Abstract: Catalytic reactions which involve the cleavage of an sp3 C−H bond adjacent to a nitrogen atom in N-2-pyridynyl alkylamines are described. The use of Ru3(CO)12 as the catalyst results in the addition of the sp3 C−H bond across the alkene bond to give the coupling products. A variety of alkenes, including terminal, internal, and cyclic alkenes, can be used for the coupling reaction. The presence of directing groups, such as pyridine, pyrimidine, and an oxazoline ring, on the nitrogen of the amine is critical for a successful reaction. This result indicates the importance of the coordination of the nitrogen atom to the ruthenium catalyst. In addition, the nature of the substituents on the pyridine ring has a significant effect on the efficiency of the reaction. Thus, the substitution of an electron-withdrawing group on the pyridine ring as well as a substitution adjacent to the sp2 nitrogen in the pyridine ring dramatically retards the reaction. Cyclic amines are more reactive than acyclic ones. The choice o...

Abstract: 1-En-6-ynes react with alcohols or water in the presence of PtCl2 as catalyst to give carbocycles with alkoxy or hydroxy functional groups at the side chain. The reaction proceeds by anti attack of the alkene onto the (η2-alkyne)platinum complex. The formation of the C−C and C−O bonds takes place stereoselectively by trans addition of the electrophile derived from the alkyne and the nucleophile to the double bond of the enyne. Formation of five- or six-membered carbo- or heterocycles could be obtained from 1-en-6-ynes depending on the substituents on the alkene or at the tether. Although more limited in scope, Ru(II) and Au(III) chlorides also give rise to alkoxy- or hydroxycyclization of enynes. On the basis of density functional theory (DFT) calculations, a cyclopropyl platinacarbene complex was found as the key intermediate in the process. In the presence of polar, nonnucleophilic solvents, 1-en-6-ynes are cycloisomerized with PtCl2 as catalyst. Formation of a platinacyclopentene intermediate is suppor...

Abstract: Recent developments in olefin epoxidation have shown promising results indicating that higher olefins can be directly epoxidized using molecular oxygen, or indirectly, by using molecular oxygen to generate an active and selective oxidant in situ during reaction. Indirect approaches have utilized bifunctional catalysts that combine new catalyst components that generate such oxidants as H2O2 in situ with the functional component that activates H2O2 for olefin epoxidation. This approach is currently limited by the low rates of in situ generation of H 2O2 and subsequent low rates of olefin epoxide formation. Heavily-modified, silver catalysts have also shown promise as catalysts for propylene epoxidation. These catalysts contain much higher silver and alkali and alkaline earth metal loadings than their analogs used for ethylene epoxidation and are quite different in terms of their chemical and physical properties. Currently, these compositions exhibit activities and selectivities to propylene oxide that are too low for commercial application, although further research and development may further improve catalyst performance. Silver-based catalysts have also been used to epoxidize a wide variety of higher olefins, such as 1,3-butadiene, that do not contain allylic hydrogen atoms, or higher olefins that contain non-reactive allylic hydrogen atoms, such as norbornene. Silver-based catalysts used for selective epoxidation of non-allylic, or kinetically-hindered, olefins require promoters, typically cesium, rubidium, or thallium salts, to assist in the desorption of the olefin epoxide. Such catalysts are extremely active, selective, and stable under extended reaction conditions. © 2001 Elsevier Science B.V. All rights reserved.

Abstract: The scope of the segment-coupling Prins cyclization has been investigated. The method is outlined in Scheme 1 and involves esterification of a homoallylic alcohol (1), reductive acetylation to give the alpha-acetoxy ether (3), and cyclization on treatment with a Lewis acid to produce a tetrahydropyran (4). Alkene geometries dictate the product configurations, with E-alkenes leading to equatorial substituents and Z-alkenes leading to axial substituents (Table 1). Not unexpectedly, applying the method to allylic alcohols leads to fragmentation rather than a disfavored 5-endo-trig cyclization. Dienols in which one alkene is allylic and the other alkene is homoallylic cyclize efficiently and produce the tetrahydropyrans 49-54, Table 3. Dienols with two homoallylic alkenes cyclize with modest to high regioselectively, generating tetrahydropyrans 40-45, Table 2. The relative rates for cyclization decrease in the order of vinyl > Z-alkene > E-alkene > alkyne. The configurations of the products are consistent with cyclization via a chair conformation, Figure 1. The 2-oxonia Cope rearrangement may be a factor in the regioselectivity of diene cyclizations and in the erosion of stereoselectivity with Z-alkenes. This investigation establishes the stereoselectivity and regioselectivity for a number of synthetically useful segment-coupling Prins cyclizations.

