A Totally Synthetic Peroxynitritase Model That Is a Postfunctional Suicide Catalyst

A Totally Synthetic Peroxynitritase Model That Is a Postfunctional Suicide Catalys

Dynamic <i>Cis</i><i>−</i><i>Trans </i>Bridge Isomerism in the Cyclidene Family of Dioxygen Carriers: A Bicyclic Cyclidene with a <i>Trans</i> Bridge Orientation

A new type of isomerism has been detected in the cyclidene family of lacunar dioxygen carriers, providing an additional structural variable for the control of their oxygen affinity. In those rare complexes that do not have methyl substituents on the primary macrocycle, NMR and X-ray crystallographic data indicate that, in addition to their usual cis orientation, the bridges can also adopt a trans orientation. In the crystal structure of [Co(C8MeHH[16]cyclidene)](PF6)2·3CH3OH, the bridge has this trans orientation with one end in the “lid-on” configuration while the other end is “lid-off”. The trans orientation of the bridge is identified as the principal cause of the decreased dioxygen affinity of such unsubstituted cyclidenes

Mechanistic Insight from Energy and Volume Profiles for CO Binding to a Lacunar Iron(II) Cyclidene Complex

Mechanistic Insight from Energy and Volume Profiles for CO Binding to a Lacunar Iron(II) Cyclidene Comple

Dicompartmental Ligands with Hexa- and Tetradentate Coordination Sites: One-Step Synthesis of Ligands and Metal Complexes and Their X-ray Structure Analysis

Examples of interesting ligands previously requiring lengthy synthetic procedures have been prepared by one-step routes, opening the way to more extensive studies of their complexes and to possible applications. New dicompartmental ligands bearing picolyl pendant arms on the amine nitrogen donors have been synthesized, via the Mannich condensation, from 5-substituted salicylaldehydes, formaldehyde, and N,N‘-bis(2-pyridylmethyl)-1,2-diaminoethane. The protonated acyclic ligand salt, two mononuclear complexes of a macrocyclic ligand with a second compartment featuring a Schiff base, and one of the decomposition products resulting from a retro-Mannich reaction have been structurally characterized. The ligand salt (L1b) has an extended conformation with the ethylenediamine fragment displaying the trans configuration, very different from that of the corresponding closed-site macrocyclic complexes NiH2(L2b)2+ and ZnH2(L2b)2+. The mononuclear macrocyclic complex NiH2(L2b)2+ has a much smaller ligand twist than the corresponcing Zn(II) complex. The degree of ligand distortion is correlated with the M−N bond length between the metal ions and the pyridine nitrogens; longer M−N bonds cause the pyridine rings to tilt and twist the phenolic rings out of the main ligand plane. The ability of the macrocyclic ligand L2b to accommodate a second metal ion has been demonstrated by successful synthesis of dinuclear complexes. The free carbonyl groups of the open-site ligand were transformed into oxime groups, and the corresponding dinuclear bis(nickel) complex has been prepared. Acetal formation by the free carbonyl groups of ligand and retro-Mannich rearrangements are found to be possible pathways for the decomposition of this family of dicompartmental ligands

Organic Acids Tunably Catalyze Carbonic Acid Decomposition

Density functional theory calculations predict that the gas-phase decomposition of carbonic acid, a high-energy, 1,3-hydrogen atom transfer reaction, can be catalyzed by a monocarboxylic acid or a dicarboxylic acid, including carbonic acid itself. Carboxylic acids are found to be more effective catalysts than water. Among the carboxylic acids, the monocarboxylic acids outperform the dicarboxylic ones wherein the presence of an intramolecular hydrogen bond hampers the hydrogen transfer. Further, the calculations reveal a direct correlation between the catalytic activity of a monocarboxylic acid and its pKa, in contrast to prior assumptions about carboxylic-acid-catalyzed hydrogen-transfer reactions. The catalytic efficacy of a dicarboxylic acid, on the other hand, is significantly affected by the strength of an intramolecular hydrogen bond. Transition-state theory estimates indicate that effective rate constants for the acid-catalyzed decomposition are four orders-of-magnitude larger than those for the water-catalyzed reaction. These results offer new insights into the determinants of general acid catalysis with potentially broad implications

