Mechanistic Study of Palladium-Catalyzed Oxidative C-H/C-H Coupling of Polyfluoroarenes with Simple Arenes

Pd-catalyzed oxidative C-H/C-H coupling reaction is an emerging type of C-H acti-vation reaction, which attracts great interests because both reaction partners do not re-quire pre-functionalization. In the present study, we employed DFT methods to investigatethe mechanism of Pd(OAc)₂-catalyzed oxidative C-H/C-H coupling of pentafluoroben-zene with benzene. Four possible pathways were examined in the C-H activation part: path A benzene-pentafluorobenzene mechanism (C-H activation of benzene occurs before the C-H activation of pentafluorobenzene), path B pentafluorobenzene-benzene mechanism (C-H activation of benzene occurs after the C-H activation of pentafluorobenzene), path C benzene-pentafluorophenylsilver mechanism (C-H activation of benzene and subsequenttransmetalation with pentafluorophenyl silver complex), path D pentafluorophenylsilver-benzene mechanism (transmetalation with pentafluorophenyl silver complex and subsequent C-H activation of benzene). Based on the calculations, the sequences of two C-H activation steps are found to be different in the oxidative couplings of same substrates (i.e. pentaflu-orobenzene and benzene) in different catalytic systems, where the additive Ag salts played a determinant role. In the absence of Ag salts, the energetically favored pathway is path B (i.e. the C-H activation of pentafluorobenzene takes place before the C-H cleavage of benzene). In contrast, with the aid of Ag salts, the coordination of pentafluorophenylsilver to Pd center could occur easily with a subsequent C-H activation of benzene in the second step, and the second step significantly raises the whole reaction barrier. Alternatively, in thepresence of Ag salts, the kinetically preferred mechanism is path C (i.e. the C-H activation of benzene takes place in the first step followed by transmetalation with pentafluorophenyl-silver complex), which is similar to path A. The calculations are consistent with the H/D exchange experiment and kinetic isotope effects. Thus the present study not only offers a deeper understanding of oxidative C-H/C-H coupling reaction, but also provides helpful insights to further development of more efficient and selective oxidative C-H/C-H coupling reactions.     

Conversion of methane to methanol at the mononuclear and dinuclear copper sites of particulate methane monooxygenase (pMMO): a DFT and QM/MM study

Methane hydroxylation at the mononuclear and dinuclear copper sites of pMMO is discussed using quantum mechanical and QM/MM calculations. Possible mechanisms are proposed with respect to the formation of reactive copper-oxo and how they activate methane. Dioxygen is incorporated into the Cu(I) species to give a Cu(II)-superoxo species, followed by an H-atom transfer from a tyrosine residue near the monocopper active site. A resultant Cu(II)-hydroperoxo species is next transformed into a Cu(III)-oxo species and a water molecule by the abstraction of an H-atom from another tyrosine residue. This process is accessible in energy under physiological conditions. Dioxygen is also incorporated into the dicopper site to form a (mu-eta(2):eta(2)-peroxo)dicopper species, which is then transformed into a bis(mu-oxo)dicopper species. The formation of this species is more favorable in energy than that of the monocopper-oxo species. The reactivity of the Cu(III)-oxo species is sufficient for the conversion of methane to methanol if it is formed in the protein environment. Since the sigma orbital localized in the Cu-O bond region is singly occupied in the triplet state, this orbital plays a role in the homolytic cleavage of a C-H bond of methane. The reactivity of the bis(mu-oxo)dicopper species is also sufficient for the conversion of methane to methanol. The mixed-valent bis(mu-oxo)Cu(II)Cu(III) species is reactive to methane because the amplitude of the sigma singly occupied MO localized on the bridging oxo moieties plays an essential role in C-H activation.     

Oxidative cyclization of 2-arylphenols to dibenzofurans under Pd(II)/peroxybenzoate catalysis

2-Arylphenols undergo intramolecular C-H bond activation/C-O bond formation to afford dibenzofuran derivatives under palladium catalysis in the presence of tert-butyl peroxybenzoate as an oxidant. Kinetic isotope effect experiments indicated that C-H bond cleavage is the rate-limiting step of the reaction.     

