- Nitrogen Dioxide Catalyzed Aerobic Oxidative Cleavage of C(OH)–C Bonds of Secondary Alcohols to Produce Acids
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Stable organic nitroxyl radicals are an important class of catalysts for oxidation reactions, but their wide applications are hindered by their steric hinderance, high cost, complex operation, and separation procedures. Herein, NO2 in DMSO is shown to effectively catalyze the aerobic oxidative cleavage of C(OH)?C bonds to form a carboxylic group, and NO2 was generated in situ by decomposition of nitrates. A diverse range of secondary alcohols were selectively converted into acids in excellent yields in this transition-metal-free system without any additives. Preliminary results also indicate its applicability to depolymerize recalcitrant macromolecular lignin. Detail studies revealed that NO2 from nitrates promoted the reaction, and NO2 served as hydrogen acceptor and radical initiator for the tandem oxidative reaction.
- Liu, Mingyang,Zhang, Zhanrong,Song, Jinliang,Liu, Shuaishuai,Liu, Huizhen,Han, Buxing
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- Regioselective aliphatic carbon-carbon bond cleavage by a model system of relevance to iron-containing acireductone dioxygenase
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Mononuclear Fe(II) complexes ([(6-Ph2TPA)Fe(PhC(O)C(R)C(O)Ph)]X (3-X: R = OH, X = ClO4 or OTf; 4: R = H, X = ClO4)) supported by the 6-Ph2TPA chelate ligand (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-(2-pyridylmethyl)amine) and containing a β-diketonate ligand bound via a six-membered chelate ring have been synthesized. The complexes have all been characterized by 1H NMR, UV-vis, and infrared spectroscopy and variably by elemental analysis, mass spectrometry, and X-ray crystallography. Treatment of dry CH3CN solutions of 3-OTf with O2 leads to oxidative cleavage of the C(1)-C(2) and C(2)-C(3) bonds of the acireductone via a dioxygenase reaction, leading to formation of carbon monoxide and 2 equiv of benzoic acid as well as two other products not derived from dioxygenase reactivity: 2-oxo-2- phenylethylbenzoate and benzil. Treatment of CH3CN/H2O solutions of 3-X with O2 leads to the formation of an additional product, benzoylformic acid, indicative of the operation of a new reaction pathway in which only the C(1)-C(2) bond is cleaved. Mechanistic studies show that the change in regioselectivity is due to the hydration of a vicinal triketone intermediate in the presence of both an iron center and water. This is the first structural and functional model of relevance to iron-containing acireductone dioxygenase (Fe-ARD′), an enzyme in the methionine salvage pathway that catalyzes the regiospecific oxidation of 1,2-dihydroxy-3-oxo-(S)- methylthiopentene to form 2-oxo-4-methylthiobutyrate. Importantly, this model system is found to control the regioselectivity of aliphatic carbon-carbon bond cleavage by changes involving an intermediate in the reaction pathway, rather than by the binding mode of the substrate, as had been proposed in studies of acireductone enzymes.
- Allpress, Caleb J.,Grubel, Katarzyna,Szajna-Fuller, Ewa,Arif, Atta M.,Berreau, Lisa M.
