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Bhide R, Phun GS, Ardo S. Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant. J Phys Chem A 2024; 128:4177-4188. [PMID: 38752741 DOI: 10.1021/acs.jpca.3c08228] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/31/2024]
Abstract
Assessing the validity of a driving-force-dependent kinetic theory for a unimolecular elementary reaction step is difficult when the observed reaction rate is strongly influenced by properties of the preceding or following elementary reaction step. A well-known example occurs for bimolecular reactions with weak orbital overlap, such as outer-sphere electron transfer, where bimolecular collisional encounters that precede a fast unimolecular electron-transfer step can limit the observed rate. A lesser-appreciated example occurs for bimolecular reactions with stronger orbital overlap, including many proton-transfer reactions, where equilibration of an endergonic unimolecular proton-transfer step results in a relatively small concentration of reaction products, thus slowing the rate of the following step such that it becomes rate limiting. Incomplete consideration of these points has led to discrepancies in interpretation of data from the literature. Our reanalysis of these data suggests that proton-transfer elementary reaction steps have a nonzero intrinsic free energy barrier, implying, in the parlance of Marcus theory, that there is non-negligible nuclear reorganization. Outcomes from our analyses are generalizable to inner-sphere electron-transfer reactions such as those involved in (photo)electrochemical fuel-forming reactions.
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Affiliation(s)
- Rohit Bhide
- Department of Chemistry, University of California Irvine, Irvine, California 92697, United States
| | - Gabriel S Phun
- Department of Chemistry, University of California Irvine, Irvine, California 92697, United States
| | - Shane Ardo
- Department of Chemistry, University of California Irvine, Irvine, California 92697, United States
- Department of Chemical & Biomolecular Engineering, University of California Irvine, Irvine, California 92697, United States
- Department of Materials Science & Engineering, University of California Irvine, Irvine, California 92697, United States
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2
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Zhang X, Cheng S, Chen C, Wen X, Miao J, Zhou B, Long M, Zhang L. Keto-anthraquinone covalent organic framework for H 2O 2 photosynthesis with oxygen and alkaline water. Nat Commun 2024; 15:2649. [PMID: 38531862 DOI: 10.1038/s41467-024-47023-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2023] [Accepted: 03/18/2024] [Indexed: 03/28/2024] Open
Abstract
Hydrogen peroxide photosynthesis suffers from insufficient catalytic activity due to the high energy barrier of hydrogen extraction from H2O. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework which is able to directly synthesize H2O2 (4784 μmol h-1 g-1 at λ > 400 nm) from oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents. The strong alkalinity resulted in the formation of OH-(H2O)n clusters in water, which were adsorbed on keto moieties within the framework and then dissociated into O2 and active hydrogen, because the energy barrier of hydrogen extraction was largely lowered. The produced hydrogen reacted with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by O2 to produce H2O2. This study ultimately sheds light on the importance of hydrogen extraction from H2O for H2O2 photosynthesis and demonstrates that H2O2 synthesis is achievable under alkaline conditions.
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Affiliation(s)
- Xiangcheng Zhang
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
| | - Silian Cheng
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
| | - Chao Chen
- School of Ecological and Environmental Science, Key Laboratory for Urban Ecological Processes and Eco-Restoration, East China Normal University, Shanghai, 200241, China
| | - Xue Wen
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
| | - Jie Miao
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
| | - Baoxue Zhou
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
| | - Mingce Long
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China.
| | - Lizhi Zhang
- School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China.
