Oxidation of organic films by beams of hydroxyl radicals

We have studied the oxidation of self-assembled monolayers (SAMs) of alkanes and alkenes with a thermal beam of OH radicals. The target films were produced by bonding alkane thiols and alkene thiols to a gold surface and the SAMs are mounted in a vacuum chamber at a base pressure of 10-9 Torr. Hydroxyl radicals were produced by a corona discharge in an Ar/H2O2/water mixture. The resultant molecular beam was scanned by an electrostatic hexapole and the OH radicals [4 (+/- 1) x 1011 OH radicals cm-2 sec-1] were focused onto the target SAM. All of the hydroxyl radicals impinging on the SAM surface are rotationally (J' ' 相似文献   

Understanding the Anomalous Alkane Selectivity of ZIF-7 in the Separation of Light Alkane/Alkene Mixtures

C2 and C3 alkanes are selectively adsorbed from mixtures over the corresponding alkenes on the zeolite imidazolate framework ZIF-7 through a gate-opening mechanism. As a result, the direct production of the pure alkene upon adsorption and the pure alkane upon desorption in packed columns is possible. Herein, a detailed investigation of the step-wise adsorption and separation of alkanes and alkenes is presented, together with a rigorous performance assessment. A molecular picture of the gate-opening mechanism underlying the unprecedented selectivity towards alkane adsorption is proposed based on DFT calculations and a thermodynamic analysis of the adsorption-desorption isotherms.     

Parahydrogen-Induced Polarization in Hydrogenation Reactions Mediated by a Metal-Free Catalyst

We report nuclear spin hyperpolarization of various alkenes achieved in alkyne hydrogenations with parahydrogen over a metal-free hydroborane catalyst (HCAT). Being an intramolecular frustrated Lewis pair aminoborane, HCAT utilizes a non-pairwise mechanism of H₂ transfer to alkynes that normally prevents parahydrogen-induced polarization (PHIP) from being observed. Nevertheless, the specific spin dynamics in catalytic intermediates leads to the hyperpolarization of predominantly one hydrogen in alkene. PHIP enabled the detection of important HCAT-alkyne-H₂ intermediates through substantial ¹H, ¹¹B and ¹⁵N signal enhancement and allowed advanced characterization of the catalytic process.     

Polymeric cofactors which accelerate homogeneous rhodium(I) and ruthenium(II) catalyzed hydrogenations of alkenes

Polymeric reagents prepared by exchanging silver(I) for H⁺ on a macroreticular polystyrene sulfonate ion exchange resin are shown to be capable of selectively absorbing triphenylphosphine from solutions of triphenylphosphine complexes of rhodium(I) and ruthenium(II). Absorption of triphenylphosphine during alkene hydrogenations catalyzed by RhCl(PPh₃)₃, RuCl₂(PPh₃)₃ and RuHCl(PPh₃)₃ led to increased hydrogenation rates in hydrogenation of 1-hexene and other alkenes. Addition of this silver(I) polystyrene sulfonate to alkene hydrogenations catalyzed by HRh(CO) (PPh₃)₃, RuH₂(PPh₃)₃ and RuH(OCOCH₃) (PPh₃)₃ also led to modest rate accelerations. Catalyst activations seen in these alkene hydrogenations were shown to be due in some cases to triphenylphosphine absorption. In other cases, HCl or HCl plus triphenylphosphine absorption was responsible for the formation of a more active catalyst solution.     

Migratory Insertion of Alkenes into Metal–Oxygen and Metal–Nitrogen Bonds

The insertion of an unsaturated ligand into a M? C or M? H bond proceeds through migratory insertion, a fundamental organometallic reaction. Recent literature documents evidence of the migratory insertion of alkenes into an M? O and M? N bonds for alkene alkoxylation and alkene amination reactions, respectively. Herein we provide an overview of the literature and a perspective on how these recent experiments relate to classic experiments on C? O and C? N bond formation with alkene complexes of the late transition metals.     