Abstract: The mechanism of the palladium-catalyzed ring opening of oxabicyclic alkenes with dialkylzinc has been studied. Experiments which rule out a π-allyl mechanism were carried out. Trapping carbometalated products and synthesis and successful reaction of alkyl palladium species provided strong evidence in favor of an enantioselective carbopalladation as the key step in the mechanism. The studies also suggest that a cationic palladium species is responsible for carbopalladation of the alkene. The combination of palladium and dialkylzinc is unique in that the dialkylzinc functions both in the transmetalation to palladium and as a Lewis acid in forming the reactive cationic palladium species.

Abstract: The Pauson--Khand reaction represents a one-step Co(2)(CO)(8)-catalyzed synthesis of cyclopentenone through [2 + 2 + 1] assembly of one molecule each of alkene, alkyne, and carbon monoxide. Density functional studies (B3LYP/631LAN) on the reaction pathway of the Pauson--Khand (PK) reaction reported here for the first time provides valuable information on the structures and energetics of various intermediates and transition states. The PK reaction consists of olefin insertion, CO insertion, and reductive elimination steps. The olefin insertion step was found to be an irreversible step that determines the stereo- and regiochemistry of the overall reaction. The following steps are low activation energy processes and reversible. The bond-forming events occur only on one of the two metal atoms, while the second metal atom not only acts as an anchor that fixes the metal cluster to the organic substrate but also exerts electronic influences on the reaction at the first atom.

Abstract: Ozone-alkene reactions generate stabilized Criegee intermediates (of the form R1R2COO), which are believed to react with water molecules to form organic hydroperoxides, hydrogen peroxide and carboxylic acids. These reactions are thought to be significant sources of these environmentally important compounds, yet both the yields of stabilized Criegee intermediates and the branching ratios from their reaction with water are not well known. The formation of hydrogen peroxide and organic hydroperoxides was investigated in the gas phase ozonolysis of ethene, trans-2-butene, and 2,3-dimethyl-2-butene for relative humidities (RH) from 0 and 80% by gas chromatography with flame ionization detection and high-performance liquid chromatography with fluorescence detection. Additionally, yields of acetaldehyde and acetic acid from trans-2-butene and acetone from 2,3-dimethyl-2-butene were measured. The reactions of stabilized Criegee intermediates with water were found to proceed almost entirely via organic hydroperoxide or hydrogen peroxide formation with little acid formation. Stabilized Criegee intermediate yields of 0.39, 0.24, and 0.10 were obtained for ethene, trans-2-butene, and 2,3-dimethyl-2-butene, respectively.

Abstract: Electron-rich carbene precursors 2 and 3, containing the imidazolidin-2-ylidene moiety with one (2) and two (3) pendent N-(2,4,6-trimethylbenzyl) groups, on reaction with [RuCl2(arene)]2 lead to ruthenium(II) complexes 5 and 6 containing the chelating 8-electron mixed arene–carbene ligand; the X-ray diffraction crystal structure of RuCl2{η1-CN[CH2( η6-2,4,6-Me3C6H2)]CH2CH2N(CH2CH2OMe)} 6, was established. These complexes are precursors of the unstable ruthenium-allenylidene intermediates 7 and 8, but are active catalysts either for selective catalytic alkene metathesis or cycloisomerization, depending on the nature of the 1,6-diene.

Abstract: (Butadiene)zirconocene adds B(C6F5)3 at a terminal diene carbon atom to yield the zirconocene−(μ-hydrocarbyl)−borate betaine Cp2Zr[C4H6-B(C6F5)3] (4). The dipolar complex 4 contains a distorted π-allyl moiety and features an additional stabilizing Zr−F−C(arene) coordination. Under kinetic control, an isomeric betaine system is formed, characterized by an internal Zr+···CH2[B]- ion-pair interaction, that rearranges to 4 upon heating. A great variety of ansa-metallocene(butadiene) complexes and related systems cleanly form analogous metallocene−(μ-conjugated diene)−borate betaines upon treatment with B(C6F5)3 and related Lewis acids. Most of these systems represent very active homogeneous single-component Ziegler−Natta catalysts for α-olefin polymerization and copolymerization. In addition, these betaine catalysts are ideally suited for carrying out mechanistic studies in active Ziegler−Natta catalyst systems. They allow for an experimental observation of the first alkene insertion step at the active single...