Cu(ClO(4))(2)-mediated arene C-H bond halogenations of azacalixaromatics using alkali metal halides as halogen sources

Regiospecific halogenation of azacalix[1]arene[3]pyridines at the lower rim position of the benzene ring was achieved from their cross-coupling reaction with cost-effective alkali metal halides through the Cu(ClO(4))(2)-mediated aerobic aryl C-H activation, which gave structurally well-defined aryl-Cu(III) intermediates, and a subsequent C-X bond formation reaction under very mild conditions.     

Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.     

Evidence for a C-H...pi type weak interaction: 1 : 1 complex of styrene with acetylene studied by mass selective high-resolution UV spectroscopy and ab initio calculations

The 1 : 1 complex of styrene with acetylene has been studied by mass selective low- and high-resolution UV resonance-enhanced two-photon ionisation (R2PI) spectroscopy combined with genetic-algorithm-based computer-aided fit of the spectra with partial rotational resolution, and high level ab initio quantum chemistry calculations. Two stable conformeric geometries of the 1 : 1 complex of styrene and acetylene have been theoretically found: one with acetylene binding to styrene as a proton donor, and one with acetylene acting as a proton acceptor. From the analysis of the vibronic structure of the S1<-- S0 spectrum and the fit of the highly resolved spectrum of the 0 origin band of the complex it is shown that the favoured conformation is the one in which acetylene binds to the benzene ring of styrene through formation of a non-conventional hydrogen bond of C-H...pi type with no marked change of the transition moment orientation of styrene. The styrene moiety remains planar and the acetylene molecule is tilted by a small angle of 4 degrees relative to the C6 symmetry axis of the benzene ring, most likely due to the reduced symmetry of the benzene ring pi electrons rather than to a direct interaction with the vinyl group.     

Experimental and theoretical study on the structure and formation mechanism of [C6H5Cu(m)]- (m = 1-3)

The important intermediate phenyl-copper metal complexes [C(6)H(5)Cu(m)]- (m = 1-3), which are produced from the reactions between copper metal clusters formed by laser ablation and the benzene molecules seeded in argon carrier gas, are studied by photoelectron spectroscopy(PES) and density functional theory (DFT). Their structures and bonding patterns are investigated, which results in the conclusion that C(6)H(5) groups bond perpendicularly on copper clusters through Cu-C sigma bond. The formation mechanism of these complexes has been studied at B3LYP//6-311G(d, p)/Lanl2dz level. Direct insertion reaction between [Cu(m)]- and C(6)H(6) yields intermediate complex [C(6)H(5)Cu(m)H]-, and then eliminates the H atom, or releases the H atom to other neutral Cu atoms or anionic Cu ions via H abstraction reaction. The first step is the rate-limiting step with C-H activation and cleavage, and H abstraction by neutral Cu atom is the most energetically favorable pathway for the final step. Moreover, the complex [C(6)H(5)Cu(2)]- is ascertained to be easier to be generated than [C(6)H(5)Cu(3)]- and [C(6)H(5)Cu]-, which are in excellent agreement with the experimental results.     

Regiospecific functionalization of azacalixaromatics through copper-mediated aryl C-H activation and C-O bond formation

Regiospecific functionalization of tetraazacalix[1]arene[3]pyridines at the lower rim position of the benzene ring was achieved conveniently from their cross-coupling reaction with both aliphatic alcohols including chiral primary and secondary alcohols and phenol derivatives through the Cu(ClO(4))(2)-mediated aerobic aryl C-H activation, which gave structurally well-defined aryl-Cu(III) intermediates and a subsequent C-O bond formation reaction under very mild conditions.     

Synthesis, properties, and redox behavior of mono-, bis-, and tris[1,1,4,4,-tetracyano-2-(1-azulenyl)-3-butadienyl] chromophores binding with benzene and thiophene cores

Mono-, bis-, tris-, and tetrakis(1-azulenylethynyl)benzene and mono- and bis(1-azulenylethynyl)thiophene derivatives 5-10 have been prepared by Pd-catalyzed alkynylation of ethynyl arenes with 1-iodoazulene derivative or the 1-ethynylazulene derivative with tetraiodobenzene and iodothiophenes under Sonogashira-Hagihara conditions. Compounds 5-10 reacted with tetracyanoethylene in a [2+2] cycloaddition reaction to afford the corresponding 1,1,4,4,-tetracyano-2-(5-isopropyl-3-methoxycarbonyl-1-azulenyl)-3-butadienyl chromophores 12-16 in excellent yields, except for the reaction of the tetrakis(1-azulenylethynyl)benzene derivative. 1,1,4,4,-Tetracyano-2,3-bis(1-azulenyl)butadiene (17) was also prepared by the similar reaction of bis(1-azulenyl)acetylene (11) with tetracyanoethylene (TCNE). The redox behavior of novel azulene derivatives 12-17 was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed multistep electrochemical reduction properties. Moreover, a significant color change was observed by visible spectroscopy under electrochemical reduction conditions.     