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- Functional models of α-keto acid dependent nonheme iron oxygenases: Synthesis and reactivity of biomimetic iron(II) benzoylformate complexes supported by a 2,9-dimethyl-1,10-phenanthroline ligand
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Two biomimetic iron(II) benzoylformate complexes, [LFeII(BF) 2] (2) and [LFeII(NO3)(BF)] (3) (L is 2,9-dimethyl-1,10-phenanthroline and BF is monoanionic benzoylformate), have been synthesized from an iron(II)-dichloro complex [LFeIICl 2] (1). All the iron(II) complexes have been structurally and spectroscopically characterized. The iron(II) center in 2 is coordinated by a bidentate NN ligand (2,9-dimethyl-1,10-phenanthroline) and two monoanionic benzoylformates to form a distorted octahedral coordination geometry. One of the benzoylformates binds to the iron in 2 via both carboxylate oxygens but the other one binds in a chelating bidentate fashion via one carboxylate oxygen and the keto oxygen. On the other hand, the iron(II) center in 3 is ligated by one NN ligand, one bidentate nitrate, and one monoanionic chelating benzoylformate. Both iron(II) benzoylformate complexes exhibit the facial NNO donor environment in their solid-state structures. Complexes 2 and 3 are stable in noncoordinating solvents under an inert atmosphere, but react with dioxygen under ambient conditions to undergo oxidative decarboxylation of benzoylformate to benzoate in high yields. Evidence for the formation of an iron(IV)-oxo intermediate upon oxidative decarboxylation of benzoylformate was obtained by interception and labeling experiments. The iron(II) benzoylformate complexes represent the functional models of α-keto acid dependent oxygenases.
- Das, Oindrila,Chatterjee, Sayanti,Paine, Tapan Kanti
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- Catalytic O2activation with synthetic models of α-ketoglutarate dependent oxygenases
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An iron complex bearing the facially capping tridentate 1,4,7-triazacyclononane ligand mimics structural and functional features of alpha-ketoglutarate (α-KG) dependent enzymes, and engages in enzyme-like catalytic O2 activation coupled to α-ketoacid decarboxylation, oxygenating sulfides. This system constitutes a rare case of non-enzymatic catalytic O2 activation, cycling between FeII and FeIV(O).
- Sánchez-Eguía, Brenda N.,Serrano-Plana, Joan,Company, Anna,Costas, Miquel
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- Phosphorus-Based Organocatalysis for the Dehydrative Cyclization of N-(2-Hydroxyethyl)amides into 2-Oxazolines
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A metal-free, biomimetic catalytic protocol for the cyclization of N-(2-hydroxyethyl)amides to the corresponding 2-oxazolines (4,5-dihydrooxazoles), promoted by the 1,3,5,2,4,6-triazatriphosphorine (TAP)-derived organocatalyst tris(o-phenylenedioxy)cyclotriphosphazene (TAP-1) has been developed. This approach requires less precatalyst compared to the reported relevant systems, with respect to the phosphorus atom (the maximum turnover number (TON) ~30), and exhibits a broader substrate scope and higher functional-group tolerance, providing the functionalized 2-oxazolines with retention of the configuration at the C(4) stereogenic center of the 2-oxazolines. Widely accessible β-amino alcohols can be used in this approach, and the cyclization of N-(2-hydroxyethyl)amides provides the desired 2-oxazolines in up to 99% yield. The mechanism of the reaction was studied by monitoring the reaction using spectral and analytical methods, whereby an 18O-labeling experiment furnished valuable insights. The initial step involves a stoichiometric reaction between the substrate and TAP-1, which leads to the in situ generation of the catalyst, a catechol cyclic phosphate, as well as to a pyrocatechol phosphate and two possible active intermediates. The dehydrative cyclization was also successfully conducted on the gram scale.
