Author: Lee D. McPhersonPublisher:
ISBN:
Category :
Languages : en
Pages : 244
Book Description
Reactivity and Mechanisms of Oxazoline- and Thiazoline-based Rhenium(V) Catalysts for Oxygen-atom and Imido Transfer Reactions
Author: Lee D. McPhersonRhenium-catalyzed Oxygen-atom Transfer Reactions
Author: Eric C. BrownPublisher:
ISBN:
Category : Rhenium catalysts
Languages : en
Pages : 406
Book Description
In situ reduction of hydrido-tris-(3,5-dimethylpyrazolyl)borato(trioxo) rhenium(V) with triphenylphosphine or triethylphosphite leads to a reactive rhenium(V) species that catalytically deoxygenates epoxides at 75-105°C. The reaction is stereospecific, except for trans- and cis-butene oxide which formed minor amounts of the opposite isomer. A variety of different functional groups were tolerated and even epoxides that reacted slowly could be pushed to greater than 95% conversion given extended time and/or higher temperature. The absence of clustering processes shows how the choice of ligand can have a major influence on the design of the catalytic cycle. The rhenium(V) species formed from reduction of Tp'ReO3 was identified as Tp'Re(O)(OH)2. Tp'Re(O)(OH)2 reacted with ethanol and HCl to form ethoxide and hydroxo chloride complexes, respectively. In addition, Tp'Re(O)(OH)2 was an excellent catalytic and stoichiometric reagent for the deoxygenation of epoxides and sulfoxides. Loss of water from Tp'Re(O)(OH)2 to form the catalytically active species Tp'Re02 was shown to be a necessary preequilibrium process. The kinetic behavior of the catalytic system is complex. First-order behavior in [Re][subscript T], zero-order dependence in [PPh3] and saturation behavior for epoxide were observed. The reversible formation of a coordinated epoxide complex was proposed to explain the saturation behavior. The epoxide complex was shown experimentally and computationally to engage in two separate reactions: ring expansion to form a syn-diolate complex, and direct fragmentation to alkene and trioxide. A steady-state concentration of diolate is eventually reached explaining a "burst" of alkene production prior to generation of a pseudo-zero-order catalytic system. The diolate formed is the syn-isomer, which is the kinetically formed product. Direct epoxide fragmentation is the primary source of alkene. This process was determined to be four times faster than ring expansion for cis-stilbene oxide. The synthesis and characterization of a tethered-epoxide Cp* rhenium trioxide complex has been achieved. Reduction of this complex leads to an unsaturated rhenium(V) species that is immediately complexed by the tethered epoxide. Experimental data and molecular mechanics modeling support intramolecular coordination of the epoxide to the rhenium center. These results confirm that the coordinate epoxide is a viable intermediate in rhenium-catalyzed epoxide deoxygenations.
Mechanistic Studies and Synthetic Applications of Oxygen Atom Transfer Reactions Catalyzed by Rhenium(V) Dithiolato Complexes and Methyltrioxorhenium (MTO)
Author: Ying WangPublisher:
ISBN:
Category :
Languages : en
Pages : 230
Book Description
In the case of oxygen atom transfer reaction between tert-butyl hydroperoxide and sulfide catalyzed by Re(V) dithiolato compounds, an induction period is observed due to the slow ligand exchange step. Reaction schemes are proposed to interpret the kinetic data. In both cases, the active intermediates are Re(VII) dioxo species, which were detected at the low temperature. Organic disulfides with both alkyl and aryl substituents are oxidized by hydrogen peroxide when CH3ReO33 (MTO) is used as a catalyst. Thiosulfinate is formed in the first step about an hour with nearly quantitative yield. Kinetics studies of the first oxidation reaction established that two peroxorhenium compounds are the active forms of the catalyst. Rate constants were obtained and a mechanism was proposed in which the electron-rich sulfur attacks the peroxo oxygen of intermediates.
Reactions and Mechanisms of Rhenium Catalyzed Oxygen Atom Transfer
Author: Joachin Jude AriasDissertation Abstracts International
Author: Publisher:
ISBN:
Category : Dissertations, Academic
Languages : en
Pages : 788
Book Description
Catalytic Applications of Rhenium Compounds and Hydrogen Atom Transfer Reactions of Substituted Phthalimide N-oxyl Radicals
Author: Yang CaiPublisher:
ISBN:
Category :
Languages : en
Pages : 254
Book Description
In this work, methyltrioxorhenium (MTO) was found to be an active catalyst for two reactions: one is the reduction of hydronium ions by Eu[Subscript aq]2 to evolve H2; the other is reduction of perchlorate ions to chloride ions by Eu[Subscript aq]2+ or Cr[Subscript aq]2+ in acidic solution. Kinetic studies were carried out and reaction mechanisms were proposed to agree with all the experimental data. In the hydrogen evolution reaction, a rhenium(V) hydride complex was postulated in the scheme to generate H2 by a proton-hydride reaction. Under similar conditions, Cr2+ ions do not evolve H2, despite E0[Subscript Cr][Difference]E0[Subscript Eu]. In addition, no H2 formation was observed in the presence of perchlorate ions because the reaction between methyldioxorhenium (MDO) and perchlorate ions has a much faster rate than that of hydrogen evolution. A six-coordinate rhenium(V) compound MeReO(edt)(bpym) was prepared, characterized, and investigated for oxygen atom transfer reactions between picoline N-oxide and triarylphosphines. We found it is a good catalyst for the reaction, even though it is less active than those five-coordinate rhenium(V) dithiolato compounds. The kinetics showed that the reaction has a first-order dependence on both rhenium and picoline N-oxide. Triarylphosphines were found to inhibit the reaction, and those phosphines with more electron-donating groups in para positions had slower reaction rates. This study proves a hypothesis: there should be a steric requirement for the potential catalyst in the oxygen transfer reactions, which is the necessary existence of an open coordination site on rhenium center. In the last chapter, we studied three different types of reactions of phthalimide N-oxyl radicals (PINO·) and N-hydroxylphthalimide (NHPI) derivatives. First, the self-decomposition of PINO· follows second-order kinetics. However, when excess of 4-Me-NHPI are used in the system, it was found that H-atom abstraction competes with the self-decomposition of 4-Me-PINO·. Second, the hydrogen atom self-exchange reactions between PINO· and substituted NHPI were found to follow H-atom transfer rather than the stepwise electron-proton transfer pathway. Last, the investigations of hydrogen abstraction from para-xylene and toluene by PINO· show large kinetic isotope effects, with the reaction becoming slower when the ring substituent on PINO· is more electron donating.
