Nucleophilic dephosphorylation of p-nitrophenyl diphenyl phosphate in cationic micellar media

The kinetics of nucleophilic dephosphorylation of p-nitrophenyl diphenyl phosphate by hydroxamate ions (R'(C=O)N(RO-)) have been investigated in aqueous cationic micellar media at pH 9.12 and 27 degrees C. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. N-Substituted hydroxamate ion shows higher reactivity over the unsubstituted hydroxamate ions in cationic micellar media. The kinetic data are discussed in terms of the pseudophase ion exchange model.     

Stereospecific Construction of Chiral Tertiary and Quaternary Carbon by Nucleophilic Cyclopropanation with Bis(iodozincio)methane

The reaction of a ketone having a leaving group at the α‐position, such as α,β‐epoxy ketone or α‐sulfonyloxy ketone, with bis(iodozincio)methane affords a zinc alkoxide of cyclopropanol. The reaction proceeds by nucleophilic addition of the dizinc to the carbonyl group and a sequential intramolecular nucleophilic substitution of the introduced iodozinciomethyl group to the adjacent electrophilic carbon that has a leaving group. When an optically active α,β‐epoxy ketone or α‐sulfonyloxy ketone is treated with the dizinc, a zinc alkoxide of cyclopropanol having a chiral tertiary or quaternary carbon in the cyclopropane ring is obtained, and the obtained zinc alkoxide of cyclopropanol acts as a chiral homoenolate. When it is treated with an electrophile in the presence of copper cyanide, it gives an optically active α‐tertiary or ‐quaternary ketone that retains high optical purity.     

Nucleophilic debenzoylation of p‐nitrophenyl benzoate in cationic micellar media

Pseudo‐first‐order rate constants for the nucleophilic debenzoylation reaction of p‐nitrophenyl benzoate with various hydroxamate ions [RC = ONHO^?] were investigated in aqueous cationic micellar media at pH 7.8 and 27°C. The kinetic rate data of the reaction revealed that the nucleophilic reactivity sequence of these hydroxamate ions is generally benzohydroxamic acid > salicylhydroxamic acid > acetohydroxamic acid. The k_obs value increases upon addition of cationic surfactants to the reaction medium involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. The effect of surfactant head and tail group is discussed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 106–112, 2010     

Palladium-Catalyzed Catellani Reaction with 1,1-Bis[(pinacolato)boryl]methane as the Nucleophilic Component

We report herein the application of 1,1-bis[(pinacolato)boryl]methane as a new type of terminating reagent to realize three-component coupling reaction with aryl iodide and different electrophiles. A series of ortho-disubstituted benzylboronates can be synthesized and the ortho-substituent can be varied by using bromoacetic esters or O-benzoyl hydroxylamine as electrophiles with slightly modified reaction conditions. The reaction shows good functional group tolerance and the obtained benzyl boronate products can be easily transformed to various synthetically useful compounds.     

Coordinational activation of methane and other alkanes by metal complexes

A highly significant challenge for chemistry is the search of the selective catalytic processes for the direct methane transformation into valuable products. The review discusses activation of inert alkane molecule by coordination to a metal atom via three-center two-electron bond involving donor-acceptor and dative interactions. This coordination weakens the C—H bond and opens up new possibilities for homolytic and heterolytic methane reactions.     

Clustering and activation in reactions of CoCp with hydrogen and methane

Gas-phase clustering reactions of CoCp⁺ with H₂ and with CH₄ were investigated using temperature-dependent equilibrium experiments. In both systems, the CoCp⁺ ion was found to form strong interactions with two ligands. The first and second H₂ groups cluster to CoCp⁺ with bond energies of 16.2 and 16.8 kcal/mol, respectively, while the first and second CH₄ groups cluster to CoCp⁺ with bond energies of 24.1 and 12.1 kcal/mol, respectively. These bond energies are in good agreement with those determined by density functional theory (DFT). Molecular geometries for the four clusters determined with DFT are also presented. Weak experimental bond energies of 0.9 kcal/mol for the third H₂ and 2.2 kcal/mol for the third CH₄ clustering to CoCp⁺ suggest these ligands occupy the second solvation shell of the ion. In addition to clustering in the methane system, H₂-elimination from CoCp(CH₄)₂⁺ was observed. The mechanism for this reaction was investigated by collision-induced dissociation experiments and DFT, which suggest the predominate H₂-elimination product is (c-C₅H₆)Co⁺---C₂H₅. Theory indicates that dehydrogenation requires the active participation of the Cp ring in the mechanism. Transfer of H and CH₃ groups to the C₅-ring ligand allows the metal center to avoid the high-energy Co(IV) oxidation state required when it forms two covalent bonds in addition to its interaction with a C₅-ring ligand.     

Nucleophilic attack by oxyanions on a phosphate monoester dianion: the positive effect of a cationic general Acid

The two negative charges on a phosphate monoester RO-PO32- at neutral pH provide a considerable electrostatic barrier toward reactions with nucleophilic reagents with a negative charge on the attacking atom. Electrostatic repulsion disappears when the hydrolysis of an aryl phosphate monoester is catalyzed by a neighboring cationic general acid. The hydrolysis of 8-dimethylammonium-1-phosphate (1) is catalyzed by oxyanions, fluoride anion, and hydroxylamines at similar rates.     

