of propargyltrimethylsilane (PTMS) to 4a produces an equilibrium mixture of Cp(2)Zr(C(6)F(5))(HCCCH(2)SiMe(3))(+) (8a), 4a, and free PTMS. The NMR data for 7a,b,and 8a are consistent with highly unsymmetrical substrate coordination and substantial polarization of the substrate multiple bond with significant positive charge buildup at C(int) and negative charge buildup at C(term). PTMS binds to 4a more strongly than ATMS does. The ATMS adducts undergo nondissociative alkene face exchange ("alkene flipping"), i.e., exchange of the (C(5)H(4)R)(2)Zr(C(6)F(5))(+) unit between the two alkene enantiofaces without decomplexation of the alkene, on the NMR time scale.

Journal Article↗
PubMed2000

Titanocene-catalyzed carbosilylation of alkenes and dienes using alkyl halides and chlorosilanes

A new method for regioselective carbosilylation of alkenes and dienes has been developed by the use of a titanocene catalyst. This reaction proceeds efficiently at 0 degrees C in THF in the presence of Grignard reagents by the combined use of alkyl halides (R'-X, X = Br or Cl) and chlorotrialkylsilanes (R3''Si-Cl) as the alkylating and silylating reagents, respectively. Terminal alkenes having aryl or silyl substituents (YRC=CH2, Y = Ar or Me3Si, R = H or Me) afford addition products YRC-(SiR''3)-CH2R' in good yields, whereas 1-octene and internal alkenes were sluggish. When 2,3-disubstituted 1,3-butadienes were used instead of alkenes, alkyl and silyl units are introduced at the 1- and 4-positions giving rise to allylsilanes in high yields under similar conditions. The present reaction involves (i) addition of alkyl radicals toward alkenes or dienes, and (ii) electrophilic trapping of benzyl- or allylmagnesium halides with chlorosilanes. The titanocene catalyst plays important roles in generation of these active species, i.e., alkyl radicals and benzyl- or allylmagnesium halides.

Journal Article↗
PubMed2001

Catalytic oxyalkylation of alkenes with alkanes and molecular oxygen via a radical process using N-hydroxyphthalimide.

A novel catalytic method for the radical addition of alkanes and molecular oxygen to electron-deficient alkenes was achieved by the use of N-hydroxyphthalimide (NHPI) combined with a Co species as the catalyst. This reaction is referred to as oxyalkylation of alkenes with alkanes and O(2). For instance, the reaction of 1,3-dimethyladamantane with methyl acrylate under molecular oxygen in the presence of catalytic amounts of NHPI and Co(acac)(3) at 70 degrees C for 16 h gave oxyalkylated products in 91% yield. Other alkenes such as fumarate and acrylonitrile also serve as good acceptors of alkyl radicals and O(2) to afford the corresponding adducts in high yields. The generality of the present reaction was examined between various alkanes and alkenes under dioxygen. The behavior of Co ions during the reaction course was discussed. The present reaction involves (i) an alkyl radical generation via hydrogen abstraction of alkane by phthalimide N-oxyl generated in situ from NHPI and O(2) assisted by Co(II), (ii) the addition of the resulting alkyl radical to an electron-deficient alkene to form an adduct radical, (iii) trapping of the adduct radical by O(2) yielding a hydroperoxide, and (iv) the decomposition of the hydroperoxide by Co ions to form an adduct in which a hydroxy or a carbonyl function is incorporated.

Journal Article↗
PubMed2003

Strategies for the design of organic aziridination reagents and catalysts: transition structures for alkene aziridinations by NH transfer.

