- +: A Masked Potent Boron Lewis Acid
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The chemistry of the boron cation has been revitalized in the past decade due to its newfound application in stoichiometric and catalytic organic reactions. To extend the frontier of boron cation catalysis, we came to discover that a mesityl-substituted η5-Cp*-coordinated boron cation could serve as a powerful Lewis acid for organic catalytic transformations. The boron cation [Cp*B-Mes][B(C6F5)4] ([1][B(C6F5)4]) stabilized in a boronium-like electronic structure binds to Et3PO readily and displays an acceptor number exceeding that of B(C6F5)3 on the Gutmann-Beckett acidity scale. The steric and electronic stabilization exerted by the electron-donating Cp? renders the highly Lewis acidic boron cation an easy-to-handle catalyst for hydrodeoxygenation of aryl ketones at ambient temperature. The exceptional catalytic performance of [1]+ implies that the incorporation of a coordinatively flexible substituent at boron is critical in bringing catalytic activity and stability to boron cation catalysts.
- Tseng, Hsi-Ching,Shen, Chao-Tang,Matsumoto, Kentaro,Shih, Ding-Nan,Liu, Yi-Hung,Peng, Shie-Ming,Yamaguchi, Shigehiro,Lin, Ya-Fan,Chiu, Ching-Wen
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supporting information
p. 4516 - 4521
(2019/11/14)
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- Terminal Alkenes from Acrylic Acid Derivatives via Non-Oxidative Enzymatic Decarboxylation by Ferulic Acid Decarboxylases
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Fungal ferulic acid decarboxylases (FDCs) belong to the UbiD-family of enzymes and catalyse the reversible (de)carboxylation of cinnamic acid derivatives through the use of a prenylated flavin cofactor. The latter is synthesised by the flavin prenyltransferase UbiX. Herein, we demonstrate the applicability of FDC/UbiX expressing cells for both isolated enzyme and whole-cell biocatalysis. FDCs exhibit high activity with total turnover numbers (TTN) of up to 55000 and turnover frequency (TOF) of up to 370 min?1. Co-solvent compatibility studies revealed FDC's tolerance to some organic solvents up 20 % v/v. Using the in-vitro (de)carboxylase activity of holo-FDC as well as whole-cell biocatalysts, we performed a substrate profiling study of three FDCs, providing insights into structural determinants of activity. FDCs display broad substrate tolerance towards a wide range of acrylic acid derivatives bearing (hetero)cyclic or olefinic substituents at C3 affording conversions of up to >99 %. The synthetic utility of FDCs was demonstrated by a preparative-scale decarboxylation.
- Aleku, Godwin A.,Prause, Christoph,Bradshaw-Allen, Ruth T.,Plasch, Katharina,Glueck, Silvia M.,Bailey, Samuel S.,Payne, Karl A. P.,Parker, David A.,Faber, Kurt,Leys, David
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p. 3736 - 3745
(2018/08/03)
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- Reaction discovery using acetylene gas as the chemical feedstock accelerated by the stop-flow micro-tubing reactor system
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Acetylene gas has been applied as a feedstock under transition-metal catalysis and photo-redox conditions to produce important chemicals including terminal alkynes, fulvenes, and fluorinated styrene compounds. The reaction discovery process was accelerated through the use of stop-flow micro-tubing reactors. This reactor prototype was developed by joining elements from both continuous micro-flow and conventional batch reactors, which was convenient and effective for gas/liquid reaction screening. Notably, the developed transformations were either inefficient or unsuccessful in conventional batch reactors. Its success relies on the unique advantages provided by this stop-flow micro-tubing reactor system.
