CONFORMATIONAL STUDY OF (R)-(-)-2,2,2-TRIFLUORO-1-(9-ANTHRYL)ETHANOL (PIRKLE ALCOHOL) BY DYNAMIC NMR

Observation of anisochronous H-1 and C-13 NMR signals in (R)-(-)-2,2,2-trifluoro-1-(9-anthryl)ethanol at low temperature indicates that restricted rotation around the C(sp2)-C(sp3) bond occurs. From the coalescence temperature data and the corresponding chemical shift difference, the free energy of activation for rotation was evaluated to be 14.5 kcal mol-1 at 320 K in deuteriochloroform. These results, together with MM2 calculations, indicate that the ground-state conformation is that in which the trifluoromethyl group is almost orthogonal to the anthracene ring. The transition state will correspond then to the conformation in which the CF3 group eclipses the aromatic nucleus. Complete H-1 and C-13 NMR assignments of the system at the frozen ground state (340 K) were made by homo- and heteronuclear COSY experiments and NOE difference spectroscopy.