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Artykuły w czasopismach na temat "Homolytic Cleavage"
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Verma, Piyush Kumar, Naresh Kumar Meher i K. Geetharani. "Homolytic cleavage of diboron(4) compounds using diazabutadiene derivatives". Chemical Communications 57, nr 64 (2021): 7886–89. http://dx.doi.org/10.1039/d1cc02881b.
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Homolytic cleavage of diboron was achieved using diazabutadiene derivatives (DABs). The cleavage is accompanied by the formation of new π-bonds and the geometry of the product is highly dependent on the substituents on the DAB units.
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Nome, Faruk, Marcos Caroli Rezende, Claudia Maria Sabóia i Arlindo Clemente Da Silva. "Kinetics of the thermolysis of para-substituted benzylcobalamins and derivatives". Canadian Journal of Chemistry 65, nr 9 (1.09.1987): 2095–99. http://dx.doi.org/10.1139/v87-347.
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The thermolysis of five para-substituted benzylcobalamins was studied at different temperatures. In the presence of KCN the observed rates increase with the cyanide concentration until a constant value is attained at high [CN−]. In both series the homolytic cleavage of the Co—C bond is slightly dependent on the para-substituent of the benzyl moiety, with ρ values of −0.1 and −0.2 when 5,6-dimethylbenzimidazole (Bzm) and cyanide are the axial ligands respectively. The Co—C bond is weakened by electron-donating axial ligands and the homolytic cleavage rates increase in the order H2O < Bzm < CN.
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Bajaj, Ashima, Rishu Khurana i Md Ehesan Ali. "Quantum interference and spin filtering effects in photo-responsive single molecule devices". Journal of Materials Chemistry C 9, nr 34 (2021): 11242–51. http://dx.doi.org/10.1039/d1tc02200h.
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Li, Yan, Kartik Chandra Mondal, Peter Stollberg, Hongping Zhu, Herbert W. Roesky, Regine Herbst-Irmer, Dietmar Stalke i Heike Fliegl. "Unusual formation of a N-heterocyclic germylene via homolytic cleavage of a C–C bond". Chem. Commun. 50, nr 25 (2014): 3356–58. http://dx.doi.org/10.1039/c4cc00251b.
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Siegel, Marshall M., i Norman B. Colthup. "Molecular Orbital Study of Remote Charge Site Decompositions in the Collision-Induced Decomposition Mass Spectra of Fatty Acid Carboxylate Anions". Applied Spectroscopy 42, nr 7 (wrzesień 1988): 1214–21. http://dx.doi.org/10.1366/0003702884429887.
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Molecular orbital calculations were used to study the energetics of four different mechanisms used to explain the collision-induced decomposition mass spectra of saturated fatty acid carboxylate anions produced by fast atom bombardment and chemical ionization. The most abundant homologous series of anions, terminally unsaturated carboxylate anions, arose from the concerted cleavage of gauche segments of the hydrocarbon backbone via a sixatom transition state. A series of anions of lower abundance arose by homolytic cleavage of anti segments of the hydrocarbon backbone into two radical fragments. The loss of methane from the parent anion is produced by the concerted cleavage of the terminal methyl group via a four-atom transition state. The computed activation energies for the reaction mechanisms were in the following order: sixatom transition state < four-atom transition state ≪ homolytic cleavage of hydrocarbon backbone. Dehydration of the parent anion is rationalized to occur by loss of a carboxylate oxygen and two hydrogen atoms on the alpha carbon from the carboxylate carbon.
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Hosseini, Seyedeh Nargess, Jeffrey R. Johnston i F. G. West. "Evidence for heterolytic cleavage of a cyclic oxonium ylide: implications for the mechanism of the Stevens [1,2]-shift". Chemical Communications 53, nr 94 (2017): 12654–56. http://dx.doi.org/10.1039/c7cc07716e.