- Soleymani Movahed, Farzaneh,Foo, Siong Wan,Mori, Shogo,Ogawa, Saeko,Saito, Susumu
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supporting information
p. 243 - 257
(2021/12/17)
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- A functional model for quercetin 2,4-dioxygenase: Geometric and electronic structures and reactivity of a nickel(II) flavonolate complex
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Quercetin 2,4-dioyxgenase (QueD) has been known to catalyze the oxygenative degradation of flavonoids and quercetin. Recent crystallographic study revealed a nickel ion occupies the active site as a co-factor to support O2 activation and catalysis. Herein, we report a nickel(II) flavonolate complex bearing a tridentate macrocyclic ligand, [NiII(Me3-TACN)(Fl)(NO3)](H2O) (1, Me3-TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane, Fl = 3-hydroxyflavone) as a functional model for QueD. The flavonolatonickel(II) complex was characterized by using spectrometric analysis including UV–vis spectroscopy, electrospray ionization mass spectrometer (ESI-MS), infrared spectroscopy (FT-IR) and 1H nuclear magnetic resonance spectroscopy (NMR). The single crystal X-ray structure of 1 shows two isomers with respect to the direction of a flavonolate ligand. Two isomers commonly are in the octahedral geometry with a bidentate of flavonolate and a monodentate of nitrate as well as a tridentate binding of Me3-TACN ligand. The spin state of 1 is determined to be a triplet state based on the Evans' method. Interestingly, electronic configuration of 1 from density functional theory (DFT) calculations revealed that the two singly occupied molecular orbitals (SOMOs) lie energetically lower than the highest (doubly) occupied molecular orbital (HOMO), that is so-called the SOMO-HOMO level inversion (SHI). The HOMO shows an electron density localized in the flavonolate ligand, indicating that flavonolate ligand is oxidized first rather than the nickel center. Thermal degradation of 1 resulted in the formation of benzoic acid and salicylic acid, which is attributed to the oxygenation of flavonolate of 1.
- Jeong, Donghyun,Sun, Seungwon,Moon, Dohyun,Cho, Jaeheung
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- Bioinspired oxidation of oximes to nitric oxide with dioxygen by a nonheme iron(II) complex
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The ability of two iron(II) complexes, [(TpPh2)FeII(benzilate)] (1) and [(TpPh2)(FeII)2(NPP)3] (2) (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, NPP-H = α-isonitrosopropiophenone), of a monoanionic facial N3 ligand in the O2-dependent oxidation of oximes is reported. The mononuclear complex 1 reacts with dioxygen to decarboxylate the iron-coordinated benzilate. The oximate-bridged dinuclear complex (2), which contains a high-spin (TpPh2)FeII unit and a low-spin iron(II)–oximate unit, activates dioxygen at the high-spin iron(II) center. Both the complexes exhibit the oxidative transformation of oximes to the corresponding carbonyl compounds with the incorporation of one oxygen atom from dioxygen. In the oxidation process, the oxime units are converted to nitric oxide (NO) or nitroxyl (HNO). The iron(II)–benzilate complex (1) reacts with oximes to afford HNO, whereas the iron(II)–oximate complex (2) generates NO. The results described here suggest that the oxidative transformation of oximes to NO/HNO follows different pathways depending upon the nature of co-ligand/reductant.
- Bhattacharya, Shrabanti,Lakshman, Triloke Ranjan,Sutradhar, Subhankar,Tiwari, Chandan Kumar,Paine, Tapan Kanti
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- 1,4-Conjugate addition/esterification of: Ortho -quinone methides in a multicomponent reaction
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A novel 1,4-conjugate addition/esterification of ortho-quinone methides in a multicomponent reaction has been developed. In this protocol, numerous carboxylic acids, ynamides and in situ generated ortho-quinone methides could assemble rapidly to constitut
- Chen, Renjie,Liu, Yu,Cui, Sunliang
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supporting information
p. 11753 - 11756
(2018/11/21)
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- Aliphatic C-H Bond Halogenation by Iron(II)-α-Keto Acid Complexes and O2: Functional Mimicking of Nonheme Iron Halogenases
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α-Ketoglutarate-dependent nonheme halogenases catalyze the halogenation of aliphatic C-H bonds in the biosynthesis pathway of many natural products. An iron(IV)-oxo-halo species has been established as the active oxidant in the halogenation reactions. With an objective to emulate the function of the nonheme halogenases, two iron(II)-α-keto acid complexes, [(phdpa)Fe(BF)Cl] (1) and [(1,4-tpbd)Fe2(BF)2Cl2] (2) (where phdpa = N,N-bis(2-pyridylmethyl)aniline, 1,4-tpbd = N,N,N′,N'-tetrakis(2-pyridylmethyl)benzene-1,4-diamine, and BF = benzoylformate), have been prepared. The iron complexes are capable of carrying out the oxidative halogenation of aliphatic C-H bonds using O2 as the terminal oxidant. Although the complexes are not selective toward C-H bond halogenation, they are the only examples of nonheme iron(II)-α-keto acid complexes mimicking the activity of nonheme halogenases. The dinuclear complex (2) exhibits enhanced reactivity toward C-H bond halogenation/hydroxylation.