- Xue, Fei,Deng, Hongping,Xue, Chengwen,Mohamed, Dara Khairunnisa Binte,Tang, Karen Yuanting,Wu, Jie
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p. 3623 - 3627
(2017/07/11)
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- New cofactor supports α,β-unsaturated acid decarboxylation via 1,3-dipolar cycloaddition
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The bacterial ubiD and ubiX or the homologous fungal fdc1 and pad1 genes have been implicated in the non-oxidative reversible decarboxylation of aromatic substrates, and play a pivotal role in bacterial ubiquinone (also known as coenzyme Q) biosynthesis or microbial biodegradation of aromatic compounds, respectively. Despite biochemical studies on individual gene products, the composition and cofactor requirement of the enzyme responsible for in vivo decarboxylase activity remained unclear. Here we show that Fdc1 is solely responsible for the reversible decarboxylase activity, and that it requires a new type of cofactor: a prenylated flavin synthesized by the associated UbiX/Pad1. Atomic resolution crystal structures reveal that two distinct isomers of the oxidized cofactor can be observed, an isoalloxazine N5-iminium adduct and a N5 secondary ketimine species with markedly altered ring structure, both having azomethine ylide character. Substrate binding positions the dipolarophile enoic acid group directly above the azomethine ylide group. The structure of a covalent inhibitor-cofactor adduct suggests that 1,3-dipolar cycloaddition chemistry supports reversible decarboxylation in these enzymes. Although 1,3-dipolar cycloaddition is commonly used in organic chemistry, we propose that this presents the first example, to our knowledge, of an enzymatic 1,3-dipolar cycloaddition reaction. Our model for Fdc1/UbiD catalysis offers new routes in alkene hydrocarbon production or aryl (de)carboxylation.
- Payne, Karl A. P.,White, Mark D.,Fisher, Karl,Khara, Basile,Bailey, Samuel S.,Parker, David,Rattray, Nicholas J.W.,Trivedi, Drupad K.,Goodacre, Royston,Beveridge, Rebecca,Barran, Perdita,Rigby, Stephen E. J.,Scrutton, Nigel S.,Hay, Sam,Leys, David
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supporting information
p. 497 - 501
(2015/07/02)
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- A transition-metal-free Heck-type reaction between alkenes and alkyl iodides enabled by light in water
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A transition-metal-free coupling protocol between various alkenes and non-activated alkyl iodides has been developed by using photoenergy in water for the first time. Under UV irradiation and basic aqueous conditions, various alkenes efficiently couple with a wide range of non-activated alkyl iodides. A tentative mechanism, which involves an atom transfer radical addition process, for the coupling is proposed.
- Liu, Wenbo,Li, Lu,Chen, Zhengwang,Li, Chao-Jun
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supporting information
p. 6170 - 6174
(2015/06/08)
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- Ruthenium-catalyzed synthesis of β-oxo esters in aqueous medium: Scope and limitations
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The ability of the hydrosoluble ruthenium(ii) complexes [RuCl 2(η6-arene)(PTA)] 3a-d, [RuCl2(η 6-arene)(PTA-Bn)] 4a-d, [RuCl2(η6-arene) (DAPTA)] 5a-d, [RuCl2(η6-arene)(TPPMS)] 6a-d (arene = C6H6, p-cymene, 1,3,5-C6H3Me 3, C6Me6) to promote the atom-economic formation of β-oxo esters, by addition of carboxylic acids to terminal propargylic alcohols in water has been explored. Scope, limitations and catalyst recycling have been evaluated using the most active catalyst [RuCl 2(η6-C6H6)(TPPMS)], 6a.
- Cadierno, Victorio,Francos, Javier,Gimeno, Jose
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experimental part
p. 135 - 143
(2010/05/18)
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- Stannylated polynorbornenes as new reagents for a clean stille reaction
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New functionalized polynorbornenes have been obtained in good yields by vinylic copolymerization of norbornene with a (norbornenyl)Sn-Bu2Cl monomer, catalyzed by [Ni(C6F5)2(SbPh 3)2]. Subsequent functionalization produces a wide variety of polymers with different -SnBu2R groups (R = aryl, vinyl, alkynyl). The polymers can be used as R-transfer reagents in Stille couplings, thereby providing easy workup and separation of the polymeric tin byproducts from the coupling products. Tin contents of around 0.05 wt% are found in the Stille products. The stannylated polymers can be recycled and reused with good efficiency.
- Carrera, Nora,Gutierrez, Enrique,Benavente, Rut,Villavieja, M. Mar,Albeniz, Ana C.,Espinet, Pablo
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scheme or table
p. 10141 - 10148
(2009/11/30)
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- Snapshots of a Stille reaction
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The main sequential intermediates involved in a real catalytic cycle of the Stille reaction (the coupling of ROTf with CH2=CHSnBu3 catalyzed by [PdR(OTf)(dppe)]; R = aryl) are observed and characterized unequivocally before the coupling product is released.