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Hou, Bo, David Benito-Alifonso, Richard Webster, David Cherns, M. Carmen Galan i David J. Fermín. "Rapid phosphine-free synthesis of CdSe quantum dots: promoting the generation of Se precursors using a radical initiator". J. Mater. Chem. A 2, nr 19 (2014): 6879–86. http://dx.doi.org/10.1039/c4ta00285g.
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Shu, Xing-Zhong, i Xiaobo Pang. "Titanium: A Unique Metal for Radical Dehydroxylative Functionalization of Alcohols". Synlett 32, nr 13 (4.03.2021): 1269–74. http://dx.doi.org/10.1055/a-1406-0484.
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AbstractThe dehydroxylative functionalization of alcohols is synthetic appealing, but it remains a long-term challenge in the synthetic community. Low-valent titanium has shown the power to produce carbon radicals from alcohols via homolytic cleavage of the C–OH bonds and thus offers the potential to overcome this problem. In this perspective manuscript, we summarized the recent advance on radical dehydroxylative transformation of alcohols either promoted or catalyzed by titanium. The limitation and outlook of the studies in this field are also provided.1 Introduction2 Recent Developments in Dehydroxylative Functionalization of Alcohols2.1 Stoichiometric Titanium Complexes Mediated Homolysis of Alcohols2.2 Radical Dehydroxylative Functionalization of Alcohols by Ti Catalysis3 Summary and Outlook
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Tang, Wai-Kit, Chun-Ping Leong, Qiang Hao i Chi-Kit Siu. "Theoretical examination of competitive β-radical-induced cleavages of N–Cα and Cα–C bonds of peptides". Canadian Journal of Chemistry 93, nr 12 (grudzień 2015): 1355–62. http://dx.doi.org/10.1139/cjc-2015-0208.
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Selective cleavages of N–Cα and Cα–C bonds of β-radical tautomers of amino acid residues in radical peptides have been examined theoretically by means of the density functional theory at the M06-2X/6-311++G(d,p) level. The majority of the bond cleavages are homolytic via β-scission. Their energy barriers depend largely on the ability of the radical being stabilized in the transition structures and the availability of a mobile proton in the vicinity of the β-radical center. The N–Cα bond is less favorably cleaved than the Cα–C bond (except Ser and Thr) for systems without a mobile proton. It is because, firstly, the homolytic cleavage is less favorable for the more polar N–Cα bond than for the less polar Cα–C bond. Secondly, a less stable σ-radical localized on the amide nitrogen atom of the incipient N-terminal fragment is formed for the former, while a more stable radical delocalized in a π*(CO)-like orbital of the incipient C-terminal fragment is formed for the latter. In the presence of a mobile proton N-terminal to the β-radical center, some degrees of heterolytic cleavage character, as preferred by the polar N–Cα bond, are observed. Consequently, its barrier is reduced. If the mobile proton is located at the C-terminal amide oxygen of the β-radical center, the Cα–C bond cleavage will be significantly suppressed. It is because the radical in the incipient C-terminal fragment becomes more localized as a σ-radical on the carbon atom of its protonated amide group. With basic amino acid residues, the Cα–C bond cleavage can be reactivated. Heterolytic cleavage of the polar N–Cα bond can be largely facilitated if a mobile proton N-terminal to the β-radical center is available and the radical in the incipient C-terminal fragment is sufficiently stabilized, for instance, by the aromatic side chain of Trp and Tyr. Therefore, cleavages of the N–Cα bond induced by the β-radical tautomer of Trp and Tyr are often preferred as compared with cleavages of the Cα–C bond in peptide radical cations containing mobile protons.
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LIU, MIN HSIEN, i GEN FA ZHENG. "COMPUTATIONAL STUDY OF UNIMOLECULAR DECOMPOSITION MECHANISM OF RDX EXPLOSIVE". Journal of Theoretical and Computational Chemistry 06, nr 02 (czerwiec 2007): 341–51. http://dx.doi.org/10.1142/s0219633607002952.