- Jana, Rahul Dev,Sheet, Debobrata,Chatterjee, Sayanti,Paine, Tapan Kanti
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p. 8769 - 8777
(2018/08/17)
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- Silver(I)-Catalyzed Widely Applicable Aerobic 1,2-Diol Oxidative Cleavage
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The oxidative cleavage of 1,2-diols is a fundamental organic transformation. The stoichiometric oxidants that are still predominantly used for such oxidative cleavage, such as H5IO6, Pb(OAc)4, and KMnO4, generate stoichiometric hazardous waste. Herein, we describe a widely applicable and highly selective silver(I)-catalyzed oxidative cleavage of 1,2-diols that consumes atmospheric oxygen as the sole oxidant, thus serving as a potentially greener alternative to the classical transformations.
- Zhou, Zhong-Zhen,Liu, Mingxin,Lv, Leiyang,Li, Chao-Jun
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supporting information
p. 2616 - 2620
(2018/02/13)
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- Reductive Activation of O2 by Non-Heme Iron(II) Benzilate Complexes of N4 Ligands: Effect of Ligand Topology on the Reactivity of O2-Derived Oxidant
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A series of iron(II) benzilate complexes (1-7) with general formula [(L)FeII(benzilate)]+ have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N4 ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me3-TPA in 2), N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N1,N1-dimethyl-N2,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me2-iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N1,N2-dimethyl-N1,N2-bis(2-pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N1,N2-dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me2-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1-3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron-oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4-6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O2-derived iron-oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, and 6-Me2-BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.
- Chakraborty, Biswarup,Jana, Rahul Dev,Singh, Reena,Paria, Sayantan,Paine, Tapan Kanti
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p. 359 - 371
(2017/01/13)
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- Conversion of nitroalkanes into carboxylic acids via iodide catalysis in water
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We report a new method for the conversion of nitroalkanes into carboxylic acids that achieves this transformation under very mild conditions. Catalytic amounts of iodide in combination with a simple zinc catalyst are needed to give good conversions into the corresponding carboxylic acids.
- Marcé, Patricia,Lynch, James,Blacker, A. John,Williams, Jonathan M. J.
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p. 1013 - 1016
(2016/01/16)
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- Fragmentation of Protonated N-(3-Aminophenyl)Benzamide and Its Derivatives in Gas Phase
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An ion of m/z 110.06036 (ion formula [C6H8NO]+; error: 0.32 mDa) was observed in the collision induced dissociation tandem mass spectrometry experiments of protonated N-(3-aminophenyl)benzamide, which is a rearrangement product ion purportedly through nitrogen-oxygen (N–O) exchange. The N–O exchange rearrangement was confirmed by the MS/MS spectrum of protonated N-(3-aminophenyl)-O18-benzamide, where the rearranged ion, [C6H8NO18]+ of m/z 112 was available because of the presence of O18. Theoretical calculations using Density Functional Theory (DFT) at B3LYP/6-31?g(d) level suggest that an ion-neutral complex containing a water molecule and a nitrilium ion was formed via a transition state (TS-1), followed by the water molecule migrating to the anilide ring, eventually leading to the formation of the rearranged ion of m/z 110. The rearrangement can be generalized to other protonated amide compounds with electron-donating groups at the meta position, such as, –OH, –CH3, –OCH3, –NH(CH3)2, –NH-Ph, and –NHCOCH3, all of which show the corresponding rearranged ions in MS/MS spectra. However, the protonated amide compounds containing electron-withdrawing groups, including –Cl, –Br, –CN, –NO2, and –CF3, at the meta position did not display this type of rearrangement during dissociation. Additionally, effects of various acyl groups on the rearrangement were investigated. It was found that the rearrangement can be enhanced by substitution on the ring of the benzoyl with electron-withdrawing groups. [Figure not available: see fulltext.]