Low-temperature synthesis of amorphous carbon nanocoils via acetylene coupling on copper nanocrystal surfaces at 468 K: a reaction mechanism analysis

A new type of amorphous helical carbon nanofibers has been synthesized using copper nanocatalysts and an acetylene gas source at atmospheric pressure. The nanofibers are grown at 468 K, which is the lowest temperature by ordinary metal-catalyzed thermal chemical vapor deposition of hydrocarbon, and exhibit a symmetric growth mode in the form of twin helices. IR, XRD, Raman, and C/H molar ratio analyses reveal a polymer-like structure with a weak trans-polyacetylene feature. The nanofibers are a mixture of solid polymers and a small amount of carbon. A reaction mechanism has been proposed on the basis of the previous studies of acetylene adsorption, desorption properties, and surface reactions on copper (111), (110), and (001) planes under ultrahigh-vacuum (UHV) conditions as well as the results obtained in our study. The reaction mechanism of acetylene on copper single-crystal surfaces under UHV conditions indeed reflects the reaction mechanism under practical catalytic conditions at atmospheric pressure. The nanofibers grow mainly via acetylene coupling to solid polymers on copper nanocrystal surfaces. Acetylene also couples to yield small amounts of liquid oligomers and gaseous products, and undergoes slight carbon deposition during the fiber growth.     

甲硫醇、甲醇在金属表面上的分解反应性能比较研究

用键级守恒-Morse势法比较了XH₃SH、CH₃OH在Ni(111)、Pt(111)和Cu(111)面上的热分解反应性能.结果表明,由Ni至Pt再至Cu,CH₃SH中C-S键断裂几率降低,而S-H键裂相对几率增加,而在Cu上仅发生S-H键裂.CH₃OH在Ni上易解离为CH₃和CH₃O-,在Pt、Cu上则较难解离.按Ni、Pt、Cu的顺序,CH₃S-选择性断裂C-S键的几率增加,而CH₃O-则选择性断裂C-H键的几率增加.CH₃XH、CH₃X-(X=O,S)解离活化能垒依Ni、Pt、Cu的顺序增加,CH₃OH在Ni上最终解离为CO,而在Cu上则解离为H₂CO.     

Experimental and modeling of oxidation of acetylene,propyne, allene and 1,3‐butadiene

The ignition delays of insaturated hydrocarbons‐oxygen‐argon mixtures were measured behind shock waves in the cases of acetylene, propyne, allene and 1,3‐butadiene. Reflected shock waves permitted to obtain temperatures from 1000–1650 K and pressures from 8.5 to 10.0 atm. A particular effort has been made to build a detailed mechanism of the reactions of C₃‐C₄ unsaturated species and benzene. This mechanism is based on the most recent kinetic data values published in the literature and is consistent with the thermochemistry. This mechanism has been validated by comparing the results of our simulations to the experimental results obtained in our shock tube experiments and to profiles of radicalar and molecular species measured in three premixed flames (acetylene [1–2] and 1,3‐butadiene [3]) coming from the literature. The main reaction pathways have been derived in the case of the oxidation of these four insaturated hydrocarbons and for the formation of benzene. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 361–379, 1999     

Microscopic models of PdZn alloy catalysts: structure and reactivity in methanol decomposition