Abstract: Butylation problems ironed out: 3-Pentynyl ethers react with butyllithium at -20°C in toluene, upon addition of a catalytic amount of a cheap iron(III) salt, to afford (E)-4-methyl-3-octenyl ethers in high yields. Stereochemically defined tetrasubstituted alkenes were also obtained by the subsequent addition of electrophiles (E+ , see scheme; acac = acetylacetonate, Bn=benzyl).

Abstract: Inexpensive Al 2 O 3 can be used as a simple catalyst for alkene epoxidation, using anhydrous hydrogen peroxide as oxidant. This system is active and selective in the epoxidation of several alkenes. Besides the epoxidation of the terpenes limonene and α-pinene, we studied the epoxidation of cyclohexene and cyclooctene, as well as α-olefins, such as 1-octene and 1-decene. Productivities of up to 4.3 g products per gram catalyst were obtained and the catalyst was recycled without significant loss of activity.

Abstract: A catalyst comprising a promoted mixed metal oxide is useful for the vapor phase oxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated carboxylic acid and for the vapor phase ammoxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated nitrile.

Abstract: Fe–Co-based metal/oxide composite materials (Co0.730Fe0.270)0.82 [Co0.35Fe2.65O4], (Co0.950Fe0.050)0.62 [Co0.31Fe2.69O4] and (Co0.900Fe0.100) [Co0.10Fe2.90O4] were prepared to be used in Fischer–Tropsch (F–T) synthesis in view of producing light alkenes. The metal is an iron–cobalt alloy of bcc structure in the former and of fcc structure in the two latter and the oxide, a cobalt containing magnetite. The metal, obtained directly by preparation method, is embedded in the oxide phase. Then, catalysts were not subjected to a reducing pre-treatment. Although, catalysts were not reduced, the two former materials are efficient as catalysts in F–T reaction to produce C2–C4 (50 wt.%) with high alkene:alkane ratio and a low selectivity of CO2 (25% molar at CO conversion of 5%); the third catalyst presents a typical distribution of hydrocarbons obtained with cobalt catalyst in F–T reaction. The possible modifications of the catalysts during the catalytic reaction are studied by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). High resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) show the surface composition under test is identical for the first two catalysts, although their initial bulk composition is different. The characterisation results, obtained by combined methods (XRD, electron-microscopy, surface analysis) provide explanations to individual catalytic behaviour of the three catalysts studied.

Abstract: A variety of terminal alkenes are produced in excellent yields by the rhodium(I)-catalyzed methylenation of aldehydes using TMSCHN2 and PPh3 [Eq. (1)]. These mild reaction conditions allowed the conversion of enolizable substrates and the chemoselective methylenation of aldehydes over ketones. TMS=trimethylsilyl.

Abstract: Silylboryl reagents for organic synthesis: 1-silyl-1-boryl-2-alkenes (2) were prepared efficiently by gem-silylborylation of α-chloroallyllithium compounds from (dimethylphenylsilyl)(pinacolato)borane (1; see scheme, LDA=lithium diisopropylamide) and were demonstrated to allylate acetals and aldehydes in the presence of a Lewis acid to produce (E)-4-alkoxy-alkenylboronates. Heating the reagents with aldehydes in the absence of Lewis acid afforded (Z)-4-hydroxy-alkenylsilanes stereospecifically.

Abstract: The scope of the segment-coupling Prins cyclization has been investigated. The method is outlined in Scheme 1 and involves esterification of a homoallylic alcohol (1), reductive acetylation to give the alpha-acetoxy ether (3), and cyclization on treatment with a Lewis acid to produce a tetrahydropyran (4). Alkene geometries dictate the product configurations, with E-alkenes leading to equatorial substituents and Z-alkenes leading to axial substituents (Table 1). Not unexpectedly, applying the method to allylic alcohols leads to fragmentation rather than a disfavored 5-endo-trig cyclization. Dienols in which one alkene is allylic and the other alkene is homoallylic cyclize efficiently and produce the tetrahydropyrans 49-54, Table 3. Dienols with two homoallylic alkenes cyclize with modest to high regioselectively, generating tetrahydropyrans 40-45, Table 2. The relative rates for cyclization decrease in the order of vinyl > Z-alkene > E-alkene > alkyne. The configurations of the products are consistent with cyclization via a chair conformation, Figure 1. The 2-oxonia Cope rearrangement may be a factor in the regioselectivity of diene cyclizations and in the erosion of stereoselectivity with Z-alkenes. This investigation establishes the stereoselectivity and regioselectivity for a number of synthetically useful segment-coupling Prins cyclizations.