A mathematical relationship between the number of isomers of alkenes and alkynes: a result established from the enumeration of isomers of alkenes from alkyl biradicals

A simple and fast algorithm for enumerating alkene isomers is described. This algorithm is based on a 2-step hypothetical chemical reaction. In the first step, an alkane molecule loses two protons becoming an alkyl biradical. In the second step, two alkyl biradicals react to form an alkene molecule. By using two sequential recursive algorithms, the number of computational terms in the enumeration process is much less than when using the Henze–Blair algorithm. By using this simple algorithm, the numbers of constitutional isomers of alkenes and alkynes for carbon content greater than 30 are easily enumerated. In addition, through this algorithm, a mathematical relationship between the number of constitutional isomers of alkenes and alkynes has been established for the first time. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heteroatom‐Free Arene‐Cobalt and Arene‐Iron Catalysts for Hydrogenations

75 years after the discovery of hydroformylation, cobalt catalysts are now undergoing a renaissance in hydrogenation reactions. We have evaluated arene metalates in which the low‐valent metal species is—conceptually different from heteroatom‐based ligands—stabilized by π coordination to hydrocarbons. Potassium bis(anthracene)cobaltate 1 and ‐ferrate 2 can be viewed as synthetic precursors of quasi‐“naked” anionic metal species; their aggregation is effectively impeded by (labile) coordination to the various π acceptors present in the hydrogenation reactions of unsaturated molecules (alkenes, arenes, carbonyl compounds). Kinetic studies, NMR spectroscopy, and poisoning studies of alkene hydrogenations support the formation of a homogeneous catalyst derived from 1 which is stabilized by the coordination of alkenes. This catalyst concept complements the use of complexes with heteroatom donor ligands for reductive processes.     

B(C6F5)3‐Catalyzed Transfer of Dihydrogen from One Unsaturated Hydrocarbon to Another

A transition‐metal‐free transfer hydrogenation of 1,1‐disubstituted alkenes with cyclohexa‐1,4‐dienes as the formal source of dihydrogen is reported. The process is initiated by B(C₆F₅)₃‐mediated hydride abstraction from the dihydrogen surrogate, forming a Brønsted acidic Wheland complex and [HB(C₆F₅)₃]^?. A sequence of proton and hydride transfers onto the alkene substrate then yields the alkane. Although several carbenium ion intermediates are involved, competing reaction channels, such as dihydrogen release and cationic dimerization of reactants, are largely suppressed by the use of a cyclohexa‐1,4‐diene with methyl groups at the C1 and C5 as well as at the C3 position, the site of hydride abstraction. The alkene concentration is another crucial factor. The various reaction pathways were computationally analyzed, leading to a mechanistic picture that is in full agreement with the experimental observations.     

Aromatization of a C4 alkane/alkene/hydrogen mixture obtained by catalytic dehydrogenation of isobutane

The aim was to establish whether there is competition between the dehydrogenation of an alkane to an alkene and its aromatization in the presence of hydrogen. The aromatization of the model compound isobutane was studied in two steps, firstly its catalytic dehydrogenation and secondly the direct thermal cracking of the dehydrogenation mixture at atmospheric pressure and at temperatures at which substantial aromatization of alkenes occurs. The role of hydrogen was studied by cracking mixtures of known composition. In all instances, the yields in aromatic hydrocarbons, although lower than expected, were higher than those obtained with the alkane. The inhibitory effect of hydrogen on aromatization was demonstrated.     

Biomimetic Alkene Epoxidation and Alkane Hydroxylation with Sodium Periodate Catalyzed by Mn(III)-salen Supported on Amberlite IRA-200

Summary. The Mn(III)-salen, containing phosphonium groups at the 5,5′-positions of the salen ligand supported on Amberlite IRA-200 via electrostatic binding was used for the oxidation of alkenes and alkanes with sodium periodate at room temperature in the presence of imidazoles as axial ligands, and the effect of solvent, different axial ligands, and various oxygen donors was investigated. This heterogenized catalyst shows high catalytic activity in alkene epoxidation and alkane hydroxylation. It showed high selectivity in the epoxidation of stilbenes, α-pinene, and (R)-(+)-limonene, and exhibits a particular ability to epoxidize linear alkenes. The stability and reusability of this new heterogenized metallo-salen complex was also investigated. The catalyst was characterized by FTIR, UV-Vis, SEM, and thermal analysis.     