We review systematic experimental and theoretical efforts that explored formation, structure and reactivity of PdZn catalysts for methanol steam reforming, a material recently proposed to be superior to the industrially used Cu based catalysts. Experimentally, ordered surface alloys with a Pd : Zn ratio of approximately 1 : 1 were prepared by deposition of thin Zn layers on a Pd(111) surface and characterized by photoelectron spectroscopy and low-energy electron diffraction. The valence band spectrum of the PdZn alloy resembles closely the spectrum of Cu(111), in good agreement with the calculated density of states for a PdZn alloy of 1 : 1 stoichiometry. Among the issues studied with the help of density functional calculations are surface structure and stability of PdZn alloys and effects of Zn segregation in them, and the nature of the most likely water-related surface species present under the conditions of methanol steam reforming. Furthermore, a series of elementary reactions starting with the decomposition of methoxide, CH(3)O, along both C-H and C-O bond scission channels, on various surfaces of the 1 : 1 PdZn alloy [planar (111), (100) and stepped (221)] were quantified in detail thermodynamically and kinetically in comparison with the corresponding reactions on the surfaces Pd(111) and Cu(111). The overall surface reactivity of PdZn alloy was found to be similar to that of metallic Cu. Reactive methanol adsorption was also investigated by in situ X-ray photoelectron spectroscopy for pressures between 3 x 10(-8) and 0.3 mbar.     

Direct carbonylation at a C-H bond in the benzene ring of 2-phenyloxazolines catalyzed by Ru(3)(CO)(12). Scope, limitations, and mechanistic aspects

The ruthenium-catalyzed carbonylation at a C-H bond in the benzene ring of a 2-phenyloxazoline is described. The reaction of 2-phenyloxazolines with CO and ethylene in toluene in the presence of a catalytic amount of Ru(3)(CO)(12) resulted in propionylation at an ortho C-H bond in the benzene ring. The presence of the oxazoline ring on the benzene ring is essential for the carbonylation to proceed. Other heterocycles, such as oxazine, oxazole, and thiazoline rings, also served as acceptable directing groups as did the oxazoline ring. A wide functional group compatibility was observed. The site selectivity of the carbonylation was examined using meta-substituted phenyloxazolines. It was found that the carbonylation took place exclusively at the less-hindered C-H bond, irrespective of the nature of substituents, indicating that the site selectivity was determined by steric factors. The reaction was also applicable, not only to a benzene ring, but also to naphthyl and thiophenyl rings. Olefins such as propene and trimethylvinylsilane in place of ethylene could also be used in the carbonylation reaction, while other olefins, such as 1-hexene, tert-butylethylene, vinylcyclohexane, isoprene, 1,5-hexadiene, cyclohexene, 1, 5-cyclooctadiene, styrene, methyl acrylate, vinyl acetate, allyltrimethylsilane, and triethoxyvinylsilane did not afford the coupling products. An equilibrium between 2-phenyloxazolines, carbon monoxide, and olefins exists on one hand and the corresponding ketones on the other hand, and product composition is governed by the equilibrium thermodynamics of the system. The results of deuterium labeling experiments suggest that the catalysis involves a reversible C-H bond cleavage and that the rate-determining step is not the cleavage of a C-H bond. The results of kinetic study of the effects of CO pressure show that the reaction rate accelerates with decreasing CO pressure.     

Assessment of the intermediacy of arylpalladium carboxylate complexes in the direct arylation of benzene: evidence for C-H bond cleavage by "ligandless" species

Palladium-catalyzed direct arylations of benzene have been proposed to occur by the generation of a phosphine-ligated arylpalladium pivalate complex LPd(Ar)(OPiv) and reaction of this complex with benzene. We have isolated an example of the proposed intermediate and evaluated whether this complex does react with benzene to form the biaryl products of direct arylation. In contrast to the proposed mechanism, no biaryl product was formed from cleavage of the benzene C-H bond by LPd(Ar)(OPiv). However, reactions of LPd(Ar)(OPiv) with benzene and additives that displace or consume the phosphine ligand formed the arylated products in good yield, suggesting that a "ligandless" arylpalladium(II) carboxylate complex undergoes the C-H cleavage step. Consistent with this conclusion, we found that reactions catalyzed by Pd(OAc)(2) without a ligand occur faster than, and with comparable selectivities to, reactions catalyzed by Pd(OAc)(2) and a phosphine ligand.     

Phase transfer Pd(O) catalyzed polymerization reactions. I. Synthesis of 1,2-(4,4′;-dialkoxyaryl) acetylene monomers and 1,4-Bis[2-(4′,4″-dialkoxyphenyl)ethynyl]benzene derivatives by phase transfer Pd(O)/Cu(I) catalyzed coupling reactions

Several 1,2-(4,4'-dialkoxyaryl)acetylene monomers containing similar or dissimilar substituents were prepared by one of three variations of a one pot phase transfer Pd(O)/Cu(I) catalyzed coupling of aryl halides with a protected acetylene. The three steps within a single flask involved first coupling of the appropriate 1-halo-4-alkoxybenzene derivative with 2-methyl-3-butyn-2-ol, followed by cleavage of the carbinol group to form an aryl acetylide, and finally a second coupling of an aryl halide with the aryl acetylide. Best results were obtained when elevated temperatures and solid-liquid phase transfer reaction conditions were used. Some 1,4-bis[2-(4',4”-dialkoxyphenyl)ethynyl]benzene compounds were also prepared.     