- Zu, Chengli,Mukhopadhyay, Sukrit,Hanley, Patrick S.,Xia, Shijing,Bell, Bruce M.,Grigg, David,Gilbert, Jeffrey R.,O’Brien, John P.
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p. 917 - 926
(2016/05/02)
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- Anion Effects in Oxidative Aliphatic Carbon-Carbon Bond Cleavage Reactions of Cu(II) Chlorodiketonate Complexes
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Aliphatic oxidative carbon-carbon bond cleavage reactions involving Cu(II) catalysts and O2 as the terminal oxidant are of significant current interest. However, little is currently known regarding how the nature of the Cu(II) catalyst, including the anions present, influence the reaction with O2. In previous work, we found that exposure of the Cu(II) chlorodiketonate complex [(6-Ph2TPA)Cu(PhC(O)CClC(O)Ph)]ClO4 (1) to O2 results in oxidative aliphatic carbon-carbon bond cleavage within the diketonate unit, leading to the formation of benzoic acid, benzoic anhydride, benzil, and 1,3-diphenylpropanedione as organic products. Kinetic studies of this reaction revealed a slow induction phase followed by a rapid decay of the absorption features of 1. Notably, the induction phase is not present when the reaction is performed in the presence of a catalytic amount of chloride anion. In the studies presented herein, a combination of spectroscopic (UV-vis, EPR) and density functional theory (DFT) methods have been used to examine the chloride and benzoate ion binding properties of 1 under anaerobic conditions. These studies provide evidence that each anion coordinates in an axial position of the Cu(II) center. DFT studies reveal that the presence of the anion in the Cu(II) coordination sphere decreases the barrier for O2 activation and the formation of a Cu(II)-peroxo species. Notably, the chloride anion more effectively lowers the barrier associated with O-O bond cleavage. Thus, the nature of the anion plays an important role in determining the rate of reaction of the diketonate complex with O2. The same type of anion effects were observed in the O2 reactivity of the simple Cu(II)-bipyridine complex [(bpy)Cu(PhC(O)C(Cl)C(O)Ph)ClO4] (3).
- Saraf, Sushma L.,Mi?aczewska, Anna,Borowski, Tomasz,James, Christopher D.,Tierney, David L.,Popova, Marina,Arif, Atta M.,Berreau, Lisa M.
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p. 6916 - 6928
(2016/07/26)
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- Halide-promoted dioxygenolysis of a carbon-carbon bond by a copper(II) diketonate complex
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A mononuclear Cu(II) chlorodiketonate complex was prepared, characterized, and found to undergo oxidative aliphatic carbon-carbon bond cleavage within the diketonate unit upon exposure to O2 at ambient temperature. Mechanistic studies provide evidence for a dioxygenase-type C-C bond cleavage reaction pathway involving trione and hypochlorite intermediates. Significantly, the presence of a catalytic amount of chloride ion accelerates the oxygen activation step via the formation of a Cu-Cl species, which facilitates monodentate diketonate formation and lowers the barrier for O2 activation. The observed reactivity and chloride catalysis is relevant to Cu(II) halide-catalyzed reactions in which diketonates are oxidatively cleaved using O2 as the terminal oxidant. The results of this study suggest that anion coordination can play a significant role in influencing copper-mediated oxygen activation in such systems.
- Allpress, Caleb J.,Mi?aczewska, Anna,Borowski, Tomasz,Bennett, Jami R.,Tierney, David L.,Arif, Atta M.,Berreau, Lisa M.
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supporting information
p. 7821 - 7824
(2014/06/23)
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- Transition-metal-free and chemoselective NaOtBu-O 2-mediated oxidative cleavage reactions of vic-1,2-diols to carboxylic acids and mechanistic insight into the reaction pathways
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A method for efficient oxidative cleavage of vic-1,2-diols using a NaO tBu-O2 system resulted in the formation of carboxylic acids in high yields. The present protocol is an eco-friendly alternative to a conventional transition-metal-based method. This new strategy allows large-scale production with nonchromatographic purification while also suppressing competitive reaction pathway such as benzilic acid rearrangement.