B3LYP/6-31G* transition structures for aziridination of various alkenes by substituted oxaziridines and diaziridinum salts were located. Oxaziridines substituted with electron-withdrawing groups have activation energies for nitrogen transfer similar to those calculated for epoxidation by various known organic oxidants. These transition states are relatively insensitive to alkene substituents, but highly electron deficient alkenes were calculated to have low activation energies. N-Trimethylsilyl-derived oxaziridines are predicted to be good targets for alkene aziridination reagents. Activation energies calculated for aziridination by diaziridinium salts are generally lower in energy. Aziridinations of electron-rich and highly electron deficient alkenes by diaziridinium salts are predicted to be rapid. N-Methyl, N-trifluoroacetyl, and N-trimethylsilyl derivatives showed reasonable activation energies for nitrogen transfer.

Aziridines↗
PubMed1998

Ruthenium meso-Tetrakis(2,6-dichlorophenyl)porphyrin Complex Immobilized in Mesoporous MCM-41 as a Heterogeneous Catalyst for Selective Alkene Epoxidations.

A ruthenium complex of meso-tetrakis(2,6-dichlorophenyl)porphyrin, [Ru(II)(TDCPP)(CO)(EtOH)], is immobilized into mesoporous MCM-41 molecular sieves; the supported Ru catalyst can effect highly selective heterogeneous alkene epoxidations using 2,6-dichloropyridine N-oxide as terminal oxidant. Aromatic and aliphatic alkenes can be efficiently converted to their epoxides in good yields and selectivities, and cis-alkenes such as cis-stilbene, cis-beta-methylstyrene, and cis-beta-deuteriostyrene are epoxidized stereospecifically. Oxidation of cycloalkenes, e.g., norbornene and cyclooctene, can be carried out effectively using the heterogeneous Ru-catalyzed reaction while these alkenes are unreactive in the zeolite-based titanium silicate (TS-1)-catalyzed conditions (Murugavel, R.; Roesky, H. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 477). On the other hand, the Ru/M-41(m) catalyst displays size selectivity in the (+)-limonene oxidation where the terminal C=C bond (vs internal trisubstituted C=C bond) becomes more readily oxidized. Bulky 3,4,6-tri-O-benzyl-D-glucal has failed to react under the heterogeneous Ru-catalyzed conditions, whereas the smaller acetyl derivative is converted to a 3:1 mixture of alpha- and beta-glycal epoxides. The Ru/M-41(m) catalyst can be used repeatedly, and 67% of its initial activity is retained after 11 691 turnovers (three runs). The loss of activity is attributed to catalyst leaching and/or deactivation. On the basis of Hammett correlation (rho(+) = -0.72, R = 0.997) and product studies (cyclohexene and cis-alkenes as the substrates), a reactive dioxoruthenium(VI) porphyrin intermediate is not favored. An oxoruthenium(V) complex or oxoruthenium(IV) porphyrin cation radical could be the key intermediate for this highly selective epoxidation reaction.

Journal Article↗
PubMedDate not supplied

The influence of glutathione and detoxifying enzymes on DNA damage induced by 2-alkenals in primary rat hepatocytes and human lymphoblastoid cells.

The reaction of 2-alkenals with GSH to form GSH conjugates by Michael addition is a major detoxification pathway. The reaction proceeds at a much higher rate under catalysis by glutathione S-transferase (GST) than the non-enzymatic reaction. Oxidation of 2-alkenals to the corresponding acids by cytosolic and microsomal fraction of rat liver also contributes to detoxification. Primary rat hepatocytes rich in GSH and proficient for GST and other metabolizing enzymes consume much more alkenal than human lymphoblastoid cells (Namalva cells), that are poor in GSH and in metabolic activities. In Namalva cells DNA single strand breaks were induced by much lower concentrations of acrolein, crotonaldehyde and (E)-2-hexenal than in primary rat hepatocytes. In both cell systems intracellular GSH depletion by 2-alkenals proceeds in a dose dependent manner, approaching about 20% of pretreatment level before DNA damage becomes detectable. GSH conjugates of (E)-2-hexenal and (2E,6Z)-2,6-nonadienal induce DNA damage in Namalva cells at high concentrations (1.5 mM). In the absence of GSH these conjugates decompose slowly into aldehyde and GSH. Although the rate of decomposition is only about 10(-4) times that of Michael adduct formation, such GSH conjugates could potentially function as transport molecules for 2-alkenals, if they reach tissues low in GSH and GST.