Abstract: The decomposition of different hydrocarbons (CH4, C2H6, C2H4, C2H2, C3H8, and C3H6) over Ni (5 wt.%)/SiO2 catalysts was carried out. The initial rates of decomposition of the hydrocarbons, the kinetic curves of the decomposition and the kinetic curves of the hydrogenation of deposited carbon into methane depended on the types of hydrocarbons. In addition, the catalytic life of the Ni/SiO2 catalyst was also dependent on the types of hydrocarbons, i.e. the life was longer according to the order, alkanes>alkenes⪢acetylene. The carbons deposited on the catalyst were characterized by SEM and Raman spectroscopy. The appearances of the deposited carbons were different among alkanes, alkenes, and acetylene, i.e. a zigzag fiber structure from methane, and a rolled fiber structure from alkenes and acetylene. From Raman spectra of the deposited carbons, it was found that the degree of graphitization of deposited carbon was higher in the order, alkanes>alkenes>acetylene. These results suggest that the mechanism of decomposition of hydrocarbons and the growth mechanism of carbon fibers on the catalyst were different among alkanes, alkenes and acetylene.

Abstract: Gas-phase and surface-bound products were determined for the reaction of ozone with self assembled monolayers of alkanes and terminal alkenes serving as proxies for atmospheric organic aerosols. The organic surfaces were characterized using infrared (IR) spectroscopy (direct absorption and attenuated total reflection) as well as contact angle measurements with water before and after the reaction with ozone. The contact angle of the organic surfaces was reduced by ∼20° owing to the reaction. Following the reaction, IR absorption due to the presence of carbonyls and carboxylic acids was observed on the surface. Gas-phase products were determined using infrared spectroscopy immediately above the reaction surface. Under dry conditions, gas-phase formaldehyde yields of 0.5±0.1 for organic monolayers of allyltrichlorosilane (C3=) and octenyltrichlorosilane (C8=) terminal alkenes were observed, in good agreement with the yields observed for gas phase ozonolysis of terminal alkenes. Surfaces of n-octane (C8) as well as processed alkene surfaces were nonreactive toward ozone. The reaction mechanism of ozone with the surface alkenes is discussed. Finally, the possible implications for the chemistry of organic aerosols are discussed and studied using a box model and realistic atmospheric scenarios.

Abstract: The synthesis of 1,1-disubstituted alkenes typically involves reactions that lack atom economy such as olefination protocols. The use of various ruthenium complexes to effect the addition of terminal alkynes to alkenes is explored as an atom economical strategy. Two new ruthenium complexes have been discovered that effect this reaction at ambient temperature, cyclopentadienylruthenium (triphenylphosphine) camphorsulfonate and cyclopentadienylruthenium tris(acetonitrile) hexafluorophosphate. Using these complexes as catalysts, reactions proceed at ambient temperature in acetone or DMF, respectively. Regioselectivity favoring the formation of a 1,1-disubstituted over a 1,2-disubstituted alkene typically ranges from 9:1 to >25:1. The reaction demonstrates extraordinary chemoselectivity-even di- and trisubstituted alkenes such as present in the products do not compete with the starting monosubstituted alkene. Free hydroxyl groups as well as silyl and PMB ethers are tolerated as are ketones, esters, and amides. The mechanism of the reaction is believed to invoke formation of a metallacyclopentene. To account for the chemo- and regioselectivity, the initial formation of the metallacycle is believed to be reversible. While formation of the 2,5-disubstituted ruthenacyclopentene, which produces the linear product, is believed to be kinetically preferred, the rate of beta-hydrogen elimination from the 2,4-disubstituted ruthenacyclopentene, which produces the branched product, is believed to be faster. Thus, the competition between the rate of beta-hydrogen elimination and cycloreversion rationalizes the results.