- Kim, Sun Min,Kim, Dong Wan,Yang, Jung Woon
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supporting information
p. 2876 - 2879
(2014/06/23)
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- Identification of an acyl-enzyme intermediate in a meta-cleavage product hydrolase reveals the versatility of the catalytic triad
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Meta-cleavage product (MCP) hydrolases are members of the α/β-hydrolase superfamily that utilize a Ser-His-Asp triad to catalyze the hydrolysis of a C-C bond. BphD, the MCP hydrolase from the biphenyl degradation pathway, hydrolyzes 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to 2-hydroxypenta-2,4-dienoic acid (HPD) and benzoate. A 1.6 A resolution crystal structure of BphD H265Q incubated with HOPDA revealed that the enzyme's catalytic serine was benzoylated. The acyl-enzyme is stabilized by hydrogen bonding from the amide backbone of 'oxyanion hole' residues, consistent with formation of a tetrahedral oxyanion during nucleophilic attack by Ser112. Chemical quench and mass spectrometry studies substantiated the formation and decay of a Ser112-benzoyl species in wild-type BphD on a time scale consistent with turnover and incorporation of a single equivalent of 18O into the benzoate produced during hydrolysis in H218O. Rapid-scanning kinetic studies indicated that the catalytic histidine contributes to the rate of acylation by only an order of magnitude, but affects the rate of deacylation by over 5 orders of magnitude. The orange-colored catalytic intermediate, ESred, previously detected in the wild-type enzyme and proposed herein to be a carbanion, was not observed during hydrolysis by H265Q. In the newly proposed mechanism, the carbanion abstracts a proton from Ser112, thereby completing tautomerization and generating a serinate for nucleophilic attack on the C6-carbonyl. Finally, quantification of an observed pre-steady-state kinetic burst suggests that BphD is a half-site reactive enzyme. While the updated catalytic mechanism shares features with the serine proteases, MCP hydrolase-specific chemistry highlights the versatility of the Ser-His-Asp triad.
- Ruzzini, Antonio C.,Ghosh, Subhangi,Horsman, Geoff P.,Foster, Leonard J.,Bolin, Jeffrey T.,Eltis, Lindsay D.
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experimental part
p. 4615 - 4624
(2012/04/23)
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- Stepwise oxygenations of toluene and 4-nitrotoluene by a fungal peroxygenase
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Fungal peroxygenases have recently been shown to catalyze remarkable oxidation reactions. The present study addresses the mechanism of benzylic oxygenations catalyzed by the extracellular peroxygenase of the agaric basidiomycete Agrocybe aegerita. The peroxygenase oxidized toluene and 4-nitrotoluene via the corresponding alcohols and aldehydes to give benzoic acids. The reactions proceeded stepwise with total conversions of 93% for toluene and 12% for 4-nitrotoluene. Using H218O2 as the co-substrate, we show here that H2O2 is the source of the oxygen introduced at each reaction step. A. aegerita peroxygenase resembles cytochromes P450 and heme chloroperoxidase in catalyzing benzylic hydroxylations.
