Synthesis and diatropicity of a trans-10c-methyl-10b,10c-dihydropyrene-containing cyclophane: A novel aromatic molecule with a (1,3)cyclophane within the π-cloud of a [14]annulene

Abstract

1,4-Bis(bromomethyl)-2,6-dimethylbenzene (17) was obtained by direct bromomethylation of α-bromomesitylene and used for the preparation of 14,18-dimethyl-2,11-dithia[3.3](1,3)(1,4)-cyclophane (19). Desulfurization of 19 to afford 12,16-dimethyl[2.2](1,4)cyclophane (22) could be achieved via two routes. Selective bromination of the methyl groups in 22 followed by coupling with 2,6-bis(mercaptomethyl)toluene led to the double-layered anti-dithiacyclophane 26. Ring contraction via a Wittig rearrangement-Hofmann elimination sequence gave the double-layered cyclophanediene 14 which underwent valence isomerization to afford the title compound 15. A diatropicity study of 15 suggests that it retains 99% of the ring current of the parent system 1. Protons of the methylene group directly attached to the dihydropyrene moiety in 15 are, however, shifted about 1 ppm downfield compared with those of the corresponding methylene protons in related dihydropyrenes with internal butyl and but-4-enyl groups. This is attributed to a deviation of the methylene carbon concerned in 15 from the central axis of delocalization of the aromatic π-cloud in the dihydropyrene.