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A TRIR, TREPR and Computational Study on the Reactivity and Structure of the 2,2,2-Trifluoroethoxycarbonyl Radical

Authors

  • Christoph Kolano,

    1. Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitatsstrasse 150, D-44801 Bochum, Germany
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    • Current address: Physikalisch-Chemisches Institut der Universität Zürich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland.

  • Götz Bucher,

    Corresponding author
    1. Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitatsstrasse 150, D-44801 Bochum, Germany
    • To whom correspondence should be addressed: Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44801 Bochum, Germany. Fax: +49-234-321-4353; e-mail: Goetz.Bucher@rub.de, c.kolano@pci.unizh.ch

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  • Dirk Grote,

    1. Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitatsstrasse 150, D-44801 Bochum, Germany
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  • Olaf Schade,

    1. Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitatsstrasse 150, D-44801 Bochum, Germany
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  • Wolfram Sander

    1. Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, Universitatsstrasse 150, D-44801 Bochum, Germany
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  • This paper is part of a special issue dedicated to Professor J. C. (Tito) Scaiano on the occasion of his 60th birthday.

ABSTRACT

The 2,2,2-trifluoroethoxycarbonyl radical, 3b, has been generated by pulsed irradiation of 9-fluorenone oxime 2,2,2-trifluoroethyl oxalate 1b in carbon tetrachloride and acetonitrile solution. It was characterized by time-resolved electron paramagnetic resonance spectroscopy (EPR) and infrared spectroscopy. The radical has a lifetime in the range of microseconds and can be detected within the rise time of our time-resolved equipment before undergoing recombination or reactions with the solvent. No decarbonylation or decarboxylation was observed. In the presence of oxygen, the radical is quenched to yield the 2,2,2-trifluoroethoxycarbonylperoxy radical 4b, which has again a lifetime in the range of several microseconds. Time-resolved electron paramagnetic resonance spectroscopy (TREPR) allowed for the detection of a 1: 1: 1 triplet of the fluorene-9-iminyl radical 7 at g = 2.0032 and a 1: 3: 3: 1 quartet with additional hyperfine splitting (HFS) due to proton coupling at g = 2.001 for the trifluoroethoxy-carbonyl radical 3b. Calculations indicate that alkoxycarbonyl radicals can exist in conformations that are s-trans or s-cis with respect to the R-O-C(O)·dihedral. A comparison of experimental TREPR spectra with simulations indicates that the s-trans conformer is observed in the case of the ethoxycarbonyl radical, 3a. In the case of the trifluorethoxycarbonyl radical, 3b, however, the additional proton HFS observed shows that it is the s-cis conformer that is formed. As calculations give evidence for a fairly high activation enthalpy for s-cis-s-trans interconversion of alkoxycarbonyl radicals, this discrepancy is likely due to differing conformational preferences of the precursor molecules.

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