- Kinne, Matthias,Zeisig, Christian,Ullrich, Rene,Kayser, Gernot,Hammel, Kenneth E.,Hofrichter, Martin
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- Electronic substituent effects on the cleavage specificity of a non-heme Fe2+-dependent β-diketone dioxygenase and their mechanistic implications
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Acinetobacter johnsonii acetylacetone dioxygenase (Dke1) is a non-heme Fe(II)-dependent dioxygenase that cleaves C-C bonds in various β-dicarbonyl compounds capable of undergoing enolization to a cis-β-keto enol structure. Results from 18O labeling experiments and quantitative structure-reactivity relationship analysis of electronic substituent effects on the substrate cleavage specificity of Dke1 are used to distinguish between two principle chemical mechanisms of reaction: one involving a 1,2-dioxetane intermediate and another proceeding via Criegee rearrangement. Oxygenative cleavage of asymmetrically substituted β-dicarbonyl substrates occurs at the bond adjacent to the most electron-deficient carbonyl carbon. Replacement of the acetyl group in 1-phenyl-1,3-butanedione by a trifluoro-acetyl group leads to a complete reversal of cleavage frequency from 83% to only 8% fission of the bond next to the benzoyl moiety. The structure-activity correlation for Dke1 strongly suggests that enzymatic bond cleavage takes place via nucleophilic attack to generate a dioxetane, which then decomposes into the carboxylate and α-keto-aldehyde products. Copyright
- Straganz, Grit D.,Hofer, Hannes,Steiner, Walter,Nidetzky, Bernd
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p. 12202 - 12203
(2007/10/03)
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- Efficient method for the preparation of inverted alkyl carboxylates and phenyl carboxylates via oxidation-reduction condensation using 2,6-dimethyl-1,4-benzoquinone or simple 1,4-benzoquinone
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Oxidation-reduction condensation using in situ formed alkoxydiphenylphosphines, 2,6-dimethy-1,4-benzoquinone, and carboxylic acids provides a useful method for the preparation of inverted tertiary alkyl carboxylates from the corresponding chiral tertiary alcohols under mild and neutral conditions. Similarly, it has afforded alkyl carboxylates successfully in good-to-high yields by the combined use of alkoxydiphenylphosphines having primary, secondary, or tertiary alkoxy groups, carboxylic acids, and simple 1,4-benzoquinone. When chiral secondary or tertiary alcohols are used, the corresponding inverted secondary or tertiary alkyl carboxylates are also obtained in good-to-high yields. In addition, a convenient method for the preparation of phenyl carboxylates in high yields has been established by utilizing oxidation-reduction condensation in toluene at 110 °C using phenoxydiphenylphosphines in situ-formed from phenols and chlorodiphenylphosphine, 2,6-dimethyl-1,4-benzoquinone, and carboxylic acids.
- Shintou, Taichi,Fukumoto, Kentaro,Mukaiyama, Teruaki
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p. 1569 - 1579
(2007/10/03)
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- Copper-mediated oxygenolysis of flavonols via endoperoxide and dioxetan intermediates
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[Cu(phen)2(fla)]ClO4 was prepared by treating [Cu(CH3CN)4]-ClO4 with flavonol (flah) in the presence of 1,10-phenanthroline (phen) as a co-ligand. Its oxygenation in DMF (or CH3CN) solution at elevated temperature gave the (O-benzoylsalicylato)copper(II) complex [Cu(phen)2(O-bs)]ClO4 (bs = benzoylsalicylato) and carbon monoxide via an endoperoxide intermediate. Crystallographic characterisation of [Cu(phen)2(O-bs)]ClO4 as the CH2Cl2 solvate [triclinic, space group P1, a = 10.499(3) A, b = 12.556(4) A, c = 17.094(5), A α = 72.69(2), β = 89.35(2), γ = 69.19(2)° v = 1999.7(10) A3, Z = 2, R1 = 0.0962] shows that the molecule has a distorted trigonal-bipyramidal structure (T = 