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This study investigated the RDX (1,3,5-Trinitro-1,3,5-triazine) molecule to elucidate its possible decomposition species and the corresponding energies by performing the density-functional theory (DFT) calculations. Reasonable decomposition mechanisms are proposed based on the bond energy calculated using the differential overlap (INDO) program, which yields the weakest bonding site for reference and determines the site of easy cleavage. Computational results indicate that the activation energy of direct cis-form HONO elimination is lower than that of direct trans-form HONO elimination and that of a two-stage elimination of two forms of HONO ( N – N bond fission combined with C – H bond breaking) in the initial decomposition step, which are 213.9 kJ/mol and 93.8–101.8 kJ/mol, respectively. Two possible pathways are proposed; (1) N – N bond homolytic cleavage followed by elimination of cis-form HONO, and (2) N – N bond homolytic cleavage followed by elimination of trans-form HONO.
Rozprawy doktorskie na temat "Homolytic Cleavage"
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Wu, Yi-Wen, i 吳翌彣. "The Study of Electronic Structures of Five-coordinate Saddled Iron(III) Porphyrin Radical Cation and O-O Bond Homolytic Cleavage of (OETPP)FeIIIO(H)OtBu". Thesis, 2018. http://ndltd.ncl.edu.tw/handle/n6af3q.
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碩士國立中興大學
化學系所
106
In this study, paramagnetic NMR spectroscopy, X-ray diffraction, magnetic susceptibility and DFT calculation are employed to elucidate the strong bonding interaction between iron(III) dx2-y2 and porphyrin a2u orbitals of [Fe(OMTPP∙)Cl]SbCl6 and [Fe(OETPP∙)Cl]SbCl6. Their crystal structures clearly indicate that their saddle deformations are increased compared to those prior to oxidations. Their 1H NMR data present the S = 2 states for such one-electron oxidation states, and demonstrate that their structures in solutions remain strong saddle deformations. In light of 1H NMR spectroscopy of [Fe(TPP∙)Cl]SbCl6 resembling to the above cases, we postulate that the ring structure of [Fe(TPP∙)Cl]+ may possess strongly saddle to have great bonding interaction between iron(III) dx2-y2 and porphyrin a2u orbitals. In the DFT calculations, the degrees of phenyl ring rotation and saddled deformation will also affect their NMR spectra. In another topic, we observe O-O bond homolytic cleavage of (OETPP)FeIIIO(H)OtBu formed in the reaction of Fe(OETPP)ClO4 with TBHP and its activation parameters (ΔH≠ = 47(2) kJ mol-1, ΔS≠ = 83(9)J mol-1K-1) is measured by low-temperature UV-vis spectral data. The corresponding one-electron oxidation product is identified as [Fe(OETPP∙)OH]+, which is an isoelectronic structure as oxoiron(IV) porphyrin, by UV-vis, NMR spectroscopy and ESI-MS spectrometry. According to the experiments of NMR and ESI-MS, we also find that ·OtBu radical will convert Fe(OETPP)ClO4 to [Fe(OETPP∙)OtBu]+. Furthermore, these related iron(III) porphyrin radical cations can be carried out one more electron oxidation to isoporphyrins, an isoelectronic structure as Compound I. These iron(III) saddled isoporphyrins are shown to be reactive for highly selective chlorination of cyclohexene.
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Verma, Piyush Kumar. "Cobalt-nhc Complexes and Diazabutadienes in Activation of Mono/Diboron Compounds and Their Application in C-b Coupling Reactions". Thesis, 2021. https://etd.iisc.ac.in/handle/2005/5241.
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Boronic acid(esters) have been well recognized as an indispensable coupling partner in the Suzuki-Miyaura cross coupling reactions producing a vast spectrum of molecules, applicable in the diverse field ranging from medicinal to materials sciences.[1] Transition metal catalyzed synthesis of boronic esters from diborons with the assistance of bases is a well-established methodology[2]. In this thesis, the cobalt-N-Heterocyclic carbene complexes catalyzed borylation of organic compounds and interaction of diazabutadienes with diboron compounds will be discussed.