Autoxidation of Substituted Phenols Catalyzed by Cobalt Schiff Base Complexes in Supercritical Carbon Dioxide

This first study of O2 oxidation (autoxidation) of substituted phenols catalyzed by a dioxygen carrier in supercritical carbon dioxide (scCO2) provides additional insights into the established mechanism of reactions that have been much studied in conventional solvents. As has been long believed, the cobalt(II) dioxygen carriers of the class represented by [{N,N‘-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato(2−)}cobalt(II)], Co(salen*), show both oxidase and oxygenase activities during oxygenation of substituted phenols in scCO2. The catalytic autoxidation of 2,6-di-tert-butylphenol (DTBP) and 3,5-di-tert-butylphenol (35-DTBP) in scCO2 was studied by analysis of products in batch reactions with carefully controlled variables, in the presence of a large excess of O2, at 207 bar of total pressure and a reaction temperature of 70 °C. The oxidation of 35-DTBP yielded only traces of products under the same experimental conditions that converted DTBP totally to a mixture of the oxygenation product 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and the related product of radical coupling 3,5,3‘,5‘-tetra-tert-butyl-4,4‘-diphenoquinone (TTDBQ). The effects on conversion of DTBP to products and on selectivity between the two products were studied for variations in temperature and the concentrations of catalyst, oxygen, and methylimidazole. Selectivity in favor of the O-transfer product DTBQ over the self-coupling of the phenoxy radical was observed upon changing the oxygen concentration. In contrast, selectivity remained unaffected over a wide range of temperatures and catalyst concentrations. The oxygen dependence of both the conversion and selectivity showed saturation effects identifying the dioxygen complex as the effective oxidant in both the initial radical formation step and the oxygenation of that radical. No direct reaction is observed between the electrophilic phenoxy radical and O2

Kinetic Investigations of <i>p</i>‑Xylene Oxidation to Terephthalic Acid with a Co/Mn/Br Catalyst in a Homogeneous Liquid Phase

Kinetic investigations of the liquid phase oxidation of <i>p</i>-xylene (<i>p</i>X) to terephthalic acid (TPA) with Co/Mn/Br catalyst were performed in a stirred 50 mL Parr reactor at 200 °C and 15 bar pressure under conditions wherein product precipitation is avoided. The oxidant (O₂) was introduced by sparging into the liquid phase at constant gas-phase O₂ partial pressure. Apparent kinetic rate constants, estimated by regressing experimental conversion data to a pseudo-first order lumped kinetic model, are at least an order of magnitude greater than those reported in the literature for similar catalytic reactions. We attribute this difference to the presence of gas–solid and liquid–solid mass transfer resistances in the previous studies wherein the TPA product precipitates as it forms, trapping intermediate products and slowing down their oxidation rates. Our results also indicate that it is not possible to completely eliminate the gas–liquid mass transfer limitations associated with the fast intermediate oxidation steps, even when operating without solids formation and at high stirrer speeds. Other types of reactor configurations are therefore needed to better overcome gas–liquid mass transfer limitations. Systematic studies of bromide concentration effects show that the observed reaction rates become zero order in bromide concentration at sufficiently high bromide levels where the elimination of intermediate 4-(bromomethyl)­benzoic acid by oxidation is favored. Further, the rate constants do not show any statistically significant dependence on <i>p</i>X concentration as suggested in other reports involving the traditional three-phase gas–liquid–solid reaction system. This again confirms that the formation of a solid phase hinders the overall oxidation rate, resulting in much smaller apparent rate constants

Comparative Economic and Environmental Assessments of H₂O₂‑based and Tertiary Butyl Hydroperoxide-based Propylene Oxide Technologies