A new route to zinc-blende CdSe nanocrystals: mechanism and synthesis

We report the possible mechanism of forming of CdSe nanocrystals in the high boiling point solvents with long alkane chains and a novel Non-TOP-Based route to zinc-blende CdSe nanocrystals. A new mechanism shows that there exits a redox reaction in the long alkane chain solvents: Se is reduced to H2Se gas; at the same time, the long alkane chains are oxidated to alkene chains; then, the Cd complex reacts with H2Se to form CdSe nanocrystals. Possible chemical reaction equations involved in the process of forming the CdSe nanocrystals have been discussed. The alkene chain and H2Se were detected respectively by a series of experiments to support the new mechanism. Under the guidance of this mechanism, we have developed a much cheaper and greener Non-TOP-Based route for the synthesis of a size series of high-quality zinc-blende (cubic) CdSe nanocrystals. Low-cost, green, and environmentally friendlier reagents are used, without use of expensive solvents such as trioctylphosphine (TOP) or tributylphosphine (TBP). The new route enables us to achieve high-quality CdSe nanocrystals with sharp ultraviolet and visible (UV-vis) absorption peaks, controllable size (2.0-5.0 nm), bright photoluminescence (PL), narrow PL full width of half-maximum (fwhm) (29-48 nm), and high PL quantum yield (up to 60%) without any size sorting.     

Product distributions of Fischer-Tropsch synthesis over Co/AC catalyst

The product distributions of Fischer-Tropsch synthesis over Co/AC catalyst are investigated under different reaction conditions in an integral fixed bed reactor.It is found that the product distributions deviate from the ASF distribution.The deviation from ASF distribution is analyzed by taking the readsorption of alkenes and the following secondary reaction into consideration.It is noted that the contents of alcohol,alkene and alkane decline with the increasing carbon number,showing a slighter declining tendency of alkanes than those of alkenes and alcohols.It is also found that high temperature,space velocity,H2/CO in feed gas and low pressure are preferential for light hydrocarbons and alcohols while against the chain propagation.The effect of space velocity on the product distributions especially on the light products is not obvious.It is noticed that low temperature,space velocity,H2/CO and high pressure lead to high contents of alcohols;high temperature,H2/CO and low space velocity lead to high contents of alkanes.The effect of pressure on the amounts of alkanes is not significant;high space velocity and low temperature,pressure,H2/CO are preferential for alkenes.     

pi-Acidic alkene ligand effects in Pd-catalysed cross-coupling processes: exploiting the interaction of dibenzylidene acetone (dba) and related ligands with Pd(0) and Pd(II)

pi-Acidic alkene (olefin) ligands positively influence Pd-catalysed cross-coupling processes, interacting with both palladium(0) and palladium(ii) species, in some cases stabilising key catalytic intermediates. Rates of oxidative addition and reductive elimination are both affected. In certain cases, beta-hydrogen elimination can be slowed down by pi-acidic alkenes, which opens up new reaction pathways (e.g. interception of sigma-alkylpalladium(ii) species by appropriate nucleophiles). pi-Acidic alkene ligands can act independently or in a synergistic fashion with another two-electron donor ligand (e.g. amine, phosphine or N-heterocyclic carbene). The purpose of this perspective article is to highlight the impressive results that can be obtained using pi-acidic alkene ligands, with a particular focus on dibenzylidene acetone (dba) derivatives. Other types of alkene ligands, e.g. macrocyclic alkenes, are also discussed.     

