Tobias A Nigst

Ludwig-Maximilian-University of Munich, München, Bavaria, Germany

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Publications (13)58.15 Total impact

  • Raman Tandon, Tobias A. Nigst, Hendrik Zipse
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    ABSTRACT: The influence of the length of alkyl substituents on various kinetic, thermodynamic, and spectroscopic properties of 4-(dialkylamino)pyridines has been determined by using a combination of experimental and theoretical methods. The chain-length dependence of these properties has subsequently been analyzed by using a quantitative model for through-bond inductive effects.
    Annalen der Chemie und Pharmacie 08/2013; 2013(24). · 3.10 Impact Factor
  • Tobias A. Nigst, Herbert Mayr
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    ABSTRACT: The kinetics of the reactions of carbanions, carboxylate anions, and primary and secondary amines with acylating agents were studied in acetonitrile and dimethyl sulfoxide (DMSO) under first-order conditions by UV/Vis spectrophotometry and conductimetry. Whereas reactions of a large variety of nucleophiles with ordinary carbanions and Michael acceptors generally follow the linear free energy relationship lg k2 (20 °C) = sN(N + E), separate correlation lines for different types of nucleophiles were found when (lg k2)/sN was plotted against N for the reactions with acyl derivatives 2–5. Even when nucleophilic attack is rate determining, the reactivities of acylating agents towards nucleophiles cannot be described by a single set of nucleophile-independent electrophilicity parameters.
    Annalen der Chemie und Pharmacie 04/2013; 2013(11). · 3.10 Impact Factor
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    ABSTRACT: New heterocyclic derivatives of 9-azajulolidine have been synthesized and characterized with respect to their nucleophilicity and Lewis basicity. The Lewis basicity of these bases as quantified through their theoretically calculated methyl-cation affinities correlate well with the experimentally measured reaction rates for addition to benzhydryl cations. All newly synthesized pyridines show exceptional catalytic activities in benchmark acylation reactions, which correlate only poorly with Lewis basicity or nucleophilicity parameters. A combination of Lewis basicity with charge and geometric parameters in the framework of a three-component quantitative structure-activity relationship (QSAR) model is, however, highly predictive.
    Chemistry - A European Journal 03/2013; · 5.93 Impact Factor
  • Tobias A Nigst, Anna Antipova, Herbert Mayr
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    ABSTRACT: The kinetics of the reactions of amines, hydrazines, hydrazides, and hydroxylamines with benzhydrylium ions and quinone methides were studied in acetonitrile and water by UV-vis spectroscopy, using conventional spectrometers and stopped-flow and laser-flash techniques. From the second-order rate constants k(2) of these reactions, the nucleophilicity parameters N and s(N) were determined according to the linear free energy relationship log k(2) = s(N)(N + E). While methyl groups increase the reactivities of the α-position of hydrazines, they decrease the reactivities of the β-position. Despite the 10(2) times lower reactivities of amines and hydrazines in water than in acetonitrile, the relative reactivities of differently substituted amines and hydrazines are almost identical in the two solvents. In both solvents hydrazine has a reactivity similar to that of methylamine. This observation implies that replacement of one hydrogen in ammonia by Me increases the nucleophilicity more than introduction of an amino group, if one takes into account that hydrazine has two reactive centers. Plots of log k(2) versus the corresponding equilibrium constants (log K) or Brønsted basicities (pK(aH)) do not show enhanced nucleophilicities (α-effect) for either hydrazines or hydroxylamine relative to alkylamines.
    The Journal of Organic Chemistry 09/2012; 77(18):8142-55. · 4.56 Impact Factor
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    ABSTRACT: A Lewis acid-triggered zincation allows the regioselective metalation of various chromones and quinolones. In the absence of MgCl(2), a C(3) zincation is observed, whereas in the presence of MgCl(2) or a related Lewis acid, C(2) zincation occurs. Applications to a natural flavone, isoflavone, and quinolone are shown.
    Journal of the American Chemical Society 08/2012; 134(33):13584-7. · 10.68 Impact Factor
  • Tobias A Nigst, Johannes Ammer, Herbert Mayr
