T:Toxic;
106-44-5Relevant articles and documents
Butanolysis of 4-methylbenzenediazonium ions in binary n-BuOH/H 2O mixtures and in n-BuOH/SDS/H2O reverse micelles. Effects of solvent composition, acidity and temperature on the switch between heterolytic and homolytic dediazoniation mechanisms
Fernandez-Alonso, Alejandra,Pastoriza Gallego, Ma Jose,Bravo-Diaz, Carlos
, p. 5304 - 5312 (2010)
We investigated the effects of solvent composition, acidity and temperature on the switch between heterolytic and homolytic mechanisms in the course of the butanolysis of 4-methylbenzenediazonium (4MBD) ions in binary BuOH/H 2O mixtures and in reverse micelles, RMs, composed of n-BuOH, H 2O and sodium dodecyl sulfate, SDS, by employing a combination of spectrometric (UV/vis) and chromatographic (HPLC) techniques. In reaction mixtures with high n-BuOH percentages, S-shaped variations of kobs with acidity, defined hereafter as -log([HCl]), are obtained with rate enhancements of up to ～370-fold on going from -log([HCl]) = 2 to 6, with inflection points at -log[HCl] ～ 4. HPLC analyses of the reaction mixtures show that the substitution product 4-cresol, ArOH and the reduction product toluene, ArH, are formed competitively. The variation of their yields with acidity is also S-shaped, so that at high acidities (-log[HCl] obs and of the product yields with acidity are found in the -log[HCl] = 3-5 range, suggesting that a turnover in the dediazoniation mechanism takes place under acidic conditions. The results can be interpreted in terms of two competitive reaction pathways, one heterolytic, involving a rate-determining formation of an extremely reactive aryl cation that traps the nucleophiles available in its solvation shell leading to the formation of substitution products (DN + AN mechanism) and a second route where the BuOH reacts with 4MBD to yield an unstable O-adduct of the type Ar-NN-O-R (diazo ether) in a rapid pre-equilibrium step that initiates a radical process leading to the formation of the reduction product ArH (O-coupling mechanism). The results illustrate how the heterolytic and homolytic mechanisms can be switched by just changing the acidity of the solution. Kinetic analyses of the variations of kobs with acidity at different temperatures allowed us to separate kobs into the components for the heterolytic pathway, kHET, and that for the homolytic one, kHOM, to determine relevant thermodynamic parameters for both reaction pathways and for the equilibrium constant K for the formation of the O-adduct Ar-NN-O-R.
CoMo sulfide-catalyzed hydrodeoxygenation of lignin model compounds: An extended reaction network for the conversion of monomeric and dimeric substrates
Jongerius, Anna L.,Jastrzebski, Robin,Bruijnincx, Pieter C.A.,Weckhuysen, Bert M.
, p. 315 - 323 (2012)
In the present work, extensive hydrodeoxygenation (HDO) studies with a commercial sulfided CoMo/Al2O3 catalyst were performed on a library of lignin model compounds at 50 bar hydrogen pressure and 300 °C in dodecane, using a batch autoclave system. The catalyst was activated under hydrogen atmosphere prior to the reaction, and the spent catalyst was analyzed using thermogravimetric analysis. An extended reaction network is proposed, showing that HDO, demethylation, and hydrogenation reactions take place simultaneously. HDO of mono-oxygenated substrates proved to be difficult at the applied conditions. Starting from most positions in the network, phenol, and cresols are therefore the main final products, suggesting the possibility of convergence on a limited number of products from a mixture of substrates. HDO of dimeric model compounds mimicking typical lignin linkages revealed that coumaran alkyl ethers and β-O-4 bonds can be broken, but 5-5′ linkages not.