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3
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Hoffmann H, Tausch MW. Intermolecular Photoredox Coupling: Alternative to Norrish Type II Reaction and Yang Cyclization in Ketones with γ‐C−H Bonds. European J Org Chem 2021. [DOI: 10.1002/ejoc.202100434] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Heiko Hoffmann
- Fachbereich Naturwissenschaften und Technik Provadis School of International Management and Technology AG Industriepark Höchst, Gebäude B 835 65926 Frankfurt am Main Germany
| | - Michael W. Tausch
- Fakultät für Mathematik und Naturwissenschaften Bergische Universität Wuppertal, Gebäude V.11.027 Gaußstraße 20 42119 Wuppertal Germany
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4
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Shangguan J, Hensley AJR, Gradiski MV, Pfriem N, McEwen JS, Morris RH, Chin YHC. The Role of Protons and Hydrides in the Catalytic Hydrogenolysis of Guaiacol at the Ruthenium Nanoparticle–Water Interface. ACS Catal 2020. [DOI: 10.1021/acscatal.0c01963] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Junnan Shangguan
- Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto M5S 3E5, Canada
| | - Alyssa J. R. Hensley
- Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto M5S 3E5, Canada
- The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman Washington 99164, United States
| | | | - Niklas Pfriem
- Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto M5S 3E5, Canada
| | - Jean-Sabin McEwen
- The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman Washington 99164, United States
- Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
- Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States
- Department of Biological Systems Engineering, Washington State University, Pullman, Washington 99164, United States
| | - Robert H. Morris
- Department of Chemistry, University of Toronto, Toronto M5S 3H6, Canada
| | - Ya-Huei Cathy Chin
- Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto M5S 3E5, Canada
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5
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Wu T, Zhu C, Han D, Kang Z, Niu L. Highly selective conversion of CO 2 to C 2H 6 on graphene modified chlorophyll Cu through multi-electron process for artificial photosynthesis. NANOSCALE 2019; 11:22980-22988. [PMID: 31769773 DOI: 10.1039/c9nr07824j] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Artificial photosynthesis is a promising strategy for converting carbon dioxide into hydrocarbon fuels through solar energy as it is clean, economical and environmentally friendly. Herein, we developed a selective and stable photocatalyst for CO2 photocatalytic reduction into C2H6 through a multi-electron transfer pathway without the external sacrificial regents. The core component of this composite catalyst was extracted from a silkworm excrement and modified to make chlorophyll Cu (Chl-Cu), which contained a porphyrin structure as an antenna for light absorption and a Cu cation as an active centre. We found that C2 hydrocarbons such as C2H2, C2H4, and C2H6 tended to generate on chlorophyll-a/graphene. After substituting Mg2+ with Cu2+ cations in the centre of the porphyrin and modifying with graphene, only C2H6 was detected in the 18 hours reaction. This photocatalyst presented an outstanding activity and selectivity for the photocatalytic CO2 reduction (CO2RR) with a C2H6 yield rate at 68.23 μmol m-2 h-1 under visible light irradiation and an apparent quantum efficiency of 1.26% at 420 nm. In this system, the porphyrin rings were excited to produce electron-hole pairs by light. The photo-induced holes oxidized water to produce oxygen while graphene worked as an adsorption centre and electron acceptor for the CO2 reduction.
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Affiliation(s)
- Tongshun Wu
- Centre for Advanced Analytical Science, c/o School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P.R. China.
| | - Cheng Zhu
- Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, PR China.
| | - Dongxue Han
- Centre for Advanced Analytical Science, c/o School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P.R. China. and State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China
| | - Zhenhui Kang
- Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, PR China.
| | - Li Niu
- Centre for Advanced Analytical Science, c/o School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P.R. China. and State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China
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6
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Prates ET, Crowley MF, Skaf MS, Beckham GT. Catalytic Mechanism of Aryl-Ether Bond Cleavage in Lignin by LigF and LigG. J Phys Chem B 2019; 123:10142-10151. [DOI: 10.1021/acs.jpcb.9b06243] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Erica Teixeira Prates
- National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80403, United States
- Institute of Chemistry and Center for Computing in Engineering and Sciences, University of Campinas, Campinas, São Paulo 13084-862, Brazil
| | - Michael F. Crowley
- Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80403, United States
| | - Munir S. Skaf
- Institute of Chemistry and Center for Computing in Engineering and Sciences, University of Campinas, Campinas, São Paulo 13084-862, Brazil
| | - Gregg T. Beckham
- National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80403, United States
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7
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Bearne SL. The role of Brønsted base basicity in estimating carbon acidity at enzyme active sites: a caveat. Org Biomol Chem 2019; 17:7161-7165. [PMID: 31317156 DOI: 10.1039/c9ob00863b] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Many enzymes catalyze the abstraction of a proton from a carbon acid substrate to initiate a variety of reactions; however, the development of a complete quantitative description of enzyme-catalyzed heterolytic cleavage of a C-H bond remains a challenge to enzymologists. To determine the pK value for such substrates bound at the active site, recent studies have estimated the equilibrium for formation of the deprotonated intermediate at the active site, however, accurate knowledge of the pK of the conjugate acid of the Brønsted base catalyst (BH+) is also required. Herein, it is shown that using the value of pK of the enzyme-substrate complex can underestimate the value of pK by an amount between zero and pδ, where pδ is the change in basicity of BH+ upon going from the enzyme-substrate complex to the enzyme-intermediate complex.
