Die Strukturchemie Der 2-Chalkogeno-1,3,4,5-Tetraisopropylimidazoline

1,3,4,5-Tetraisopropy1-2-thioimidazoline (6b) was prepared by condensation of N,N1-diisopropylthiourea and isobutyroin. 1,3,4,5-Tetraisopropy1-2-oxoimidazoline (6a), 1,3,4,5-tetraisopropy1-2selenoimidazoline (6c) and 1,3,4,5-tetraisopropy1-2-telluroimidazoline (6d) were obtained from 2,3dihydro-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 H-1 and lig{H-1} NMR study of 6b in the range of 185.5 to 423.3 K revealed a temperature-dependent rotation of the isopropyl substituents Delta H-double dagger = 15.7(9) kcal mol(-1), AS(double dagger) = -5(3) cal K-1 mol(-1), AG(double dagger) = 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 Delta H-double dagger = 13.7 kcal mol(-1), AG(double dagger) = 15.4 kcal mol(-1)) is slightly lower than that about the C-C bond from Ito G.