Structural basis for a Kolbe-type decarboxylation catalyzed by a glycyl radical enzyme
Martins, Berta M.,Blaser, Martin,Feliks, Mikolaj,Ullmann, G. Matthias,Buckel, Wolfgang,Selmer, Thorsten
, p. 14666 - 14674 (2011)
4-Hydroxyphenylacetate decarboxylase is a [4Fe-4S] cluster containing glycyl radical enzyme proposed to use a glycyl/thiyl radical dyad to catalyze the last step of tyrosine fermentation in clostridia. The decarboxylation product p-cresol (4-methylphenol) is a virulence factor of the human pathogen Clostridium difficile. Here we describe the crystal structures at 1.75 and 1.81 A resolution of substrate-free and substrate-bound 4-hydroxyphenylacetate decarboxylase from the related Clostridium scatologenes. The structures show a (βγ)4 tetramer of heterodimers composed of a catalytic β-subunit harboring the putative glycyl/thiyl dyad and a distinct small γ-subunit with two [4Fe-4S] clusters at 40 A distance from the active site. The γ-subunit comprises two domains displaying pseudo-2-fold symmetry that are structurally related to the [4Fe-4S] cluster-binding scaffold of high-potential iron-sulfur proteins. The N-terminal domain coordinates one cluster with one histidine and three cysteines, and the C-terminal domain coordinates the second cluster with four cysteines. Whereas the C-terminal cluster is buried in the βγ heterodimer interface, the N-terminal cluster is not part of the interface. The previously postulated decarboxylation mechanism required the substrate hydroxyl group in the vicinity of the active cysteine residue. In contrast to expectation, the substrate-bound state shows a direct interaction between the substrate carboxyl group and the active site Cys503, while His536 and Glu637 at the opposite side of the active site pocket anchor the hydroxyl group. This state captures a possible catalytically competent complex and suggests a Kolbe-type decarboxylation for p-cresol formation.
Characteristic Effect of Pyridine on the NIH Shift and Selectivity in the Monooxygenation of Aromatic Compounds Catalyzed by a Nonheme Iron Complex/Hydroquinones/O2 System
Funabiki, Takuzo,Toyoda, Takehiro,Yoshida, Satohiro
, p. 1279 - 1282 (1992)
The high values of the NIH and Me-NIH shifts were observed in the hydroxylation of aromatic compounds such as toluene and xylenes with O2 by the catalytic system in the title.The pyridine concentration greatly affected not only the NIH shift, but the selectivity to form phenols by hydroxylation of the aromatic ring and to form aldehydes by oxidation of the methyl group.
Raney Ni-Al alloy-mediated reduction of alkylated phenols in water
Tan, Song-Liang,Liu, Guo-Bin,Gao, Xiang,Thiemann, Thies
, p. 5 - 7 (2009)
Raney Ni-Al alloy in a dilute aqueous alkaline solution has been shown to be a very powerful reducing agent in the hydrogenation of phenol and alkylated phenols to the corresponding cyclohexanol derivatives.
Highly dispersed molybdenum carbide nanoparticles supported on activated carbon as an efficient catalyst for the hydrodeoxygenation of vanillin
He, Lili,Qin, Yu,Lou, Hui,Chen, Ping
, p. 43141 - 43147 (2015)
Characterized by XRD and TEM, highly dispersed molybdenum carbide (Mo2C) nanoparticles with a diameter of 1-4 nm were effectively synthesized on activated carbon at 700 °C. The Mo2C-based catalyst exhibited high activity and stability for the hydrodeoxygenation (HDO) of vanillin under mild conditions (100 °C, 1.0 MPa of H2, 3 h) in aqueous solution. According to the distribution of products with time, a HDO mechanism involving vanillyl alcohol as an intermediate product was proposed. Moreover, after being recycled several times, the loss of catalytic activity was negligible, which demonstrated that the Mo2C-based catalyst had the property of resistance to deactivation.
Aerobic homocoupling of phenylboronic acid on Mg-Al mixed-oxides-supported Au nanoparticles
Wang, Liang,Wang, Hong,Zhang, Wei,Zhang, Jian,Lewis, James P.,Meng, Xiangju,Xiao, Feng-Shou
, p. 186 - 197 (2013)
Au nanoparticles are highly dispersed on Mg-Al mixed oxides by anion exchange (Au/MAO-AE) and homogeneous deposition-precipitation (Au/MAO-HDP). The XRD, UV-visible, and XPS spectra demonstrate that the Au species on both samples are present as metallic Au. The Au nanoparticles are directly confirmed by the transmission electron microscopy images. Very importantly, both Au/MAO-AE and Au/MAO-HDP catalysts show superior catalytic activity, selectivities, and recyclabilities in the aerobic homocoupling of phenylboronic acid, yielding biphenyl and phenol. During this reaction, H2O molecules from the system and hydroxyl groups on Mg-Al mixed oxides strongly influence the catalytic performance. Based on the catalytic data and XPS characterizations, a mechanism for aerobic homocoupling of phenylboronic on metallic Au nanoparticles is proposed. These catalytic data are in good agreement with those obtained from theoretical calculations.
Catalysis of the Methoxyaminolysis of Phenyl Acetate by a Preassociation Mechanism with a Solvent Isotope Effect Maximum
Cox, Michael M.,Jencks, William P.
