The Structural Chemistry of the 2-Chalcogeno-1,3,4,5-tetraisopropylimidazolines

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.