(i) In the first section, Co(IMes)2Cl2 catalyzed borylation of aryl halides will be discussed. [3a] The robust protocol, operating under mild condition facilitate the borylation of a diverse range of aryl halides with great efficacy, which includes the challenging aryl chlorides. The preliminary mechanistic studies suggest that base-bis(pinacolato)diboron adduct reduces the Co(IMes)2Cl2 complex to generate Co(IMes)2Cl complex, which acts as an active catalytic species.
(ii) The second section deals with catalytic synthesis of primary and secondary alkyl boronic esters using alkyl halides. [3b] The in situ generated Co-NHC complex, in assistance with base and diboron compound, produces the corresponding borylated product from alky halides. The reaction proceeds under very mild conditions and covers a wide range of alkyl halides, including chlorides having different functional groups.
(iii) In the third section, development in selective hydroboration of vinyl arenes and aliphatic alkenes will be discussed. [3c] Catalyzed by Co(I)NHC complex, the alkene hydroboration by pinacol borane gives Markovnikov selective product with good selectivity, where the regio-selectivity is controlled by phenyl substituent. In absence of that, complete inversion in the selectivity has been observed. The preliminary mechanistic cycle suggests that the catalytic cycle proceeds via oxidative addition of pinacol borane to [Co] followed by alkene insertion and reduction elimination steps.
(iv) The last section discusses the interaction of diazabutadiene molecules with diboron compounds. [3d] The diazabutadiene derivatives have been observed to completely cleave the B-B bond of Bis(catacolato)diboron and Bis(dithiocatacolato)diboron. The preliminary findings hint towards homolytic cleavage of the B-B bond by concerted interaction of the two nitrogen atoms of diazabutadiene with the two boron atoms of the diboron from the same face.
References:
[1] Boronic Acids-Preparation and Applications in Organic Synthesis, Medicine and Materials, 2nd ed.; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2011.
[2] Neeve, E. C.; Geier, S. J.; Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse. Chem. Rev. 2016, 116, 9091-9161.
[3] (a) Verma, P. K.; Mandal, S.; Geetharani, K. ACS Catal. 2018, 8, 4049-4054. (b) Verma, P. K.; Prasad, K. S.; Varghese, D.; Geetharani, K. Org. Lett. 2020, 22, 4, 1431-1436. (c) Verma, P. K.; Setulekshmi, A. S.; Geetharani, K. Org. Lett. 2018, 20, 7840-7845. (d) Verma, P. K.; Meher, N. K.; Geetharani, K. Accepted for publication in Chem. Commun., Manuscript ID: CC-COM-06-2021-002881.R2.
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Mala, Deep. "Chemistry of Ru(II) Complexes Bearing N-heterocyclic Carbene, Hydride, and Dihydrogen Ligands : Synthesis, Mechanistic Insights, and Applications". Thesis, 2016. http://etd.iisc.ac.in/handle/2005/4162.
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Introduction
N-heterocyclic carbenes (NHC) are strong donors and weak acceptors. Also, NHC forms strong metal-carbon bonds in metal complexes and hence complexes bearing NHC ligands are in general, thermally stable. Chemistry of NHCs and their transition metal complexes has been explored extensively in catalysis. Metal hydride or dihydrogen complexes are reactive intermediates and also employed as catalysts in several catalytic reactions such as hydrogenation, transfer hydrogenation, and hydroformylation. The binding of H2 to a metal center and its cleavage for the oxidative addition to a metal center are key steps in these catalytic reactions. There has been a substantial development in the field of dihydrogen chemistry. In particular, elongated dihydrogen complexes are of significant interest due to their relevance in hydrogenation reactions. A large number of elongated dihydrogen complexes have been reported. However, there has been no systematic study for the elongation of the H–H bond in dihydrogen complexes employing NHC’s as a co-ligand.
Objectives
The objectives of this work are as follows:
i. Synthesis and characterization of ruthenium dihydrogen complexes bearing an NHC ligand.
ii. Systematic variation in the ligand environment around a metal center for the elongation of the H‒H bond in the process of oxidative addition of H2 to a metal center.
iii. To synthesize new ruthenium hydride and dihydride complexes and explore their activities in catalysis such as hydrogenation, hydrodehalogenation, and CO2 activation.