Two-dimensional Molecular Phase Transition of Alkylated-TDPB on Au(111) and Cu(111) Surfaces

The derivatives of aromatic cores bearing alkyl chains with different lengths are of potential interest in on-surface chemistry, and thus have been widely investigated both at liquid-solid interfaces and in vacuum. Here, we report on the structural evaluation of self-assembled 1,3,5-tri(4-dodecylphenyl)benzene(TDPB) molecules with increased molecular coverages on both Au(111) and Cu(111) surfaces. As observed on Au(111), rhombic and herringbone structures emerge successively depending on surface coverage. In the case of Cu(111), the same process of phase conversion is also observed, but with two distinct structures. In comparison, the self-assembled structures on Au(111) surface are packed more densely than that on Cu(111) surface under the same preparation conditions. This may fundamentally result from the higher adsorption energy of TDPB molecules on Cu(111), restricting their adjustment to optimize a thermodynamically favorable molecular packing.     

Local chemical reaction of benzene on Cu110 via STM-induced excitation

We have investigated the mechanism of the chemical reaction of the benzene molecule adsorbed on Cu(110) surface induced by the injection of tunneling electrons using scanning tunneling microscopy (STM). With the dosing of tunneling electrons of the energy 2-5 eV from the STM tip to the molecule, we have detected the increase of the height of the benzene molecule by 40% in the STM image and the appearance of the vibration feature of the nu(C-H) mode in the inelastic tunneling spectroscopy (IETS) spectrum. It can be understood with a model in which the dissociation of C-H bonds occurs in a benzene molecule that induces a bonding geometry change from flat-lying to up-right configuration, which follows the story of the report of Lauhon and Ho on the STM-induced change of benzene on the Cu(100) surface. [L. J. Lauhon and W. Ho, J. Phys. Chem. A 104, 2463 (2000)]. The reaction probability shows a sharp rise at the sample bias voltage at 2.4 V, which saturates at 3.0 V, which is followed by another sharp rise at the voltage of 4.3 V. No increase of the reaction yield is observed for the negative sample voltage up to 5 eV. In the case of a fully deuterated benzene molecule, it shows the onset at the same energy of 2.4 eV, but the reaction probability is 10(3) smaller than the case of the normal benzene molecule. We propose a model in which the dehydrogenation of the benzene molecule is induced by the formation of the temporal negative ion due to the trapping of the electrons at the unoccupied resonant states formed by the pi orbitals. The existence of the resonant level close to the Fermi level ( approximately 2.4 eV) and multiple levels in less than approximately 5 eV from the Fermi level, indicates a fairly strong interaction of the Cu-pi(*) state of the benzene molecule. We estimated that the large isotope effect of approximately 10(3) can be accounted for with the Menzel-Gomer-Redhead (MGR) model with an assumption of a shallow potential curve for the excited state.     

Cupric superoxo-mediated intermolecular C-H activation chemistry

The new cupric superoxo complex [LCu(II)(O(2)(?-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.     

Adsorption, mobility, and dimerization of benzaldehyde on Pt(111)

Building on results for the adsorption of benzene on Pt(111), the adsorption of benzaldehyde is investigated using density functional theory. Benzaldehyde is found to chemisorb preferentially with its aromatic ring in the flat-lying bridge geometry that is also preferred for benzene. Across the investigated geometries, adsorption is homogeneously weakened compared to corresponding benzene geometries. This is found to be true for very different adsorption modes, namely, η(6) and η(8) modes, the latter having metal atoms inserted in the carbonyl bond. Reorientation and diffusion of benzaldehyde is found to have low energy barriers. Aggregation of molecules in dimers bound by aryl C-H···O hydrogen bonds is investigated, and specific configurations are found to be up to 0.15 eV more favorable than optimally configured, separated adsorbates. The binding is significantly stronger than what is found for gas phase dimers, suggesting an enhancing effect of the metal interaction.