Directory of Graduate Research
Author: American Chemical Society. Committee on Professional TrainingPublisher:
ISBN:
Category : Biochemistry
Languages : en
Pages : 1932
Book Description
Faculties, publications and doctoral theses in departments or divisions of chemistry, chemical engineering, biochemistry and pharmaceutical and/or medicinal chemistry at universities in the United States and Canada.
Mechanistic Study of Oxygen Atom Transfer Catalyzed by Rhenium Compounds
Author: Xiaopeng ShanPublisher:
ISBN:
Category :
Languages : en
Pages : 424
Book Description
The investigation of oxygen atom transfer (OAT) catalyzed by transition metal complexes continues to provide chemical insight for advanced studies in bioinorganic chemistry as well as industrial applications. Unlike molybdenum(IV/VI) pairs, which received intensive interest from inorganic and bioinorganic chemists for decades, rhenium(V/VII), forming the redox loop involving two-electron or one-oxygen atom processes has only received limited attention. A family of oxorhenium(V) complexes was synthesized from methyltrioxorhenium(VII), abbreviated as MTO, that can be reduced by phosphanes, thiols or sulfides and coordinated by suitable ligands including thiolates, phosphanes, pyridines, phenolates, carboxylates and etc. An unexpected methyl transfer from rhenium to thiolate sulfur was discovered when MTO react with 1,2-ethanedithiol without the presence of a reducing reagent. Ligand displacement was found to be an essential step in OAT reactions catalyzed by rhenium(V) complexes. This allows the oxidant to access rhenium(V) and be activated by the metal subsequently. Kinetic studies of ligand exchange of MeReO(dithiolate)Py with Py or phosphanes and ReO([kappa]2-edt)([kappa]2-edtMe) with phosphanes all revealed in unique correlation behavior when series of substituted ligands were employed. Detailed investigation led us to conclude that a three-step mechanism was involved and caused this unique phenomenon. Further study of the OAT catalytic cycle led us to investigate the geometric effect on the oxidation of rhenium(V) complexes with pyridine N-oxides. Five and six coordinated rhenium(V) complexes with tridentate ligands display an entirely different rate law. The reactions of six-coordinate compounds shows first-order dependence on the concentration of water instead of pyridine N-oxide in the rate law of the reactions of five coordinated rhenium(V) compounds. Steric demand may play the key role in this difference. A catalytic OAT cycle with pyridine N-oxides and sulfide catalyzed by MeReO(PA)2, where PAH is 2-piclinic acid, was investigated. Mechanistic and isotope labeling studies were applied to trap the intermediate, from which a structure was postulated.
Metal-Catalyzed Oxidations of Organic Compounds
Author: Roger SheldonPublisher: Elsevier
ISBN: 0323150047
Category : Science
Languages : en
Pages : 447
Book Description
Metal-Catalyzed Oxidations of Organic Compounds: Mechanistic Principles and Synthetic focuses on the oxidative transformations of functional groups. This book explores oxidation as being extensively used in the laboratory synthesis of fine organic chemicals and in the manufacture of large-volume petrochemicals. Organized into two parts encompassing 13 chapters, this book starts with an overview of the mechanistic principles of oxidation–reduction in biochemical, organic, and inorganic systems. This text then proceeds with a discussion of the use of molecular oxygen, hydrogen peroxide, and alkyl hydroperoxides as primary oxidants. Other chapters explore stoichiometric oxidations with metal oxidants, which include permanganate and chromic acid. This book discusses as well the synthetic applications of catalytic oxidations as well as the technology of petrochemical oxidation. The final chapter deals with the autoxidations of sulfur, phosphorus, and nitrogen compounds. This book is intended for chemists involved in organic synthesis, catalysis, and organometallic chemistry, both in academic institutions and in industrial laboratories.
Perchlorate in the Environment
Author: Edward Todd UrbanskyPublisher: Springer Science & Business Media
ISBN: 1461543037
Category : Science
Languages : en
Pages : 299
Book Description
Based on a symposium sponsored by the Environmental Division of the American Chemical Society, Perchlorate in the Environment is the first comprehensive book to address perchlorate as a potable water contaminant. The two main topics are: analytical chemistry (focusing on ion chromatography and electrospray ionization mass spectrometry), and treatment or remediation. Also included are topics such as ion exchange, phytoremediation, bacterial reduction of perchlorate, bioreactors, and in situ bioremediation. To provide complete coverage, background chapters on fundamental chemistry, toxicology, and reulatory issues are also included. The authors are environmental consultants, government researchers, industry experts, and university professors from a wide array of disciplines.