2. 2.

Article abstract of DOI:10.1021/ja00502a031

Syntheses of methyl-substituted left bracket 2. 2 right bracket (2, 6) pyridinoparacyclophanes 15, 16, and 17, as well as their corresponding 1, 9-dienes, 19, 20, and 21, are reported. The triple-layered left bracket 2. 2 right bracket (2, 6) pyridinoparacyclophanes 23, 24, and 30 have also been prepared. The temperature-dependent NMR behavior of the simple left bracket 2. 2(2, 6) pyridinoparacyclophanes is in accord with a geometry where the pyridine ring is more or less parallel to the benzene ring, but undergoing rapid conformational flipping with an energy barrier of about 12 kcal/mol. The triple-layered cyclophane 30 shows a similar behavior, but with a slightly smaller energy barrier (11 kcal/mol). However, the apparent geometry of the corresponding 1, 9-dienes has the pyridine ring essentially perpendicular to the benzene ring. This is true also of the triple-layered cyclophane 23, where both pyridine rings are almost perpendicular to the central benzene ring. The right bracket 2. 2(2, 6) pyridinoparacyclophanes show enhanced basicity compared to simple model pyridine derivatives, whereas their 1, 9-diene analogues show greatly reduced basicity. Photoelectron spectra have been measured for a number of the left bracket 2. 2(2, 6) pyridinoparacyclophanes and their corresponding 1, 9-dienes and orbital assignments have been proposed for their lower energy ionization potentials.