- Elucidating the Role of the Boronic Esters in the Suzuki-Miyaura Reaction: Structural, Kinetic, and Computational Investigations
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The Suzuki-Miyaura reaction is the most practiced palladium-catalyzed, cross-coupling reaction because of its broad applicability, low toxicity of the metal (B), and the wide variety of commercially available boron substrates. A wide variety of boronic acids and esters, each with different properties, have been developed for this process. Despite the popularity of the Suzuki-Miyaura reaction, the precise manner in which the organic fragment is transferred from boron to palladium has remained elusive for these reagents. Herein, we report the observation and characterization of pretransmetalation intermediates generated from a variety of commonly employed boronic esters. The ability to confirm the intermediacy of pretransmetalation intermediates provided the opportunity to clarify mechanistic aspects of the transfer of the organic moiety from boron to palladium in the key transmetalation step. A series of structural, kinetic, and computational investigations revealed that boronic esters can transmetalate directly without prior hydrolysis. Furthermore, depending on the boronic ester employed, significant rate enhancements for the transfer of the B-aryl groups were observed. Overall, two critical features were identified that enable the transfer of the organic fragment from boron to palladium: (1) the ability to create an empty coordination site on the palladium atom and (2) the nucleophilic character of the ipso carbon bound to boron. Both of these features ultimately relate to the electron density of the oxygen atoms in the boronic ester.
- Thomas, Andy A.,Zahrt, Andrew F.,Delaney, Connor P.,Denmark, Scott E.
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supporting information
p. 4401 - 4416
(2018/04/05)
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- PROCESS FOR STRAIGHTENING KERATIN FIBRES WITH A HEATING MEANS AND DENATURING AGENTS
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The invention relates to a process for straightening keratin fibres, comprising: (i) a step in which a straightening composition containing at least two denaturing agents is applied to the keratin fibres, (ii) a step in which the temperature of the keratin fibres is raised, using a heating means, to a temperature of between 110 and 250° C.
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- Photoreduction of o-benzoquinones in the presence of p-bromo-N,N- dimethylaniline
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Photoreduction of o-benzoquinones in the presence of p-bromo-N,N- dimethylaniline under irradiation (λ, > 500 nm) affords the corresponding pyrocatechols and hydroxyphenyl ethers. The latter are unstable and, in turn, decompose in the dark reaction to pyrocatechols. The ratio between pyrocatechol and hydroxyphenyl ether formed upon the photoreaction is determined by the structure of o-quinone, namely, the presence and bulk of substituents in positions 3 and 6 of the ring. The yield of pyrocatechol is maximal (60-65%) if the substituents are the same (H and H, But and But) or insignificantly differ (Pri and But), regardless of its bulk.
- Chesnokov,Cherkasov,Abakumov,Kurskii,Shurygina,Mamysheva,Shavyrin
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p. 718 - 724
(2007/10/03)
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- Total synthesis and biological evaluation of the nakijiquinones
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The Her-2/Neu receptor tyrosine kinase is vastly overexpressed in about 30% of primary breast, ovary, and gastric carcinomas. The nakijiquinones are the only naturally occurring inhibitors of this important oncogene, and structural analogues of the nakijiquinones may display inhibitory properties toward other receptor tyrosine kinases involved in cell signaling and proliferation. Here, we describe the first enantioselective synthesis of the nakijiquinones. Key elements of the synthesis are (i) the reductive alkylation of a Wieland - Mieschertype enone with a tetramethoxyaryl bromide, (ii) the oxidative conversion of the aryl ring into a p-quinoid system, (iii) the regioselective saponification of one of the two vinylogous esters incorporated therein, and (iv) the selective introduction of different amino acids via nucleophilic conversion of the remaining vinylogous ester into the corresponding vinylogous amide. The correct stereochemistry and substitution patterns are completed by conversion of two keto groups into a methyl group and an endocyclic olefin via olefination/reduction and olefination/isomerization sequences, respectively. This synthesis route also gave access to analogues of nakijiquinone C with inverted configuration at C-2 or with an exocyclic instead of an endocyclic double bond. Investigation of the kinase-inhibiting properties of the synthesized derivatives revealed that the C-2 epimer 30 of nakijiquinone C is a potent and selective inhibitor of the KDR receptor, a receptor tyrosine kinase involved in tumor angiogenesis. Molecular modeling studies based on the crystal structure of KDR and a model of the ATP binding site built from a crystal structure of FGF-R revealed an insight into the structural basis for the difference in activity between the natural product nakijiquinone C and the C-2 epimer 30.
- Stahl,Kissau,Mazitschek,Huwe,Furet,Giannis,Waldmann
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p. 11586 - 11593
(2007/10/03)
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- Oxidations with Lead Tetraacetate. II. Oxidations of 2,2-Disubstituted 1,3-Benzodioxoles
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The oxidation of 2,2-disubstituted 1,3-benzodioxoles by lead tetraacetate proceeds readily to give 5-acetoxy and 5,6-dione derivatives as main products. 5,6-Diacetoxy compuonds and 5-carboxy derivatives are found in some instances as minor products.Oxidation of benzodioxoles substituted at the 5 and 6 positions with methoxyl groups or with a second dioxole ring results in oxidative loss of the dioxole ring.Relative effects of these substitutions on reactivity have been evaluated.The mechanism of ο-quinone formation is discussed and is postulated to occur via a tetraacetoxy intermediate.The 5-acetoxy derivatives and the ο-quinones, by hydrolysis and reduction respectively, serve as sources of 5-hydroxy and 5,6-dihydroxy benzodioxoles.
- Cole, Edward R.,Crank, George,Minh, H. T. Hai
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p. 527 - 543
(2007/10/02)
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- The Electron Spin Resonance Spectra of Semiquinones obtained from Some Naturally Occurring Methoxybenzoquinones
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Radical anions of methoxyquinones and related compounds were generated in a static system in alkaline media.The unpaired electron distribution in these radicals could not be satisfactorily verified by simplified SCF calculations.It is shown that a simple relationship exists between splittings in semiquinones and corresponding splittings in the closely related alkyl aryl ether radical cations.The relationship correlates very closely with the exsess charge effect which has been examined quantitatively for aromatic hydrocarbon radical ions, indicating that the same effect is operative in the oxygenated radicals, in which the splittings of the cations are ca. 20percent greater than corresponding splittings in the anions.These correlations, together with observed smooth variations of splitting patterns with substitution have permitted unambiguous assignment of the coupling constants of radicals such as the fumigatin anion or the 1,2-methylenedioxynaphthalene cation, without recourse to new computation.
- Holton, Dolores M.,Murphy, David
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p. 1757 - 1760
(2007/10/02)
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