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Affiliation(s)
- Stephen L Bearne
- Department of Biochemistry & Molecular Biology, Dalhousie University, Halifax, NS B3H 4R2, Canada. and Department of Chemistry, Dalhousie University, Halifax, NS B3H 4R2, Canada
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8
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Hydrative syntheses of amides from alkynes catalyzed by an Au(I) complex containing pyridyl-functionalized NHC ligand. J Organomet Chem 2019. [DOI: 10.1016/j.jorganchem.2019.02.011] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
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9
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Fetter CM, Morrison ZA, Nagar M, Douglas CD, Bearne SL. Altering the Y137-K164-K166 triad of mandelate racemase and its effect on the observed pK a of the Brønsted base catalysts. Arch Biochem Biophys 2019; 666:116-126. [PMID: 30935886 DOI: 10.1016/j.abb.2019.03.011] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2018] [Revised: 03/16/2019] [Accepted: 03/25/2019] [Indexed: 11/16/2022]
Abstract
Mandelate racemase (MR) catalyzes the interconversion of the enantiomers of mandelate using a two-base mechanism with Lys 166 acting as the Brønsted base to abstract the α-proton from (S)-mandelate. The resulting intermediate is subsequently re-protonated by the conjugate acid of His 297 to yield (R)-mandelate. The roles of these amino acids are reversed when (R)-mandelate is the substrate. The side chains of Tyr 137, Lys 164, and Lys 166 form a H-bonding network and the proximity of the two ε-NH3+ groups is believed to lower the pKa of Lys 166. We used site-directed mutagenesis, kinetics, and pH-rate studies to explore the roles of Lys 164 (K164 C/M) and Tyr 137 (Y137 L/F/S/T) in catalysis. The efficiency (kcat/Km) was reduced ∼3.5 × 105-fold for K164C MR, relative to wild-type MR, indicating a major role for this residue in catalysis. The efficiency of Y137F MR, however, was reduced only 25-30-fold. pH-Rate profiles (log kcat vs. pH) revealed that substitution of Tyr 137 by Phe increased the kinetic pKa of Lys 166 from 5.88 ± 0.02 to 7.3 ± 0.2. Hence, Tyr 137 plays an important role in facilitating the reduction of the pKa of the Brønsted base Lys 166 by ∼1.4 units. Interestingly, the Phe substitution also increased the kinetic pKa of His 297 from 5.97 ± 0.04 to 7.1 ± 0.1. Thus, the Tyr 137-Lys 164-Lys 166 H-bonding network plays a broader role in modulating the pKa of catalytic residues by influencing the electrostatic character of the entire active site, not only by decreasing the observed pKa value of Lys 166, but also by decreasing the pKa of His 297 by 1.1 units.
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Affiliation(s)
- Christopher M Fetter
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, B3H 4R2, Canada
| | - Zachary A Morrison
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, B3H 4R2, Canada
| | - Mitesh Nagar
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, B3H 4R2, Canada
| | - Colin D Douglas
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, B3H 4R2, Canada
| | - Stephen L Bearne
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, B3H 4R2, Canada; Department of Chemistry, Dalhousie University, Halifax, NS, B3H 4R2, Canada.
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10
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Li W, Wang Y, Xu D. Asymmetric synthesis of β-amino ketones by using cinchona alkaloid-based chiral phase transfer catalysts. Org Biomol Chem 2018; 16:8704-8709. [PMID: 30411772 DOI: 10.1039/c8ob02484g] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A highly enantioselective nucleophilic addition of ketones to imines catalyzed by chiral phase-transfer catalysts (N-quaternised cinchona alkaloid ammonium salts) has been developed, and the process affords the Mannich reaction products with tertiary stereocenters in good to high yields (up to 95%) with excellent enantioselectivities (up to 97% ee).
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Affiliation(s)
- Weihua Li
- State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Key Laboratory of Green Pesticides and Cleaner Production Technology of Zhejiang Province, Zhejiang University of Technology, Hangzhou 310014, China.
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11
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Bearne SL, St Maurice M. A Paradigm for CH Bond Cleavage: Structural and Functional Aspects of Transition State Stabilization by Mandelate Racemase. ADVANCES IN PROTEIN CHEMISTRY AND STRUCTURAL BIOLOGY 2017; 109:113-160. [PMID: 28683916 DOI: 10.1016/bs.apcsb.2017.04.007] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Mandelate racemase (MR) from Pseudomonas putida catalyzes the Mg2+-dependent, 1,1-proton transfer reaction that racemizes (R)- and (S)-mandelate. MR shares a partial reaction (i.e., the metal ion-assisted, Brønsted base-catalyzed proton abstraction of the α-proton of carboxylic acid substrates) and structural features ((β/α)7β-barrel and N-terminal α + β capping domains) with a vast group of homologous, yet functionally diverse, enzymes in the enolase superfamily. Mechanistic and structural studies have developed this enzyme into a paradigm for understanding how enzymes such as those of the enolase superfamily overcome kinetic and thermodynamic barriers to catalyze the abstraction of an α-proton from a carbon acid substrate with a relatively high pKa value. Structural studies on MR bound to intermediate/transition state analogues have delineated those structural features that MR uses to stabilize transition states and enhance reaction rates of proton abstraction. Kinetic, site-directed mutagenesis, and structural studies have also revealed that the phenyl ring of the substrate migrates through the hydrophobic cavity within the active site during catalysis and that the Brønsted acid-base catalysts (Lys 166 and His 297) may be utilized as binding determinants for inhibitor recognition. In addition, structural studies on the adduct formed from the irreversible inhibition of MR by 3-hydroxypyruvate revealed that MR can form and deprotonate a Schiff-base with 3-hydroxypyruvate to yield an enol(ate)-aldehyde adduct, suggesting a possible evolutionary link between MR and the Schiff-base forming aldolases. As the archetype of the enolase superfamily, mechanistic and structural studies on MR will continue to enhance our understanding of enzyme catalysis and furnish insights into the evolution of enzyme function.