Acrolein↗
PubMed2002

A synthesis of trisubstituted alkenes by a Ru-catalyzed addition.

Catalyzed by ruthenium trisacetonitrile hexafluorophosphate 4, the Alder-ene type reaction of alkenes and internal alkynes provides an effective way to synthesize trisubstituted alkenes. Unlike most typical olefination protocols, this reaction is atom economical, and affords trisubstituted alkenes with defined olefin geometry. The regioselectivity can be explained invoking a steric argument based on the proposed mechanism. The first C-C bond formation generally involves sterically less hindered carbons of the alkenes and alkynes. Modest to very high regioselectivity can be achieved depending on the steric difference of the two substituents of alkynes.

Alkenes↗
PubMed2006

Weak attractive interactions between methylthio groups and electron-deficient alkenes in peri-naphthalenes: a competition with conjugative effects.

The solid-state conformations of five peri-disubstituted naphthalenes bearing a methylthio group and an electron-deficient alkene indicate a weak attractive interaction between the functional groups in four cases in which out-of-plane displacements lead to a common orientation of the MeSsp(2)-C vector to the alkene bond. In some cases the interaction is not strong enough to outweigh the tendency of the alkene to conjugate with the aromatic ring, and in one case this optimisation of conjugation alone controls the molecular conformation. The methylthio group lies close to the aromatic plane in all but one example for which the plane of the sulfide group is presented to the alkene.

Alkenes↗
PubMed2002

Studies on reactions of ozone with alkenes.

In the last years, a continuous increase of the O3 concentration has been recorded in the lower atmospheric layers. Photochemical reactions with NO(x), CO and organic compounds are the main sources of O3 in the troposphere. In this work, an attempt was made to determine the impact of alkenes on the O3 concentration in the troposphere. A study on the gas-phase reactions of 03 with 1-hexene, 1-heptene and 1-nonene was made. The reactions were carried out at room temperature under atmospheric pressure. Ozone was formed by the ultraviolet radiation emitted by a mercury lamp, in order to simulate the atmospheric conditions. The changes with time in the concentration of O3, 1-alkenes and formed aldehydes were investigated. Qualitative and quantitative analyses were done by means of the gas chromatography and colorimetry. The following products were identified: pentanal from 1-hexene; hexanal from 1-heptene; oktanal from 1-nonene. For each of the reactions, HCHO was also determined as a product. The reaction rate constants were calculated and obtained in units of 10(-17) cm(-3) molecule(-1) s(-1): 1.94-0.99 for 1-hexene, 5.54-4.51 for 1-heptene and 1.54-0.76 for 1-nonene. Based on the results obtained, an explanation of O3 concentration variations in the planetary boundary layer can be given. Last year a considerable increase of O3 concentration on the roads of Western Europe was recorded. This increase could have resulted from the decrease of alkene concentration in the air due to common use of the catalytic converters in cars. The unsaturated hydrocarbons rapidly oxidize on the catalyst. In Eastern Europe, where the amount of cars equipped with catalytic converters is smaller than in Western Europe, the alkene content in the exhaust fumes results in a decrease of the O3 concentration in the troposphere.

Air Pollutants↗
PubMed1995

Metabolic transformation of halogenated and other alkenes--a theoretical approach. Estimation of metabolic reactivities for in vivo conditions.