Significant results
A series of dihydrogen complexes bearing NHC ligands of the type [RuCl(η2–H2)(CO)(IMes)(PPh3)(L)](OTf) [L = pyridine (Py), 4-methylpyridine (4MePy), acetonitrile (MeCN), pivalonitrile (Me3CCN)] have been prepared by the protonation of hydride complexes [RuHCl(CO)(IMes)(PPh3)(L)] (L = Py, 4MePy, MeCN, Me3CCN) with HOTf. The ligands (Py, 4MePy, MeCN, Me3CCN) trans to the hydride were found to be labile in all the hydride complexes. The H–H bond distances in the η2–H2 ligands of dihydrogen complexes were found to be temperature dependent. The H–H bond distances got slightly elongated (0.98-0.93 Å) with an increase in the temperature (183-233 K). Phosphine analogues, [RuCl(η2–H2)(CO)(PPh3)2(L)](OTf) [L = Py, 4MePy) of NHC dihydrogen complexes were also synthesized. The H‒H bond distances were temperature invariant (0.93 Å, 223-263 K) in these complexes.
A series of homobimetallic ruthenium hydride [{RuHCl(CO)(IMes)(PPh3)}2(NN)] (NN= 4,4′-bpy, 4,4′-dpyen, 4,4′-dpyan) [4,4′-bipyridine (4,4′-bpy); 1,2-bis(4-pyridyl)ethylene (4,4′-dpyen); 1,2-bis(4-pyridyl)ethane) (4,4′-dpyan)] complexes bearing an NHC ligand and their corresponding dihydrogen complexes of the type [{RuCl(η2–H2)(IMes)(PPh3)(CO)}2(NN)][OTf]2 (NN= 4,4′-bpy, 4,4′-dpyen, 4,4′-dpyan) have been synthesized and characterized. They are the first examples of homobimetallic dihydrogen complexes bearing NHC ligands. In addition, hydrogenation of internal and terminal alkenes was carried out using these homobimetallic hydride complexes.
NHC/PMe3 based cationic complexes of the type [RuH(CO)(IMes)(PMe3)3](X) (X = Cl, BPh4) have been synthesized and characterized using NMR spectroscopy. Partial hydrodechlorination of CHCl3 to CH2Cl2 was observed in the presence of both the complexes. In addition, these complexes were also found to be efficient for the activation of the C‒Cl bond in CH2Cl2 to form [CH2Cl.PMe3]Cl salt in the presence of excess PMe3. Notably, HD exchange in H2 molecules via CDCl3 was observed in the presence of both the complexes.
A new and efficient route has been developed for the synthesis of the ruthenium dihydride complexes [Ru(H)2(CO)(L)(PPh3)2] (L = IMes, IPr, PPh3). The reaction pathway for the synthesis of these derivatives was established by isolation of few intermediates. Insertion of CO2 into one of the Ru‒H bond of [Ru(H)2(CO)(L)(PPh3)2] (L = IMes, IPr) complexes took place to afford bidentate formate complexes [RuH(η2-O2CH)(CO)(L)(PPh3)2] (L = IMes, IPr). Structure formulation of these complexes was done using NMR spectroscopy.
Części książek na temat "Homolytic Cleavage"
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James, B. R., i M. T. Ashby. "Homolytic Cleavage to Give Metal-Hydrides". W Inorganic Reactions and Methods, 66–71. Hoboken, NJ, USA: John Wiley & Sons, Inc., 2007. http://dx.doi.org/10.1002/9780470145319.ch29.
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Kühn, F. E., C. C. Romäo i W. A. Herrmann. "Homolytic Cleavage". W Compounds of Groups 7-3 (Mn..., Cr..., V..., Ti..., Sc..., La..., Ac...), 1. Georg Thieme Verlag KG, 2003. http://dx.doi.org/10.1055/sos-sd-002-00209.