- Casado,Espinet,Gallego,Martinez-Ilarduya
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p. 339 - 340
(2007/10/03)
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- Polyfluoroaralkylamines: an improved synthesis of 4,5,6,7-tetrafluoroindole
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Tetrafluoroindole (5), a precursor for potential biologically-active compounds, was prepared previously in a four-step synthesis from C6F6.However, catalytic reduction of pentafluorophenylacetonitrile (2) to 2-pentafluorophenylethyl amine (3) is accompanied by a significant amount of a secondary amine, which, like 3, undergoes cyclization to an indoline and subsequent dehydrogenation to a new indole 8.The side-reaction in the reduction of 2 to 3 is obviated by the use of LiAlH4/AlCl3 (1:1).The final aromatization to yield 5 is vastly improved by replacing MnO2 with DDQ. - Keywords: Polyfluoroaralkylamines; Synthesis; Tetrafluoroindole; NMR spectroscopy; IR spectroscopy
- Filler, Robert,Chen, Wei,Woods, Sarah M.
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- Laser-Induced Reactions of Hexafluorobenzene and Selected Hydride Compounds
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Infrared-laser-induced reactions between C6F6 and general hydrides R-H (R = H, D, CH3, HCC, H2C=CH, and Cl) were studied by irradiating C6F6 at 1027 cm-1 in C6F6/R-H mixtures.In general, two competitive pathways involving C-F bond cleavage in C6F6 were observed as follows: (1) C6F6 + R-H C6F5H + R-F and (2) C6F6 + R-H C6F5R + HF.C6F6 decomposition also took place to a minor extent depending on the mole fraction of C6F6 and gave rise to C2F4 and C2H2.From infrared and GC/MS analysis of the product mixtures after 20-200 pulses, C6F5H was observed in all reactions except that involving D2.When D2 was used C6F5D was the major product.C6F5H was the major product in the reactions involving H2 and C2H2.In the reaction with C2H4, C6F5H was the major product derived from C6F6 though C2H2 was the major product of the reaction.The large amount of C2H2 seems to be derived from an additional sensitized decomposition of C2H4.C6F5H was present in minor amounts in the reaction with CH4 and HCl.Besides C6F5H, other monosubstituted products derived from C6F6 were also formed, generally within 20-100 pulses.Thus, C6F5CH3, C6F5CH=CH2, C6F5CCH, and C6F5Cl were produced, respectively, in the reaction of C6F6 with CH4, C2H4, C2H2, and HCl.In the first and last cases these products were the major ones observed.The results are discussed mechanistically in terms of the initial formation of the C6F5. radical and synthetically in terms of the utility of obtaining selective-laser-induced reduction of C6F6.
- Koga, Yoshinori,Chen, Ruth,Keehn, Philip M.
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p. 306 - 311
(2007/10/02)
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- Isomerization of 1-Butene Catalyzed by (η6-Arene)NiR2. A Very Active Homogeneous Catalyst System
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(η6-Arene)NiR2 (R = SiCl3, SiF3, C6F5) complexes exhibit very high catalytic activity for the isomerization of 1-butene, especially in bromobenzene solution.The isomerization is generally first order in both 1-butene and Ni complex.The catalytic activity decreases in the order SiCl3 > SiF3 > C6F5.The reaction rate in toluene solution increases with increasing 1-butene concentration below a ratio of 1-C4H8/Ni = 100 but decreases at higher ratios apparently due to induced decomposition of the Ni complex.However, the rate is not adversely affected by high 1-C4H8 concentration in bromobenzene solution.The ratio of cis-2-butene to the trans isomer is also dependent on solvent, suggesting that an important step in the mechanism is the involvement of solvent in the catalytic cycle.The reaction of ethylene with (η6-arene)Ni(SiCl3)2 gives vinyltrichlorosilane and ethyltrichlorosilane.A mechanism is proposed which involves the intermediacy of nickel hydride species generated by the insertion of 1-butene into the Ni-R bond followed by β-hydride elimination.
- Kanai, Hiroyoshi,Choe, Seok Burm,Klabunde, Kenneth J.
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p. 2019 - 2023
(2007/10/02)
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