syn-8,16-dimethylphenanthro[9,10:1′,2′][2.2]metacyclophane-1,9- diene: The first derivative of 8,16-dimethyl[2.2]metacyclophanediene failing to undergo spontaneous valence isomerization to the dihydropyrene system

Abstract

Relative stabilities of the anti and syn conformations of 9,10-di-o-tolylphenanthrene (11) were estimated by molecular mechanics calculations, the results of which helped to assign the isolated minor isomer of 13 as the syn conformer. The stereochemistry was confirmed experimentally by the successful conversion of syn-13 to the corresponding syn-thiacyclophanene 8a. Variable-temperature 1H NMR studies indicated a rigid conformation for 8a, namely IIa, with no evidence for the conversion IIa → IIb up to 150°C. A possible conformational process was, however, the tilting of the meta-bridged rings. Ring contraction of 8a also retained the syn stereochemistry and afforded only the syn isomer 19 with a pseudoaxial methylthio group. Hofmann elimination of the corresponding sulfonium salt 21 unexpectedly resulted in the isolation of the first syn-8,16-dimethyl[2.2]metacyclophanediene derivative 22, which failed to undergo valence isomerization to the dihydropyrene system thermally or photochemically under normal conditions. The significant cofacial interactions of the syn transannular rings in 22 could be illustrated readily by 1H NMR (shielded methyl protons and aryl protons of meta-bridged rings) and UV (bathochromically shifted β and p bands of meta-bridged rings) spectroscopic studies. © 1989 American Chemical Society.