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12
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Nakatsuka H, Yamamura T, Shuto Y, Tanaka S, Yoshimura M, Kitamura M. Mechanism of Asymmetric Hydrogenation of Aromatic Ketones Catalyzed by a Combined System of Ru(π-CH2C(CH3)CH2)2(cod) and the Chiral sp(2)N/sp(3)NH Hybrid Linear N4 Ligand Ph-BINAN-H-Py. J Am Chem Soc 2015; 137:8138-49. [PMID: 26046693 DOI: 10.1021/jacs.5b02350] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The combination of a Goodwin-Lions-type chiral N4 ligand, (R)-Ph-BINAN-H-Py ((R)-3,3'-diphenyl-N(2),N(2')-bis((pyridin-2-yl)methyl)-1,1'-binaphthyl-2,2'-diamine; L), with Ru(π-CH2C(CH3)CH2)2(cod) (A) (cod = 1,5-cyclooctadiene) catalyzes the hydrogenation of acetophenone (AP) to (R)-1-phenylethanol (PE) with a high enantiomer ratio (er). Almost no Ru complex forms, with A and L remaining intact throughout the reaction while generating PE quantitatively according to [PE] = k(obs)t(2). An infinitesimal amount of reactive and unstable RuH2L (B) with C2-Λ-cis-α stereochemistry is very slowly and irreversibly generated from A by the action of H2 and L, which rapidly catalyzes the hydrogenation of AP via Noyori's donor-acceptor bifunctional mechanism. A CH-π-stabilized Si-face selective transition state, CSi, gives (R)-PE together with an intermediary Ru amide, D, which is inhibited predominantly by formation of the Ru enolate of AP. The rate-determining hydrogenolysis of D completes the cycle. The time-squared term relates both to the preliminary step before the cycle and to the cycle itself, with a highly unusual eight-order difference in the generation and turnover frequency of B. This mechanism is fully supported by a series of experiments including a detailed kinetic study, rate law analysis, simulation of t/[PE] curves with fitting to the experimental observations at the initial reaction stage, X-ray crystallographic analyses of B-related octahedral metal complexes, and Hammett plot analyses of electronically different substrates and ligands in their enantioselectivities.
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Affiliation(s)
- Hiroshi Nakatsuka
- †Graduate School of Pharmaceutical Sciences, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan
| | - Tomoya Yamamura
- †Graduate School of Pharmaceutical Sciences, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan
| | - Yoshihiro Shuto
- †Graduate School of Pharmaceutical Sciences, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan
| | - Shinji Tanaka
- †Graduate School of Pharmaceutical Sciences, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan
| | - Masahiro Yoshimura
- ‡Division of Liberal Arts and Sciences, Aichi Gakuin University, Iwasaki, Nisshin 470-0195, Japan
| | - Masato Kitamura
- †Graduate School of Pharmaceutical Sciences, Graduate School of Science, and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan
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Plata RE, Singleton DA. A case study of the mechanism of alcohol-mediated Morita Baylis-Hillman reactions. The importance of experimental observations. J Am Chem Soc 2015; 137:3811-26. [PMID: 25714789 PMCID: PMC4379969 DOI: 10.1021/ja5111392] [Citation(s) in RCA: 312] [Impact Index Per Article: 34.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2014] [Indexed: 12/12/2022]
Abstract
The mechanism of the Morita Baylis-Hillman reaction has been heavily studied in the literature, and a long series of computational studies have defined complete theoretical energy profiles in these reactions. We employ here a combination of mechanistic probes, including the observation of intermediates, the independent generation and partitioning of intermediates, thermodynamic and kinetic measurements on the main reaction and side reactions, isotopic incorporation from solvent, and kinetic isotope effects, to define the mechanism and an experimental mechanistic free-energy profile for a prototypical Morita Baylis-Hillman reaction in methanol. The results are then used to critically evaluate the ability of computations to predict the mechanism. The most notable prediction of the many computational studies, that of a proton-shuttle pathway, is refuted in favor of a simple but computationally intractable acid-base mechanism. Computational predictions vary vastly, and it is not clear that any significant accurate information that was not already apparent from experiment could have been garnered from computations. With care, entropy calculations are only a minor contributor to the larger computational error, while literature entropy-correction processes lead to absurd free-energy predictions. The computations aid in interpreting observations but fail utterly as a replacement for experiment.