The nature of cationic adsorption sites in alkaline zeolites--single, dual and multiple cation sites

Gas adsorption on zeolites constitutes the base of many technological applications of these versatile porous materials. Quite often, especially when dealing with small molecules, individual extra-framework (exchangeable) cations are considered to be the adsorption site on which molecules coming from a gas phase form the corresponding adsorption complex. Nonetheless, while that can be the case in some instances, recent research work that combines variable temperature infrared spectroscopy with periodic DFT calculations showed that some types of adsorption sites involve two or more cations, which constitute dual and multiple cation sites, respectively. Adsorption complexes formed on these cationic adsorption sites differ in both structure and stability from those formed on a single cation alone. Examples concerning CO, CO(2) and H(2) adsorption on alkali and alkaline-earth metal exchanged zeolites are reviewed, with the double purpose of clarifying concepts and highlighting their relevance to practical use of zeolites in such fields as gas separation and purification, gas storage and heterogeneous catalysis.     

Substituent effects on methane activation and elimination by high-valent Zr complexes

A computational study of substituent effects on methane activation and elimination by high-valent zirconium complexes is reported. Substituent (Z) effects (in a structural, electronic, and enthalpic sense) are substantially less important for the imido (L_nZr(DOUBLE BOND)NZ) and imidolike TS than the amido (L_nM(SINGLE BOND)NHZ). For the microscopic reverse reaction, methane CH activation, it suggests that tailoring imido reactivity through electronic modification of nitrogen substituents will be difficult. Analysis of the earliest part of the reaction coordinate for methane elimination entails structural deformation of the Zr—amido to assume an appropriate geometry for elimination, which, in some cases, is directly reflected in substantially higher elimination barriers. Lower elimination barriers correlate with stronger agostic bonding, providing further support for the crucial importance of agostic bonding in facilitating alpha-elimination processes for high-valent transition-metal complexes. © 1996 John Wiley & Sons, Inc.     

A study of methane activation by modified gallium- and zinc-based catalysts

The activation of CH₄ has been probed by studying CH₄ oxidation at ambient pressure over gallium- and zinc-based catalysts prepared by precipitation and modified with Au and Pt. The unmodified gallium- and zinc-based catalysts were both active for CH₄ oxidation. Modification of these catalysts by the addition of Au and Pt, alone and in combination significantly increased the rate of CH₄ oxidation. The 1% Pt/Ga₂O₃ catalyst was the most active of the gallium-based systems. The addition of Au to ZnO markedly increased the activity compared with unmodified ZnO, whilst for Au in combination with Pt the activity was further enhanced due to a synergistic effect of the metals.     

Computational study of methane activation by mercury(II) complexes

A computational investigation of methane activation by Hg^II complexes is reported. Calculated geometries and energetics of Hg(II)-containing reactants and products are consistent with available experimental data for a wide range of diverse ligand types. Calculated reaction enthalpies and activation barriers for Hg^II complexes cover a wide range of values for different ligands. This diversity suggests that the kinetics and thermodynamics of methane activation by Hg(II) and related medium-valent complexes can be tailored through rational modification of the ligand environment. Calculated activation barriers and reaction enthalpies for methane activation by Hg(II) complexes indicates that harder, more electronegative, ligands are kinetically and thermodynamically preferred. Potential donor groups on the activating ligand can stabilize the transition state versus the ground state reactants and hence result in substantially lower methane activation barriers. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 902–911, 1998     

Intermediates in dioxygen activation by methane monooxygenase: a QM/MM study

Protein effects in the activation of dioxygen by methane monooxygenase (MMO) were investigated by using combined QM/MM and broken-symmetry Density Functional Theory (DFT) methods. The effects of a novel empirical scheme recently developed by our group on the relative DFT energies of the various intermediates in the catalytic cycle are investigated. Inclusion of the protein leads to much better agreement between the experimental and computed geometric structures for the reduced form (MMOH(red)). Analysis of the electronic structure of MMOH(red) reveals that the two iron atoms have distinct environments. Different coordination geometries tested for the MMOH(peroxo) intermediate reveal that, in the protein environment, the mu-eta2,eta2 structure is more stable than the others. Our analysis also shows that the protein helps to drive reactants toward products along the reaction path. Furthermore, these results demonstrate the importance of including the protein environment in our models and the usefulness of the QM/MM approach for accurate modeling of enzymatic reactions. A discrepancy remains in our calculation of the Fe-Fe distance in our model of HQ as compared to EXAFS data obtained several years ago, for which we currently do not have an explanation.     

Mechanism of C-O activation in dimethoxyethane cationic iron complexes

The fragmentation mechanism of iron complexes bearing a bidentate ligand, dimethoxyethane (CH(3)OCH(2)CH(2)OCH(3), labeled as DXE) has been investigated by means of FT-ICR mass spectrometry (ion-molecule reactions) and infrared multiphoton dissociation spectroscopy. Two possible reaction mechanisms were envisioned for the Fe(DXE)(+) + DXE reaction, leading to the formation of the Fe(CH(2)O)(DXE)(+) ion. The two mechanisms differ in the nature of the neutral molecules formed: CH(3)OC(2)H(5) or CH(2)=CH(2) + CH(3)OH. The combination of ion-molecule reactions, thermochemistry considerations, and IRMPD spectra leads us to suggest that the mechanism involves successive elimination of the neutrals CH(2)=CH(2) and CH(3)OH, the first step of the mechanism being the insertion of the iron atom in the O-C(central) bond.