EXPERIMENTAL AND THEORETICAL-STUDY OF DITHIA[N]METACYCLOPHANES - SYNTHESES, CHIROPTICAL PROPERTIES, AND CONFORMATIONAL-ANALYSIS

The synthesis of 8,12-dimethyl-2,5-dithia[6]metacyclophane (3a) has been achieved for the first time by cesium-assisted high dilution methods. Enantiomeric separation was performed by HPLC with enantioselective chromatography using cellulose tris(3,5-dimethylphenyl carbamate). The CD spectra of 3a and the homologous 9,13-dimethyl-2,6-dithia[7]metacyclophan (2a) are reported and compared with results of theoretical AM1/MRD-CI calculations. The geometries of the cyclophanes have been investigated with X-ray and-theoretical AM1 and ab initio SCF methods. Application of all these methods reveals the existence of several energetic low-lying conformers with different orientation of the S-(CH2)(n)-S (n = 3 and 2) chains. Comparison of experimental and theoretical CD data shows that the spectra depend strongly on the varying dihedral angles in these chains. Electronic excitations out of the sulfur 3p lone-pair orbitals are quite important in the theoretical description of the Cotton effects above 200 nm excitation wavelength. The conversion barriers (approximate to 20 kJ/mol for 2a,40-50 kJ/mol for 3a) between the conformers have been determined by temperature dependent NMR-spectroscopy and are found to be in good agreement with theoretical AM1 data.