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Affiliation(s)
- R. Erik Plata
- Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
| | - Daniel A. Singleton
- Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
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Abstract
The ene reaction is a pericyclic process in which an alkene having an allylic hydrogen atom (the ene donor) reacts with a second unsaturated species (the enophile) to form a new product with a transposed π-bond. The aromatic ene reaction, in which the alkene component is embedded in an aromatic ring, has only been reported in a few (four) instances and has proceeded in low yield (≤6%). Here we show efficient aromatic ene reactions in which a thermally generated aryne engages a pendant m-alkylarene substituent to produce a dearomatized isotoluene, itself another versatile but rare reactive intermediate. Our experiments were guided by computational studies that revealed structural features conducive to the aromatic ene process. We proceeded to identify a cascade comprising three reactions: (i) hexadehydro-Diels-Alder (for aryne generation), (ii) intramolecular aromatic ene, and (iii) bimolecular Alder ene. The power of this cascade is evident from the structural complexity of the final products, the considerable scope, and the overall efficiency of these multi-stage, reagent- and byproduct-free, single-pot transformations.
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15
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Chia M, Haider MA, Pollock G, Kraus GA, Neurock M, Dumesic JA. Mechanistic Insights into Ring-Opening and Decarboxylation of 2-Pyrones in Liquid Water and Tetrahydrofuran. J Am Chem Soc 2013; 135:5699-708. [DOI: 10.1021/ja312075r] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Affiliation(s)
- Mei Chia
- Department of Chemical and Biological
Engineering, University of Wisconsin—Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States
| | - M. Ali Haider
- Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way,
P.O. Box 400741, Charlottesville, Virginia 22904-4741, United States
| | - Gerald Pollock
- Department of Chemistry, Iowa State University, 2759 Gilman, Ames, Iowa 50011-3111,
United States
| | - George A. Kraus
- Department of Chemistry, Iowa State University, 2759 Gilman, Ames, Iowa 50011-3111,
United States
| | - Matthew Neurock
- Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way,
P.O. Box 400741, Charlottesville, Virginia 22904-4741, United States
| | - James A. Dumesic
- Department of Chemical and Biological
Engineering, University of Wisconsin—Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States
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Arumugam S, Popik VV. Dual Reactivity of Hydroxy- and Methoxy- Substituted o-Quinone Methides in Aqueous Solutions: Hydration versus Tautomerization. J Org Chem 2010; 75:7338-46. [DOI: 10.1021/jo101613t] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Affiliation(s)
- Selvanathan Arumugam
- Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
| | - Vladimir V. Popik
- Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
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Gut IG, Scheibler LC, Wirz J. Flash photolytic generation of two keto tautomers of 1-naphthol in aqueous solution: kinetics and equilibria of enolization. Photochem Photobiol Sci 2010; 9:901-7. [PMID: 20383354 DOI: 10.1039/c0pp00034e] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Benzo[b]cyclohexa-2,4-dien-1-one (4) and benzo[b]cyclohexa-2,5-dien-1-one (5), the two most stable keto tautomers of 1-naphthol (1), were generated in aqueous solution by Norrish Type II fission of 4- and 2-phenacyl-1-tetralone, respectively, and the pH-rate profiles of their enolization were measured by flash photolysis. Several isotopic exchange rates of 1 were measured in aqueous acid to determine the corresponding rate constants of ketonization. The resulting equilibrium constants for enolization are pKE(4) = -7.1 and pKE(5) = -6.2. The acidity constants of the carbon acids 4 and 5, pKa(4) = 2.1 and pKa(5) = 3.0, were then obtained from a thermodynamic cycle using pKa(1) = 9.25.
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Affiliation(s)
- Ivo Glynne Gut
- Centro Nacional de Analisis Genomico, Parc Cientific de Barcelona, C/Baldiri Reixac 4, 08028, Barcelona, Spain.
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Andrada DM, Michoff MEZ, Rossi RHD, Granados AM. Role of the hydrophobicity on the thermodynamic and kinetic acidity of Fischer thiocarbene complexes. Phys Chem Chem Phys 2010; 12:6616-24. [DOI: 10.1039/c000141d] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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19
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Richard JP, O’Ferrall RM. Biographical Essay: A. Jerry Kresge. ADVANCES IN PHYSICAL ORGANIC CHEMISTRY 2010. [DOI: 10.1016/s0065-3160(08)44013-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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20
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Kinetic studies of keto–enol and other tautomeric equilibria by flash photolysis. ADVANCES IN PHYSICAL ORGANIC CHEMISTRY 2010. [DOI: 10.1016/s0065-3160(08)44006-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register]
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21
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Chenoweth DM, Chenoweth K, Goddard III WA. Lancifodilactone G: Insights about an Unusually Stable Enol. J Org Chem 2008; 73:6853-6. [DOI: 10.1021/jo8012385] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- David M. Chenoweth
- Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
| | - Kimberly Chenoweth
- Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
| | - William A. Goddard III
- Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
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Cucinotta CS, Ruini A, Catellani A, Stirling A. Ab initio molecular dynamics study of the keto-enol tautomerism of acetone in solution. Chemphyschem 2007; 7:1229-34. [PMID: 16683282 DOI: 10.1002/cphc.200600007] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-transfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.