Olefinic hydrocarbons are metabolized in vivo by cytochrome P450-dependent monooxygenases to the corresponding epoxides. The maximum in vivo metabolic rate, which is an important toxicokinetic parameter, has been used to define the apparent rate constant (kapp) describing in vivo metabolic reactivity of alkenes. To derive kapp, the metabolic rate normalized per body weight was divided by the corresponding average alkene concentration in the body at saturation conditions of 90%. Toxicokinetic data obtained in rats for 13 compounds (ethene, 1-fluoroethene, 1,1-difluoroethene, 1-chloroethene, 1,1-dichloroethene, cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1,2-trichloroethene, perchloroethene, propene, isoprene, 1,3-butadiene and styrene) have been used to calculate kapp values. A theoretical model, based on the assumption that in vivo epoxidation can be described as a cytochrome P450-mediated electrophilic reaction, has been developed. Using the olefinic hydrocarbons as an example it has been shown that kapp can be explained solely by the following molecular parameters: ionization potential, dipole moment and pi-electron density. These molecular parameters were calculated by a quantum chemical method or were taken from the literature. Furthermore, the model was tested also by predicting kapp for isobutene, an alkene which was not used for the model development. The predicted value of kapp agrees with the one derived experimentally, demonstrating that molecular parameters of halogenated and other alkenes can be used to predict in vivo metabolic reactivity. The model presented here is a first contribution to the ultimate goal to predict toxicokinetic parameters for in vivo conditions based on physicochemical parameters of enzymes and compounds exclusively.

Alkenes↗
PubMed2005

Palladium-catalyzed ring-forming aminoacetoxylation of alkenes.

A mild, palladium(II)-catalyzed ring-forming aminoacetoxylation of alkenes is described. Treatment of a range of nitrogen nucleophiles with catalytic palladium(II) in the presence of PhI(OAc)2 as oxidant resulted in alkene aminoacetoxylation, affording a variety of nitrogen-containing heterocycles. Our studies indicate the possibility for high levels of reaction regio- and stereocontrol. It appears that this is a stereoselective trans alkene difunctionalization and thus a useful alternative to related cis-selective, metal-catalyzed alkene aminohydroxylation processes.

Acetylation↗
PubMed2003

Modes of antifungal action of (2E)-alkenals against Saccharomyces cerevisiae.

A series of aliphatic (2E)-alkenals from C(5) to C(14) were tested for their antifungal activity against Saccharomyces cerevisiae ATCC 7754. (2E)-Undecenal (C(11)) was found to be the most effective with the minimum fungicidal concentration (MFC) of 6.25 microgram/mL, followed by (2E)-decenal (C(10)) with an MFC of 12.5 microgram/mL. The time-kill curve study showed that (2E)-undecenal was fungicidal against S. cerevisiae at any growth stage, and this activity was not influenced by pH values. The (2E)-alkenals inhibited glucose-induced acidification by inhibiting the plasma membrane H(+)-ATPase. The primary antifungal action of medium-chain (C(9)-C(12)) (2E)-alkenals against S. cerevisiae comes from their ability to function as nonionic surface-active agents (surfactants), disrupting the native membrane-associated function nonspecifically. Hence, the antifungal activity of (2E)-alkenals is mediated by biophysical processes, and the maximum activity can be obtained when the balance between the hydrophilic and hydrophobic portions becomes the most appropriate.

Aldehydes↗
PubMed2000

Azidomercurations of alkenes: mercury-promoted Schmidt reactions.

Azides bearing a suitably disposed alkene, when treated with either mercuric perchlorate or mercuric trifluoromethanesulfonate, produce bicyclic iminium ions. This new version of the Schmidt reaction proceeds by capture of the mercuronium ion intermediate by the azide to produce an aminodiazonium ion, which suffers a 1,2-shift to give an iminium ion (e.g., 10 --> 16 --> 17 --> 18). Reduction of the iminium ion may then be carried out to produce an amine. Compared to earlier work on the protic acid-promoted intramolecular Schmidt reaction of azido-alkenes, the mercury-promoted Schmidt reaction has several advantages. First, the acid-promoted Schmidt reaction of azido-alkenes requires that the intermediate carbocations be tertiary, allylic, benzylic, or propargylic. The mercury-promoted method has no such limitation; thus even 1,2-disubstituted alkenes may be used. Second, the mercury-promoted method is milder, allowing the presence of acid-sensitive functionality. The protic version, typically employing trifluoromethanesulfonic acid, is limited in its functional group tolerance. Third, whereas carbocation rearrangement is often observed prior to cyclization/rearrangement in the acid-promoted Schmidt reaction, the mercury-promoted method avoids this problem. Fourth, the presence of the mercurio group during the rearrangment may alter the regioselectivity of the 1,2-migration. Finally, the mercury-bearing iminium ions that are the result of the Schmidt reaction were found to be sensitive to protodemercuration, precluding their use in other transformations.

