CONFORMATIONAL-ANALYSIS BY THE RING CURRENT METHOD - THE STRUCTURE OF 2,2,13,13-TETRAMETHYL[4.4]METACYCLOPHANE

The conformational analysis of 2,2,13,13-tetramethyl[4.4] metacyclophane (4) using X-ray crystallographic analysis, molecular mechanics calculations, and the ring current method is presented. Compound 4 shows a characteristic temperature-dependent chemical shift behavior in its H-1-NMR spectrum. Three conformers are found to play an important role in the conformational dynamic process. The method for analysis employed consists of three stages, generation of plausible structures by two programs (MMRS and MM3), estimation of the chemical shift of aromatic protons in these structures by the ring current effect, and selection of the most plausible structures by comparison of the observed and calculated chemical shifts. The C-s symmetric structure found in the crystal is the main contributor in solution below -80 degrees C. Above this temperature another structure is found to be predominant, in which the conformation of one of the bridging chains is different from that in the C-s symmetric structure.