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Affiliation(s)
- Clotilde S Cucinotta
- CNR-INFM National Center on nanoStructures and bioSystems at Surfaces (S3), Italy.
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23
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Hénin F, Létinois S, Muzart J, Wirz J. Flash-photolytic generation of dienols and dienolates from α,β-unsaturated esters and kinetics of their amine-catalyzed ketonization in nonaqueous media. Photochem Photobiol Sci 2006; 5:426-31. [PMID: 16583024 DOI: 10.1039/b601749e] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Dienols of alpha,beta-unsaturated esters were generated and observed by flash photolysis and their decay rates in acetonitrile and in hexane solutions were measured in the presence of amines or aminoalcohols. In acetonitrile, 1 ratio 1 ammonium dienolate transient intermediates were observed. In hexane, no dienolate could be detected and two moles of the inductor participated in the rate-determining step for tautomerization.
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Affiliation(s)
- Françoise Hénin
- Unité Mixte de Recherche "Réactions Sélectives et Applications", CNRS Université de Reims Champagne-Ardenne, B.P. 1039, 51687, Reims Cedex 2, France.
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De Maria P, Fontana A, Gasbarri C, Siani G. The effects of cationic and zwitterionic micelles on the keto–enol interconversion of 2-phenylacetylfuran and 2-phenylacetylthiophene. Tetrahedron 2005. [DOI: 10.1016/j.tet.2005.05.035] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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25
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Hsieh YH, Weinberg N, Yang K, Kim CK, Shi Z, Wolfe S. Hydration of the carbonyl group Acetic acid catalysis in the co-operative mechanism. CAN J CHEM 2005. [DOI: 10.1139/v05-027] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In a co-operative reaction, solvent molecules, specifically water molecules, participate actively in the mechanism to circumvent the formation of charged intermediates. This paper extends our earlier theoretical treatment of the neutral co-operative hydration of acetone to include general acid catalysis by acetic acid. As before, the predominant neutral channel employs three catalytic water molecules. The principal acetic acid catalyzed channels employ one catalytic water molecule and, in approximately equal proportions, one or both oxygens of the carboxyl group. The theoretical rate constant for general acid catalysis is calculated to be 0.49 M1s1at 298 K. This compares to an estimated experimental value of 0.30 M1s1for acetic acid catalyzed hydration of acetone at 298 K in water solvent, determined by using the18O-isotope shift in the13C NMR spectrum of 2-13C-labelled acetone as a kinetic probe. It is concluded that the notion of co-operativity can be extended to include general acid catalysis of the hydration of a carbonyl group in water solvent. This creates an obvious problem for the generally accepted view that multistep ionic mechanisms are operative in the low dielectric media that exist at the active sites of hydrolytic enzymes. The relevance of this finding to the mechanisms of action of β-lactam antibiotics has been noted.Key words: hydration, reaction mechanism, co-operativity, general acid catalysis, ab initio, SCRF,18O-isotope shift.
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Ma C, Steinmetz MG, Kopatz EJ, Rathore R. Time-resolved pH jump study of photochemical cleavage and release of carboxylic acids from α-keto amides. Tetrahedron Lett 2005. [DOI: 10.1016/j.tetlet.2004.11.167] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
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27
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Bearne SL, Spiteri RJ. Reduction of intrinsic kinetic and thermodynamic barriers for enzyme-catalysed proton transfers from carbon acid substrates. J Theor Biol 2004; 233:563-71. [PMID: 15748916 DOI: 10.1016/j.jtbi.2004.11.003] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2004] [Revised: 10/19/2004] [Accepted: 11/03/2004] [Indexed: 11/23/2022]
Abstract
Many enzymes catalyse the heterolytic abstraction of the alpha-proton from a carbon acid substrate. Gerlt and Gassman have applied Marcus formalism to such proton transfer reactions to argue that transition states for concerted general acid-general base catalysed enolization at enzyme active sites occur late on the reaction coordinate (J. Am. Chem. Soc. 115 (1993) 11552). We postulate that as an enzyme evolves, it may decrease deltaG++ for a proton transfer step associated with substrate enolization by following the path of steepest descent on the two-dimensional surface corresponding to deltaG++, as defined by Marcus formalism. We show that for an enzyme that has decreased deltaG++ following the path of steepest descent, the values of the intrinsic kinetic (deltaG++(int,E)) and thermodynamic (deltaG(E)0) barriers for proton transfer reactions on the enzyme may be predicted from the known values of deltaG++(int,N) and deltaG(N)0 for the corresponding non-enzymic reaction and the free energy of activation on the enzyme (deltaG++(E)). In addition, the enzymic transition state will occur later on the reaction coordinate than the corresponding non-enzymic transition state (i.e. x++(E)>x++(N)) if the condition (6 - square root 2)/8<x++(N)<(6 + square root 2)/8 is satisfied. For enzyme-catalysed abstraction of the alpha-proton from carbon acid substrates with high pK(a) values (e.g. pK(a) approximately 29), the free energy of activation for the non-enzymic reaction (deltaG++(N)) is dominated by deltaG(N)0. Reduction of deltaG++, via the path of steepest descent will reduce deltaG0 to a greater extent (i.e. differential binding) than deltaG++(int) if deltaG(N)0>2deltaG++(int,N).
