Z. Naturforsch. 2014, 69b, 1384 – 1394
The Structural Chemistry of the 2-Chalcogeno-1,3,4,5-tetraisopropylimidazolines
Die Strukturchemie der 2-Chalkogeno-1,3,4,5-tetraisopropylimidazoline
Martin Göhner1, Florian Herrmann1, Klaus Eichele1, Holger F. Bettinger2, Markus Ströbele1, and Norbert Kuhn1
1 Institut für Anorganische Chemie der Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany
2 Institut für Organische Chemie der Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany
Reprint requests to Prof. Dr. N. Kuhn. E-mail: norbert.kuhn@uni-tuebingen.de
Received August 13, 2014 / published online December 19, 2014
Professor Hubert Schmidbaur zum 80. Geburtstag gewidmet
1,3,4,5-Tetraisopropyl-2-thioimidazoline (6b) was prepared by condensation of N,N′-diisopropylthiourea and isobutyroin. 1,3,4,5-Tetraisopropyl-2-oxoimidazoline (6a), 1,3,4,5-tetraisopropyl-2-selenoimidazoline (6c) and 1,3,4,5-tetraisopropyl-2-telluroimidazoline (6d) were obtained from 2,3-dihydro-1,3,4,5-tetraisopropylimidazol-2-ylidene and dinitrogen oxide or selenium and tellurium, respectively. The crystal structure analyses revealed the presence of the A-type rotamer for 6a while for 6c a paddlewheel-like orientation of the isopropyl substituents was found (I type). In crystals of 6b, both A- and E-type molecules are present while in solution at room temperature the I-type rotamer is detected. A dynamic 1H and 13C{1H} NMR study of 6b in the range of 185.5 to 423.3 K revealed a temperature-dependent rotation of the isopropyl substituents (ΔH = 15.7(9) kcal mol−1, ΔS =− 5(3) cal K−1 mol−1, ΔG = 17.2(9) kcal mol−1) with I as the rotamer of lowest energy. This result is confirmed by MO calculations which indicate A and C being the next stable rotamers. The barrier of rotation of the isopropyl substituents about the C–N bond from I to C (B3LYP ΔH =13.7  kcal mol−1, ΔG =15.4  kcal mol−1) is slightly lower than that about the C–C bond from I to G.
Key words: Heterocycles, Synthesis, Crystal Structures, NMR, MO Calculations
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