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

Abstract

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.