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Affiliation(s)
- Stephen L Bearne
- Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7.
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Abstract
The kinetics of base catalyzed racemization of ibuprofen enantiomers has been studied in DMSO-water mixed medium. The dynamic equilibrium rate of keto-enol tautomerism leading to racemization of ibuprofen enantiomers, is proportional to the concentrations of base catalyst and substrate. A kinetic model capable of predicting the time course of racemization, under different base and substrate concentrations, is established and experimentally verified.
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Affiliation(s)
- X Yuchun
- Institute of Chemical Metallurgy, Chinese Academy of Sciences, 100080, Beijing, People's Republic of China.
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Chiang Y, Eustace SJ, Jefferson EA, Kresge AJ, Popik VV, Xie RQ. Flash photolysis of 4-diazoisochroman-3-one in aqueous solution. Hydration of the carbene produced by loss of nitrogen and ketonization of the enol hydration product. J PHYS ORG CHEM 2000. [DOI: 10.1002/1099-1395(200008)13:8<461::aid-poc257>3.0.co;2-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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31
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Cozens FL, Kanagasabapathy VM, McClelland RA, Steenken S. Lifetimes and UV-visible absorption spectra of benzyl, phenethyl, and cumyl carbocations and corresponding vinyl cations. A laser flash photolysis study. CAN J CHEM 1999. [DOI: 10.1139/v99-210] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Benzyl (4-MeO, 4-Me, and 4-methoxy-1-naphthylmethyl), phenethyl (4-Me2N, 4-MeO, 3,4-(MeO)2, 4-Me, 3-Me, 4-F, 3-MeO, 2,6-Me2, parent, and 4-methoxy-1-naphthylethyl) and cumyl (4-Me2N, 4-MeO, 4-Me, parent) cations have been studied by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). In most cases styrene or α-methylstyrene precursors were employed for the phenethyl and cumyl ions, the intermediate being obtained by solvent protonation of the excited state. Benzyl cations were generated by photoheterolysis of trimethylammonium and chloride precursors. While a 4-MeO substituent provides sufficient stabilization to permit observation of cations in TFE, cations with less stabilizing substituents usually require the less nucleophilic HFIP. Even in this solvent, the parent benzyl cation is too short-lived (lifetime <20 ns) to be observed. When generated in HFIP, phenethyl cations can be seen to react with unphotolyzed styrene, giving rise to dimer cations that are observed to grow in as the initial phenethyl cation decays. The dimer cations, in common with the oligomer cations seen in cationic styrene polymerization, have a λmax 15-20 nm higher than the monomer and react with both solvent and styrene several orders of magnitude more slowly. This stabilization relative to the phenethyl may reflect an interaction with the aryl group present at the gamma-carbon. Cations 4-MeOC6H4C+(R)-CH3 (R = Me, Et, i-Pr, t-Bu, cyclopropyl, C6H5, 4-MeOC6H4) were generated in TFE via the photoprotonation route. The alkyl series shows that steric effects are important in the decay reaction. The cation with R = cyclopropyl is a factor of 1.5 less reactive than the cation where R = phenyl. Several vinyl cations have also been generated by photoprotonation of phenylacetylenes. ArC+=CH2 has a reactivity very similar to that of its analog ArC+H-CH3, the vinyl cation being slightly (factors of 2-5) shorter-lived. For the various series of cations, including vinyl, substituents in the aryl ring have a consistent effect on the λmax, a shift to higher wavelength relative to hydrogen of 15 nm for 4-Me, 30 nm for 4-MeO, and 50 nm for 4-Me2N.Key words: photogenerated carbocations, carbocation lifetime, styrene, photoprotonation.
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Kresge AJ, Silverman DN. Application of Marcus rate theory to proton transfer in enzyme-catalyzed reactions. Methods Enzymol 1999; 308:276-97. [PMID: 10507009 DOI: 10.1016/s0076-6879(99)08014-3] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- A J Kresge
- Department of Chemistry, University of Toronto, Ontario, Canada
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Thermodynamic and kinetic basicities of the carbon and oxygen ends of enolates. Their role in the reductive cleavage electrochemistry of α-substituted acetophenones. J Electroanal Chem (Lausanne) 1999. [DOI: 10.1016/s0022-0728(99)00358-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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34
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Capponi M, Gut IG, Hellrung B, Persy G, Wirz J. Ketonization equilibria of phenol in aqueous solution. CAN J CHEM 1999. [DOI: 10.1139/v99-048] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.