0.96). The oxygenolysis was monitored by spectrophotometry, and the pseudo-first-order rate constant k′phen was found to be (2.47 ± 0.11) × 10-4 s-1 at 120°C. Complexes of [Cu(L)(4′R-fla)2] (L = phen, bpy, TMEDA; R = H, OCH3, CH3, Cl) were also prepared by treating the complexes Cu(4′R-fla)2 with nitrogen-containing co-ligands. Their oxygenation resulted in the corresponding complexes Cu(L)(2HOpg)2 (2HOpg =2-hydroxyphenylglyoxylate) derived by spontaneous hydrolysis of Cu(L)(bpg)2 (bpg = 2-benzoatophenylglyoxylate). The (phenylglyoxylato)-copper complexes were probably formed via 1,2-dioxetan intermediates, since the oxygenation of Cu(phen)(fla)2 showed chemiluminescence with bands at 506, 546, and 578 nm in the emission spectrum due to the decomposition of a 1,2-dioxetan species. Labelling experiments with an 18O2/16O2 mixture (1:3) showed the incorporation of both 18O atoms of 18O2 into the flavonolate ligand. The kinetics of the oxygenolysis of Cu(L)(fla)2 gave rate constants according to the rate law -d[Cu(L)(4′R-fla)2]/dt = k[Cu(L)(4′R-fla)2] [O2]: k/m-1 s-1 = (9.50 ± 0.60) × 10-2 (L = phen), (2.40 ± 0.10) × 10-2 (L = bpy), (2.00 ± 0.10) × 10-2 (L = TMEDA) m-1 s-1 at 353.16 K. The oxygenolysis of the complexes Cu(phen)(4′R-fla)2 fits a Hammett linear free energy relationship and an increase of the electron density on the copper ion makes the oxygenation reaction faster. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.
- Balogh-Hergovich, Eva,Kaizer, Jozsef,Pap, Jozsef,Speier, Gabor,Huttner, Gottfried,Zsolnai, Laszlo
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p. 2287 - 2295
(2007/10/03)
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- Kinetics and Mechanism of the Hydrolysis of N-Methyl-N-nitroamides in Aqueous Sulphuric Acid
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Pseudo first-order rate constants for the hydrolysis of N-methyl-N-nitroacetamide and various 4-substituted N-methyl-N-nitrobenzamides in sulphuric acid solutions are reported.N-Methyl-N-nitroacetamide undergoes an acid-catalysed process at all acidities studied for which the solvent deuterium isotope effect, k0H2SO4/k0D2SO4, is 0.87, and ΔS(excit.) ca. -85 (+/-10) J K-1 mol-1.These results suggest an AAc2 mechanism involving rapid pre-equilibrium protonation of the substrate followed by rate-limiting attack of water at the carbonyl C-atom to form atetrahedral intermediate which collapses, in a fast step, to the products.The N-methyl-N-nitrobenzamides, however, exhibit both non-catalysed and acid-catalysed hydrolysis.The non-catalysed pathway operates at acidities up to ca. 5 mol dm-3 H2SO4, and is characterised by a solvent deuterium isotope effect, k0H2SO4/k0D2SO4, of 1.5, ΔS(excit.) ca. -100 (+/-10) J K-1 mol-1 and a Hammett ρ value of 1.0 (+/-0.1).No catalysis by Br- is observed and the results are more consistent with a thermal rearrangement and expulsion of N2O.The acid-catalysed pathway operates at acidities >5 mol dm-3 H2SO4.The solvent deuterium isotope effect k0H2SO4/k0D2SO4 is 0.58, ΔS(excit.)>0 JK-1 mol-1 and the Hammett ρ value is -3.4 (+/-0.1).Thus a change in mechanism occurs at ca. 5 mol dm-3 H2SO4 to an Ac1 pathway involving protonation of the substrate, followed by rate-limiting cleavage of the amide C-N bond to form a benzoyl cation.The different acid-catalysed hydrolysis pathways for the N-nitroacetamide and N-nitrobenzamides is ascribed to the stabilisation afforded to the benzoyl cation as opposed to the carbonium ion.N-Nitroamides are hydrolysed exclusively via amide C-N bond cleavage whereas the correspondning N-nitrosoamides decompose via concurrent C-N and N-N (i.e. denitrosation) bond cleavage.This difference between N-nitro and N-nitroso-amides is discussed in terms of the greater stability of the NO+ group.
- Challis, Brian C.,Rosa, Eduarda,Iley, Jim,Norberto, Fatima
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p. 179 - 183
(2007/10/02)
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