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"Oxidative Degradation". W Organic Chemistry of Drug Degradation, 48–109. The Royal Society of Chemistry, 2012. http://dx.doi.org/10.1039/bk9781849734219-00048.
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This chapter contains three functional parts: an introduction (3.1), a description of several major types of autooxidative mechanisms (3.2–3.4), and a discussion of specific oxidation pathways of drugs with various functional groups and structures in relation to each type of the major autooxidative mechanisms (3.5). In Sections 3.2–3.4, the ubiquitously known Fenton reaction and the little known, but more relevant Udenfriend reaction, are discussed in terms of their roles in free radical-mediated autooxidation by activating molecular oxygen into several reactive oxygen species (ROS), that is, O2−˙/HO2˙, H2O2, and HO˙. The radical ROS then triggers radical chain reactions, in which process organic peroxyl radicals and hydroperoxides are the predominant intermediates. The latter can undergo homolytic cleavage, owing to their relatively low O–O bond dissociation energies, as well as metal ion-catalyzed heterolytic cleavage. The homolytic cleavage generates alkoxyl and hydroxyl radicals, while the heterolytic cleavage reproduces peroxyl radical. Non-radical reactions of peroxides were then discussed, in particular those responsible for the formation of N-oxide, S-oxide, and epoxide degradants. The general mechanism for a less known autooxidative degradation pathway, carbanion/enolate-mediated autooxidation (base-catalyzed autooxidation) is also discussed. This mechanism can be significant for those drug molecules containing somewhat “acidic” carbonated CHn moieties, particularly when the drugs are formulated in liquid form. In Section 3.5, more than 60 examples of drug autooxidation in real life scenarios, that is, oxidation occurring under ambient or various stability conditions, are presented and their underlying degradation mechanisms are discussed in details. These examples cover the functional groups, moieties, and structures that are commonly seen in drug molecules.
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"S-Adenosyl Methionine: One Electron and Two Electron Reaction Manifolds in Biosyntheses". W Natural Product Biosynthesis: Chemical Logic and Enzymatic Machinery, 524–68. The Royal Society of Chemistry, 2017. http://dx.doi.org/10.1039/bk9781788010764-00524.
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S-Adenosylmethionine (SAM), likely an ancient molecule in biological systems, is known for delivery of a [CH3+] equivalent to a host of cellular metabolites containing nucleophilic N, O, S, and C centers via heterolytic cleavage of the CH3–S bond in SAM. SAM can also undergo catalyzed hemolytic cleavage of that CH3–S bond by iron/sulfur-containing enzymes that use the resultant 5′-deoxyadenosyl radical for carbon-based radical chemistry on specific substrates. Bioinformatic analysis indicates >50 000 such enzymes in microbial data bases. All these iron/sulfur cluster enzymes are predicted to be sensitive to autoxidation so, in contrast/complementarity to O2-based homolytic chemistry which is by definition aerobic, the SAM radical enzymes function anaerobically.
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"Carbon Radicals". W The Chemical Biology of Carbon, 402–37. The Royal Society of Chemistry, 2023. http://dx.doi.org/10.1039/bk9781839169502-00402.
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Chapter 12 turns from heterolytic C–C bond formations in vivo to homolytic pathways at C–H and C–C bonds that involve carbon-centered radical species rather than carbanions and carbocations. One extreme is reaction of alkane and alkene carbons with high valent oxo-iron species in the active sites of oxygenases. O2 is an obligate one electron acceptor in chemical biology. In oxygenase active sites high valent iron species cleave C–H bonds of bound substrates by hydrogen atom transfer to yield carbon radicals that are capturable by an [OH˙] equivalent in a radical rebound step. At the other end of the oxygen spectrum, substrate radicals occur under anaerobic conditions. More than 700 000 predicted protein open reading frames are proposed to cleave bound S-adenosylmethionine homolytically. The resultant 5′-deoxyadenosyl radical initiates C–H bond cleavage in a nearby bound cosubstrate to set off substrate radical chemistries.