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Bearne SL, St Maurice M, Vaughan MD. An assay for mandelate racemase using high-performance liquid chromatography. Anal Biochem 1999; 269:332-6. [PMID: 10222006 DOI: 10.1006/abio.1999.4018] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mandelate racemase (EC 5.1.2.2) catalyzes the interconversion of the two stereoisomers of mandelic acid. A fixed-time assay for the quantification of mandelate racemase activity has been developed. The assay involves enzymatic conversion of R-mandelate to S-mandelate (or the reverse reaction) followed by separation and detection of the substrate and product using isocratic reversed-phase high-performance liquid chromatography on a Sumichiral OA-6100 column and absorbance detection. This method offers an economical and efficient alternative to the existing circular dichroism-based and coupled assays.
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Affiliation(s)
- S L Bearne
- Department of Biochemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada
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Hegarty AF, Dowling JP, Eustace SJ, McGarraghy M. Enolization of Aldehydes and Ketones: Structural Effects on Concerted Acid−Base Catalysis. J Am Chem Soc 1998. [DOI: 10.1021/ja9729544] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Anthony F. Hegarty
- Contribution from the Chemistry Department, University College Dublin, Belfield, Dublin 4, Ireland
| | - Joseph P. Dowling
- Contribution from the Chemistry Department, University College Dublin, Belfield, Dublin 4, Ireland
| | - Stephen J. Eustace
- Contribution from the Chemistry Department, University College Dublin, Belfield, Dublin 4, Ireland
| | - Michelle McGarraghy
- Contribution from the Chemistry Department, University College Dublin, Belfield, Dublin 4, Ireland
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Abstract
Hydrogen bonds are a key feature of chemical structure and reactivity. Recently there has been much interest in a special class of hydrogen bonds called "strong" or "low-barrier" and characterized by great strength, short distances, a low or vanishing barrier to hydrogen transfer, and distinctive features in the NMR spectrum. Although the energy of an ordinary hydrogen bond is ca 5 kcal mol-1, the strength of these hydrogen bonds may be > or = 10 kcal mol-1. The properties of these hydrogen bonds have been investigated by many experimental techniques, as well as by calculation and by correlations among those properties. Although it has been proposed that strong, short, low-barrier hydrogen bonds are important in enzymatic reactions, it is concluded that the evidence for them in small molecules and in biomolecules is inconclusive.
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Affiliation(s)
- C L Perrin
- Department of Chemistry, University of California San Diego, La Jolla 92093-0358, USA.
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Frey J, Rappoport Z. Generation and Detection of a Relatively Persistent Carboxylic Acid Enol2,2-Bis(2‘,4‘,6‘-triisopropylphenyl)ethene-1,1-diol. J Am Chem Soc 1996. [DOI: 10.1021/ja9601090] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Joseph Frey
- Contribution from the Department of Organic Chemistry, The Hebrew University, Jerusalem 91904, Israel
| | - Zvi Rappoport
- Contribution from the Department of Organic Chemistry, The Hebrew University, Jerusalem 91904, Israel
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Jefferson EA, Keeffe JR, Kresge AJ. Characterization of the indan-1-one keto–enol system. ACTA ACUST UNITED AC 1995. [DOI: 10.1039/p29950002041] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Tee OS. The Stabilization of Transition States by Cyclodextrins and other Catalysts. ADVANCES IN PHYSICAL ORGANIC CHEMISTRY 1994. [DOI: 10.1016/s0065-3160(08)60075-1] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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Andraos J, Kresge AJ, Obraztsov PA. Solvent and secondary substrate isotope effects on the acid-catalyzed ketonization of acetophenone enol in aqueous solution. J PHYS ORG CHEM 1992. [DOI: 10.1002/poc.610050607] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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Urwyler B, Wirz J. Das Tautomeriegleichgewicht zwischen Cyclopentadienyl-1-carbonsäure und Fulven-6,6-diol in wäßriger Lösung. Angew Chem Int Ed Engl 1990. [DOI: 10.1002/ange.19901020715] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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A. McClelland R, Cozens F, Steenken S. Laser flash photolysis observation of the 1-p-methoxyphenylvlnyl cation by photoprotonation of p-methoxyphenylacetylene. Comparison with the 1-p-methoxyphenethyl cation. Tetrahedron Lett 1990. [DOI: 10.1016/0040-4039(90)80157-h] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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46
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Lewis ES. Rate-equilibrium LFER characterization of transition states: The interpretation of ? J PHYS ORG CHEM 1990. [DOI: 10.1002/poc.610030102] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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