Synthesis, X-ray crystallographic, and dynamic 1H NMR studies of crown-tetrathia[3.3.3.3]metacyclophanes - Conformational control by cooperative intramolecular C-H?π interaction both in solid state and in solution

Article abstract of DOI:10.1016/j.tet.2005.07.071

Crown-tetrathia[3.3.3.3]metacyclophanes 3a-c were synthesized via intermolcular coupling reaction in 22-30% yields. X-ray crystal analysis of 3b revealed that it adopted a perpendicular conformation (3b-B or 3b-C) in which two aromatic rings were inclined to be perpendicular to the opposite aromatic rings, driving two internal methyl groups into the π-cloud of the corresponding benzene rings. Furthermore, this perpendicular structural feature led to benzylic protons of thia-bridges being in close proximity to the adjacent aromatic rings. As a result, the induced upfield shifts for the two internal methyl protons and four benzylic protons were clearly observed in dynamic ¹H NMR spectra at low temperature, indicating that the intramolecular C-H?π interaction became increasingly important at low temperature. The energy barrier for inter-conversion between 3b-B and 3b-C was estimated to be 12.1 kcal mol^-1 by using a coalescence method. The total stabilization enthalpy of the C-H?π interactions was quantitatively calculated to be 7.9±0.8 kcal mol^-1 by the dynamic NMR spectroscopy. In contrast, 3a showed two non-interconvertible conformers at room temperature, which tended to interconvert at elevated temperature, however, many conformers co-existed at low temperature.