Alkenes↗
PubMed2005

Rh-catalyzed enantioselective diboration of simple alkenes: reaction development and substrate scope.

[Reaction: see text]. The rhodium-catalyzed reaction between bis(catecholato)diboron and simple alkenes results in the syn addition of the diboron across the alkene. The resulting 1,2-bis(boronate) is subsequently oxidized to provide the 1,2-diol. In the presence of enantiomerically enriched Quinap ligand, high enantioselection in the diboration can be achieved. The reaction is highly selective for trans- and trisubstituted alkenes and can be selective for some monosubstituted alkenes as well. The development of this reaction is described as is the substrate scope and experiments that are informative about the reaction mechanism and competing pathways.

Alkenes↗
PubMed2006

Stereoselectivity of intramolecular SN' cyclizations of alkyllithium reagents on methoxy alkenes.

The cyclization of alkyllithium reagents onto methoxy alkenes has been investigated. The alkyllithium reagent was generated by reductive lithiation of an alkyl nitrile. In an unbiased substrate, a methoxy leaving group attached to a stereogenic secondary carbon atom led to the cyclization product with high optical purity. The configuration of the product demonstrated that the cyclization had proceeded with high syn-S(N)' selectivity. Previously we have shown that 2-lithiotetrahydropyran reagents cyclize to form spirocycles with the alkene cis to the pyran oxygen. Substrates were prepared to evaluate the importance of the configuration of the secondary allyl methyl ether against the alpha-alkoxy alkyllithium configuration. In the matched case (cyano acetal 38), a very selective cyclization ensued. In the mismatched case (cyano acetal 39), the spiro ether selectivity dominated. The syn-S(N)' selectivity overcame the normal E selectivity in the cyclization and accounted for the major product, Z-alkene 45. Thus the stereochemical preference in these alkyllithium cyclizations is dominated by the spiroether effect, followed by the syn-S(N)' selectivity and finally the preference for E-alkene formation.

Acetals↗
PubMed2006

Stereoselective synthesis of (Z)-alkene-containing proline dipeptide mimetics.

In peptides and proteins, the peptide bond between an amino acid and proline exists as an equilibrium mixture of the cis-imide and trans-imide due to the low energy barrier in their interconversion. This feature greatly influences the structure and function of the proline-containing peptides and proteins. Therefore, restricting the amide bond with an (E)- or (Z)-alkene should provide a promising method for elucidating the structure-activity relationships of the peptide and the proteins. In this report, the regio- and stereoselective synthesis of cis-alanylproline (Ala-Pro) type (Z)-alkene dipeptide mimetic is described. The key steps of this synthesis are to introduce a C3 unit onto a gamma-phosphoryloxy-alpha,beta-unsaturated-delta-lactam with an organocopper-mediated anti-S(N)2' reaction and subsequently construct a five-membered proline-like cyclic structure with an intramolecular Suzuki coupling reaction. Hydrolysis of the amide bond in the resulting bicyclic lactam yields the desired cis-Ala-Pro type (Z)-alkene dipeptide isostere. The presented synthetic methodology should be applicable to the general syntheses of other cis-aminoacylproline type (Z)-alkene dipeptide mimetics.

Alkenes↗