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    ABSTRACT: Laser flash irradiation of substituted N-benzhydryl pyridinium salts yields benzhydryl cations (diarylcarbenium ions) and/or benzhydryl radicals (diarylmethyl radicals). The use of 3,4,5-triamino-substituted pyridines as photoleaving groups allowed us to employ the third harmonic of a Nd/YAG laser (355 nm) for the photogeneration of benzhydryl cations. In this way, benzhydryl cations can also be photogenerated in the presence of aromatic compounds and in solvents which are opaque at the wavelength of the quadrupled Nd/YAG laser (266 nm). To demonstrate the scope and limitations of this method, the rate constants for the bimolecular reactions of benzhydryl cations with several substituted pyridines were determined in acetonitrile and with water in acetone. The obtained data agree with results obtained by stopped-flow UV-vis spectroscopic measurements. The rate constants for the reaction of the 4,4'-bis[methyl(2,2,2-trifluoroethyl)amino]benzhydrylium ion with 4-(dimethylamino)pyridine were also determined in dimethyl sulfoxide, N,N-dimethylformamide, and acetone. From the second-order rate constants, we derived the nucleophilicity parameters N and s(N) for the substituted pyridines, as defined by the linear free energy relationship, log k(2) = s(N)(N + E).
    The Journal of Physical Chemistry A 07/2012; 116(33):8494-9. · 2.77 Impact Factor
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    ABSTRACT: The structural motif within a series of tetrahydropyrimidine-based isothioureas necessary for generating high asymmetric induction in the asymmetric Steglich rearrangement of oxazolyl carbonates is fully explored, with crossover and dynamic (19)F NMR experiments used to develop a mechanistic understanding of this transformation.
    Chemistry - A European Journal 02/2012; 18(8):2398-408. · 5.93 Impact Factor
  • Tobias A. Nigst, Johannes Ammer, Herbert Mayr
    Angewandte Chemie 02/2012; 124(6).
  • Tobias A Nigst, Johannes Ammer, Herbert Mayr
    Angewandte Chemie International Edition 02/2012; 51(6):1353-6. · 11.34 Impact Factor
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    ABSTRACT: Rate and equilibrium constants for the reactions of a series of isothioureas with benzhydrylium ions have been measured photometrically. The data were employed to determine the nucleophilicities and nucleofugalities of isothioureas and compare them with those of other organocatalysts.
    The Journal of Organic Chemistry 06/2011; 76(12):5104-12. · 4.56 Impact Factor
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    ABSTRACT: The kinetics of the reactions of primary and secondary amines with benzhydrylium ions and quinone methides in acetonitrile have been studied under pseudo-first-order conditions (high excess of amines) by UV/Vis spectrophotometry. Generally, the plots of kobs versus amine concentration were linear, in line with a second-order rate law. However, for some reactions of secondary amines with quinone methides, the plots of kobs versus amine concentration showed an upward curvature, which indicates rate-limiting deprotonation of the initially formed adduct by a second molecule of amine. From the second-order rate constants k2 for the attack on the electrophiles by the amines, the nucleophilicity parameters N and s for the amines were determined from the linear free energy relationship log k2 (20 °C) = s(N + E). The rates of the reactions of the amines with benzhydrylium ions are strongly affected by solvent polarity, in sharp contrast to the analogous reactions of other neutral nucleophiles.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
    Annalen der Chemie und Pharmacie 11/2009; 2009(36):6379 - 6385. · 3.10 Impact Factor
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    ABSTRACT: The second-order rate constants of the reactions of alkyl-substituted pyrroles with a series of benzhydrylium ions were determined in acetonitrile, and the reaction products were fully characterized by NMR spectroscopy and mass spectrometry. The formation of the σ adducts is the rate-limiting step of these reactions. Because the second-order rate constants correlate linearly with the electrophilicity parameters of the benzhydrylium ions, the determination of the nucleophilicity parameters N and s according to the linear free energy relationship log k2 (20 °C) = s(N + E) was achieved. With these findings, a direct comparison of the nucleophilic reactivities of these π-excessive heterocycles with other nucleophiles became possible, and the pyrroles were integrated into the comprehensive scale of nucleophilicity, covering a range of 8–9 orders of magnitude from N-(triisopropylsilyl)pyrrole (N = 3.12), the weakest nucleophile of this series, to kryptopyrrole (3-ethyl-2,4-dimethylpyrrole, N = 11.63). Thus, highly reactive pyrroles show similar nucleophilic reactivities as enamines, whereas those of less-reactive pyrroles are comparable to allylsilanes or indoles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
    Annalen der Chemie und Pharmacie 03/2008; 2008(14):2369 - 2374. · 3.10 Impact Factor
  • European Journal of Organic Chemistry - EUR J ORG CHEM. 01/2008; 2008(14):2343-2343.