K_2O和MnO助剂对Fe／Silicalite－2催化剂CO加氢制低碳烯烃性能的影响

在Ｓｉｌｉｃａｌｉｔｅ－２分子筛担载的铁催化剂中添加ＭｎＯ和Ｋ２Ｏ助剂，可显著提高其ＣＯ加氢制低碳烯烃的选择性及催化活性．ＭｎＯ助剂主要抑制乙烯和丙烯的加氢反应而提高烯／烷比值；Ｋ２Ｏ助剂则增加催化剂对ＣＯ的吸附能力，同时抑制乙烯在催化剂表面的二次反应（主要是乙烯的歧化反应），从而有利于提高低碳烯烃的选择性及催化剂活性．     

Cobalt dinitrosoalkane complexes in the C-H functionalization of olefins

The use of cobalt dinitrosoalkane complexes in the C-H functionalization of alkenes has been demonstrated. Reaction of a series of alkenes with Me4CpCo(CO)2 in the presence of NO generates intermediate cobalt dinitrosoalkane complexes that can be deprotonated alpha to the nitrosyl group and added to various Michael acceptors. The resultant products can then undergo retrocycloaddition reactions in the presence of the original alkene to regenerate the starting cobalt dinitrosoalkane complex and release the functionalized alkene.     

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar−Ir−H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk–Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir−C bond followed by C−H reductive elimination to afford the hydrofunctionalization products site-selectively.     

Rhodium phosphine-π-arene intermediates in the hydroamination of alkenes

A detailed mechanistic study of the intramolecular hydroamination of alkenes with amines catalyzed by rhodium complexes of a biaryldialkylphosphine is reported. The active catalyst is shown to contain the phosphine ligand bound in a κ(1), η(6) form in which the arene is π-bound to rhodium. Addition of deuterated amine to an internal olefin showed that the reaction occurs by trans addition of the N-H bond across the C═C bond, and this stereochemistry implies that the reaction occurs by nucleophilic attack of the amine on a coordinated alkene. Indeed, the cationic rhodium fragment binds the alkene over the secondary amine, and the olefin complex was shown to be the catalyst resting state. The reaction was zero-order in substrate, when the concentration of olefin was high, and a primary isotope effect was observed. The primary isotope effect, in combination with the observation of the alkene complex as the resting state, implies that nucleophilic attack of the amine on the alkene is reversible and is followed by turnover-limiting protonation. This mechanism constitutes an unusual pathway for rhodium-catalyzed additions to alkenes and is more closely related to the mechanism for palladium-catalyzed addition of amide N-H bonds to alkenes.     

The reactions of O(3P) with terminal alkenes: the H2CO channel via 3,2 H-atom shift

The step-scan time-resolved FTIR emission spectroscopy is used to characterize systematically the H(2)CO channel for the reactions of O((3)P) with various alkenes. IR emission bands due to the products of CO, CO(2), and H(2)CO have been observed in the spectra. H(2)CO is identified to be the primary reaction product whereas CO and CO(2) are secondary reaction products of O((3)P) with alkenes. A general trend is observed in which the fraction yield of the H(2)CO product increases substantially as the reactant alkene varies from C(2)H(4), C(3)H(6), 1-C(4)H(8), iso-C(4)H(8), to 1-C(5)H(10). The formation mechanism of the H(2)CO is therefore elucidated to arise from a 3,2 H-atom shift followed by breaking of the C(1)-C(2) bond in the initially formed energized diradical RCH(2)CHCH(2)O*. The 3,2 H-atom shift may become the dominant process with the more rapid delocalization of the energy when the hydrocarbon chain of the alkene molecule is lengthened.     

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.     

Single-Atom Nickel on Carbon Nitride Photocatalyst Achieves Semihydrogenation of Alkynes with Water Protons via Monovalent Nickel

Prospects in light-driven water activation have prompted rapid progress in hydrogenation reactions. We describe a Ni²⁺−N₄ site built on carbon nitride for catalyzed semihydrogenation of alkynes, with water supplying protons, powered by visible-light irradiation. Importantly, the photocatalytic approach developed here enabled access to diverse deuterated alkenes in D₂O with excellent deuterium incorporation. Under visible-light irradiation, evolution of a four-coordinate Ni²⁺ species into a three-coordinate Ni⁺ species was spectroscopically identified. In combination with theoretical calculations, the photo-evolved Ni⁺ is posited as HO−Ni⁺−N₂ with an uncoordinated, protonated pyridinic nitrogen, formed by coupled Ni²⁺ reduction and water dissociation. The paired Ni−N prompts hydrogen liberation from water, and it renders desorption of alkene preferred over further hydrogenation to alkane, ensuring excellent semihydrogenation selectivity.