Use of bridge annelation to retain the syn stereochemistry of [m.n]metacyclophane derivatives: A route to an elusive syn-[2.2]metacyclophanediene

Abstract

Free-radical bromination of 9,10-di-m-tolylphenanthrene (6) afforded the anti and syn conformers of the bis(bromomethyl) derivative 7 with the barrier of interconversion estimated at 90 kJ mol-1. Sodium sulfide coupling of 7 surprisingly yielded only the syn-thiacyclophanene 8b. The anti isomer 8a is believed to be less favorable due to steric interaction between the protons at C1 and C8 and those at C7′ and C13′. The bridge annelation has induced a sufficiently high energy barrier to restrict the isomerization of the syn isomer to the anti conformation either by rotation of the aryl rings or direct flipping of the cyclophane. The syn stereochemistry was clearly retained in the respective products from Wittig rearrangement of syn-isomer 8b and Raney nickel desulfurization of syn-isomer 12-the first examples of such reactions not to result in the anti isomer as a major product. A comparison of the chemical shifts of Hi protons in the syn-isomers 4, 1b, 8b, and 14b clearly illustrates the anisotropic effect of the annelated bridge. A Hofmann elimination of the sulfonium salt derived from syn-isomer 12 afforded only the known phenanthropyrene 18. It is believed that the syn-cyclophanediene 24 formed was not sufficiently stable and tautomerized to the cis-dihydropyrene 25, which oxidized readily to the polycyclic aromatic hydrocarbon 18. © 1988 American Chemical Society.