Conformational analysis of 9,18-difluoro-2,11-diaza[3.3]metacyclophane

The conformational isomers (syn and anti) of 9,18-difluoro-2,11-diaza[3.3]m etacyclophane 1, were isolated at ambient temperature and each was identifi ed by F-19 MMR spectra. The anti isomer gradually converted to the syn isom er in solution, and kinetic measurements of this conversion afforded the Ar rhenius activation energy, E-a = 24.8 +/- 0.9 kcal/mol, in acetonitrile. Th e difference in thermodynamic stability of the syn and anti isomers in DMSO was estimated to be ca. 2 kcal/mol (298 K) on the basis of the variable te mperature NMR method. The three conformational isomers, boat-boat, chair-bo at, and chair-chair, are present in the syn isomer in solution. The relativ e stability order of the three isomers, boat-boat, chair-boat, chair-chair, is estimated by the H-1- and F-19 NMR spectra. The boat-boat isomer is pre dominant at low temperatures, but the ratio of the other two isomers, chair -boat and chair-chair, is gradually increased as the temperature is raised.