Until a decade ago, the industrial technologies for producing propylene oxide from propylene were predominantly based on variations of the venerable chlorohydrin and organic hydroperoxide processes. Within the past decade, highly selective H2O2-based propylene epoxidation technologies have been developed by Dow-BASF (HPPO process) and the University of Kansas Center for Environmentally Beneficial Catalysis (CEBC-PO process). We present comparative economic and environmental impact analyses based on plant scale simulations of the processes for an assumed 200,000 tonnes/yr of PO production capacity and employing relevant process data from the literature. The predicted capital costs for the CEBC-PO process ($228 million) and HPPO process ($275 million) are lower than the conventional PO/TBA process ($372 million). The PO production costs via the conventional PO/TBA and HPPO processes are 150.4¢/lb PO (profit 87.9¢/lb, assuming a market value of 41¢/lb for the TBA co-product and 42¢/lb for the enriched propane co-product) and 107.1¢/lb PO (profit 36.1¢/lb, assuming a market value of 42¢/lb for the enriched propane co-product), respectively. For the CEBC-PO process, the production cost is 90.6¢/lb PO (profit 30.4¢/lb), assuming a life of one year for the methyltrioxorhenium catalyst and a catalyst leaching rate of 9.3 × 10–2 lb/h (or 1.6 ppm Re in the reactor effluent). The comparative economic analysis suggests that the CEBC-PO process has potential for being economically competitive and establishes quantitative catalyst performance metrics for achieving the same. Quantitative cradle-to-gate LCA shows that the environmental impacts of producing PO by the conventional PO/TBA, HPPO, and CEBC-PO processes are of the same order of magnitude. The lower GHG emissions predicted for the HPPO and CEBC-PO technologies, compared to the PO/TBA process, lie within the prediction uncertainty of this analysis. This comparative LCA analysis traces the adverse environmental impacts to sources outside the propylene oxide plant in all three processes: fossil fuel-based energy (natural gas, transportation fuel) utilization during raw material (i-butane, propylene and hydrogen peroxide) production

Criegee Intermediate Reaction with CO: Mechanism, Barriers, Conformer-Dependence, and Implications for Ozonolysis Chemistry

Density functional theory and transition state theory rate constant calculations have been performed to gain insight into the bimolecular reaction of the Criegee intermediate (CI) with carbon monoxide (CO) that is proposed to be important in both atmospheric and industrial chemistry. A new mechanism is suggested in which the CI acts as an oxidant by transferring an oxygen atom to the CO, resulting in the formation of a carbonyl compound (aldehyde or ketone depending upon the CI) and carbon dioxide. Fourteen different CIs, including ones resulting from biogenic ozonolysis, are considered. Consistent with previous reports for other CI bimolecular reactions, the <i>anti</i> conformers are found to react faster than the <i>syn</i> conformers. However, this can be attributed to steric effects and not hyperconjugation as generally invoked. The oxidation reaction is slow, with barrier heights between 6.3 and 14.7 kcal/mol and estimated reaction rate constants 6–12 orders-of-magnitude smaller than previously reported literature estimates. The reaction is thus expected to be unimportant in the context of tropospheric oxidation chemistry. However, the reaction mechanism suggests that CO could be exploited in ozonolysis to selectively obtain industrially important carbonyl compounds

Synthesis and X-ray Crystal Structure Determination of the First Transition Metal Complexes of the Tetracycles Formed by Tetraazamacrocycle−Glyoxal Condensation: PdL*Cl₂ (L = Cyclam−Glyoxal Condensate (<b>1</b>), Cyclen−Glyoxal Condensate (<b>2</b>))

Synthesis and X-ray Crystal Structure Determination of the First Transition Metal Complexes of the Tetracycles Formed by Tetraazamacrocycle−Glyoxal Condensation:  PdL*Cl2 (L = Cyclam−Glyoxal Condensate (1), Cyclen−Glyoxal Condensate (2)