, p. 572 - 580 (1981)
General-acid catalysis of the reaction of methoxyamine with phenyl acetate by the proton, carboxylic acids, and ammonium ions follows a nonlinear Broensted curve.This curve agress quantitavely with the behavior expected for the enforced preassociation mechanism of catalysis that was predicted for this reaction.The stronger acids, including the proton, follow a Broensted slope of α=0.16 that represents rate-limiting amine attack assisted by hydrogen bonding, weaker acids react with partially rate-limiting proton transfer to the addition intermediate T+/-, and the weakest acids follow a steeper Broensted slope approaching α=1.0 that represents rate-limiting separation of the protonated intermediate T+.There is no decrease in the rate constant for catalysis by chloroacetic acid with increasing viscosity in water-glycerol mixtures; a decrease is observed for the reaction of methylamine with p-tolyl acetate catalyzed by acetate buffers, which is believed to proceed by a diffusion-controlled trapping mechanism.A sharp maximum in the solvent isotope effect at pKHA = 6.8 confirms the kinetically significant proton-transfer step in the intermediate region near ΔpK = 0.The decrease with stronger acids represents a decrease in the isotope effect for this proton-transfer step, which is largely rate limiting for acids of pKa = 4-7, but the decrease with weaker acids can be explained by the change to rate-limiting diffusional separation of T+ and A-.Two explanations are offered for the decreased isotope effect with increasing acid strengh. (1) There is a sharp change to an asymmetric structure of the transition state for the very rapid proton-transfer step, as suggested by Melander and Westheimer. (2) There is a shift to a rate-limiting change in solvation that occurs immediately either before or after the proton-transfer step with stronger acids.It is possible to fit the observed Broensted curve and isotope effect maximum with calculated rate constants that are based on a rate law and estimated rate constants for the steps of the latter mechanism.
One step phenol synthesis from benzene catalysed by nickel(ii) complexes
Muthuramalingam, Sethuraman,Anandababu, Karunanithi,Velusamy, Marappan,Mayilmurugan, Ramasamy
, p. 5991 - 6001 (2019)
Nickel(ii)complexes of N4-ligands have been synthesized and characterized as efficient catalysts for the hydroxylation of benzene using H2O2. All the complexes exhibited Ni2+ → Ni3+ oxidation potentials of around 0.966-1.051 V vs. Ag/Ag+ in acetonitrile. One of the complexes has been structurally characterized and adopted an octahedral coordination geometry around the nickel(ii) center. The complexes catalysed direct benzene hydroxylation using H2O2 as an oxygen source and afforded phenol up to 41% with a turnover number (TON) of 820. This is unprecedentedly the highest catalytic efficiency achieved to date for benzene hydroxylation using 0.05 mol% catalyst loading and five equivalents of H2O2. The benzene hydroxylation reaction possibly proceeds via the key intermediate bis(μ-oxo)dinickel(iii) species, which was characterized by HR-MS, vibrational and electronic spectral methods, for almost all complexes. The formation constant of the key intermediate was calculated to be 5.61-9.41 × 10-2 s-1 by following the appearance of an oxo-to-Ni(iii) LMCT band at around 406-413 nm. The intermediates are found to be very short-lived (t1/2, 73-123 s). The geometry of one of the catalytically active intermediates was optimized by DFT and its spectral properties were calculated by TD-DFT calculations, which are comparable to experimental spectral data. The kinetic isotope effect (KIE) values (0.98-1.05) support the involvement of nickel-bound oxygen species as an intermediate. The isotope-labeling experiments using H218O2 showed 92.46% incorporation of 18O, revealing that H2O2 is the key oxygen supplier to form phenol. The catalytic efficiencies of complexes are strongly influenced by the geometrical configuration of intermediates, and stereoelectronic and steric properties, which are fine-tuned by the ligand architecture.
Hydrolysis of aryl N-methyl-N-arylsulfonylcarbamates
Araujo,Campelo,Iley,Norberto
, p. 494 - 497 (2001)
Tertiary sulfonylcarbamates 1 were prepared by reaction of a sulfonamide anion with aryl chloroformates. These previously unreported compounds hydrolyse in aqueous media to the parent sulfonamide and phenol. The pH-rate profile shows both spontaneous and base-catalysed processes. The reaction is also catalysed by buffers. Kinetic data for the hydrolysis of these compounds by HO- are best interpreted in terms of a mechanism involving rate-limiting formation of a tetrahedral intermediate from nucleophilic attack of hydroxide ion at the carbamate carbonyl carbon atom. For the 4-nitrophenylsulfonyl compound 1h decomposition of the tetrahedral intermediate appears to be rate-limiting with the sulfonamide anion, rather than the phenoxide, functioning as the leaving group. The buffer-catalysed process is consistent with general base-catalysed attack of water at the carbamate carbonyl carbon atom.