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"Carbon-based Radicals in C–C Bond Formations in Natural Products". W Natural Product Biosynthesis: Chemical Logic and Enzymatic Machinery, 456–522. The Royal Society of Chemistry, 2017. http://dx.doi.org/10.1039/bk9781788010764-00456.
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Molecular oxygen, O2, has a limited role in primary metabolism, albeit a key one as the terminal electron acceptor in mitochondrial respiratory chains. By contrast, oxygenases are interspersed everywhere in the biosynthetic pathways to all the major classes of secondary metabolites. Because O2 is a ground state triplet molecule it is kinetically stable in the presence of organic metabolites, including cellular metabolites. Reductive activation occurs by one-electron paths, mediated either by flavin-dependent enzymes or iron-based enzymes. The iron-based oxygenases carry out homolytic cleavage of substrate C–H bonds, generate carbon-centered radicals, and can lead not only to oxygenation products but to a variety of radical-based rearrangements in product scaffolds, as in morphine, staurosporine, rebeccamycin, penicillin, and cephalosporin biosynthesis.
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"Energy of a Homolytic Cleavage of Communication OH in Replaced 2,6-di-tert.Butylphenols". W Chemistry and Physics of Complex Materials, 225–34. Apple Academic Press, 2013. http://dx.doi.org/10.1201/b16302-13.
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Martinho Simões, José A., i Manuel Minas da Piedade. "Electrochemical Measurements". W Molecular Energetics. Oxford University Press, 2008. http://dx.doi.org/10.1093/oso/9780195133196.003.0020.
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Electrochemical measurements have been playing an increasingly important role in the thermodynamic study of reactions in solution, not only because they provide data that are difficult (or even impossible) to obtain by other methods but also because these data can often be compared with the values determined for the analogous gas-phase reactions, thus yielding information on solvation energetics. Figure 16.1 was adapted from a scheme proposed by Griller et al. It summarizes the thermochemical information on the R–X bond that can be probed by electrochemical methods. The vertical arrows represent homolytic cleavages, and the horizontal arrows depict reduction or oxidation potentials. The authors have appropriately called the scheme in figure 16.1 a “mnemonic,” rather than a “thermochemical cycle,” because not all arrow combinations define thermochemical cycles. This can be made more clear by inspecting figure 16.2, where true thermochemical cycles are defined. For example, the enthalpy of reaction 7 is not the sum of the enthalpies of reactions 1 and 4 (as might be suggested by figure 16.1) but their sum minus the enthalpy of reaction 12. In fact, true thermochemical cycles in figure 16.1 can only be defined by considering parallelograms confined either to the upper or the lower part of the mnemonic. For instance, the enthalpy of reaction 7 is given by the enthalpy of reaction 4 plus the enthalpy of reaction 9 minus the enthalpy of reaction 3, but it is not equal to the enthalpy of reaction 6 minus the enthalpy of reaction 11 plus the enthalpy of reaction 10. Also, the enthalpy of reaction 1 (the homolytic dissociation of the R–X bond in the neutral molecule RX) can be given by the sum of the enthalpies or reaction 5 and 11 minus the enthalpy of reaction 3 or, for example, by the sum of the enthalpies of reactions 7 and 12 minus the enthalpy of reaction 4. The attractive feature of the mnemonic in figure 16.1 (or the thermochemical cycles in figure 16.2) is that it depicts the seven possible R–X cleavage reactions of RX, RX−, and RX+, as well as their relationships.
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Klärner, F. G., M. K. Diedrich, G. Dierkes i J. S. Gehrke. "Organic Reactions at High Pressure: the Effect of Pressure on cyclizations and Homolytic Bond Cleavage". W High Pressure Food Science, Bioscience and Chemistry, 3–11. Elsevier, 1998. http://dx.doi.org/10.1533/9781845698379.1.3.
