SYNTHESIS AND CONFORMATIONAL-ANALYSIS OF 3,3,12,12-TETRAMETHYL[4.4]PARACYCLOPHANE-2,13-DIONE

Three [4.4]paracyclophanes (2-4) were synthesized by radical cyclization using the photoinduced S(RN)1 reaction. The structure of the most stable conformer of 4 was ascertained by means of X-ray crystallographic analysis, molecular mechanics calculations, and variable-temperature NMR spectra. The energy barriers to rotation of the two benzene rings in these compounds were obtained from an analysis of the temperature-dependent signals due to the 1,3-dioxolene and aromatic ring protons. Molecular mechanics calculations predicted three stable conformers for 4. Actually, three sets of signals were observed in its low-temperature H-1 NMR spectrum. An experimental technique was used in which crystals were dissolved in precooled solvent. In this manner, NMR signals were recorded due to the single conformer found in its crystalline state, which are identical to the major set of signals present in the original spectrum. The structures due to the other two sets of signals of 4 can;be successfully assigned by comparison of the low temperature H-1 NMR spectra of the two compounds (2 and 3). The interconversion pathways among these conformers were characterized.