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Frey, Perry A., i Adrian D. Hegeman. "Decarboxylation and Carboxylation". W Enzymatic Reaction Mechanisms. Oxford University Press, 2007. http://dx.doi.org/10.1093/oso/9780195122589.003.0012.
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Decarboxylation is an essential process in catabolic metabolism of essentially all nutrients that serve as sources of energy in biological cells and organisms. The most widely known biological process leading to decarboxylation is the metabolism of glucose, in which all of the carbon in the molecule is oxidized to carbon dioxide by way of the glycolytic pathway, the pyruvate dehydrogenase complex, and the tricarboxylic acid cycle. The decarboxylation steps take place in thiamine pyrophosphate (TPP)–dependent α-ketoacid dehydrogenase complexes and isocitrate dehydrogenase. The latter enzyme does not require a coenzyme, other than the cosubstrate NAD+. Many other decarboxylations require coenzymes such as pyridoxal-5'-phosphate (PLP) or a pyruvoyl moiety in the peptide chain. Biological carboxylation is the essential process in the fixation of carbon dioxide by plants and of bicarbonate by animals, plants, and bacteria. Carboxylation by enzymes requires the action of biotin or a divalent metal cofactor, and it requires ATP when the carboxylating agent is the bicarbonate ion. The most prevalent enzymatic carboxylation is that of ribulose bisphosphate carboxylase (rubisco), which is responsible for carbon dioxide fixation in plants. The basic chemistry of decarboxylation is illustrated by mechanisms A to D in fig. 8-1. The mechanisms all require some means of accommodation for the electrons from the cleavage of the bond linking the carboxylate group to the α-carbon. In mechanism A, an electron sink at the β-carbon provides a haven for two electrons. Acetoacetate decarboxylase functions by this mechanism (see chap. 1), as well as PLP- and TPP-dependent decarboxylases (see chap. 3). In mechanism B, a leaving group at the β-carbon departs with two electrons. Mevalonate-5-diphosphate decarboxylate functions by mechanism B and is discussed in a later section. In mechanism C, a leaving group replaces the α-carbon and departs with a pair of electrons. A biological example is formate dehydrogenase, in which the leaving group is a hydride that is transferred to NAD+. In mechanism D, a free radical center is created adjacent to the α-carbon and potentiates the homolytic scission of the bond to the carboxylate group. Mechanism D requires secondary electron transfer processes to create the radical center and quench the formyl radical.
Streszczenia konferencji na temat "Homolytic Cleavage"
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Jeremić, Svetlana R., Jelena R. Đorović Jovanović, Marijana S. Stanojević Pirković i Zoran S. Marković. "THERMODYNAMICALLY INVESTIGATIONS OF FREE RADICAL SCAVENGER POTENCY OF 1,2,4-TRIHYDROXYTHIOXANTHONE". W 1st INTERNATIONAL Conference on Chemo and BioInformatics. Institute for Information Technologies, University of Kragujevac, 2021. http://dx.doi.org/10.46793/iccbi21.414j.
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The operative mechanism of the antioxidative action of 1,2,4-trihydroxythioxanthone (TX) is investigated in this contribution. Conclusions are made based on enthalpy values, as thermodynamical parameters. All calculations are done using the M06-2X/6-311++G(d,p) level of theory. To imitate polar and non-polar environments, calculations are done in water and benzene as the medium. It is found that, among three possible radicals that TX can generate, the most stable is the one obtained by homolytic cleavage of the O-H group in position 4. It was found that HAT (Hydrogen Atom Transfer) is the most plausible mechanism for that purpose in benzene. On the other hand, the most favorable mechanism in water is SPLET (Sequential Proton Loss Electron Transfer). Here is estimated the capacity of TX to deactivate hydroxyl (HO●), hydroperoxyl (HOO●) and methylperoxyl radical (CH3OO●). It is found that TX can deactivate all three free radicals following HAT and SPLET reaction mechanisms competitively, in the polar and non-polar environment. SET-PT (Single-Electron Transfer followed by Proton Transfer) is the inoperative mechanism for radicals scavenging, in the polar and non-polar environment.