An experimental estimation of aromaticity relative to that of benzene. The synthesis and NMR properties of a series of highly annelated dimethyldihydropyrenes: Bridged benzannulenes

Article abstract of DOI:10.1021/ja00110a008

The synthesis of 13 trans-dimethyldihydropyrenes (bridged [14]annulenes) fused to one or more benzene, naphthalene, phenanthrene, phenalene, or quinoxaline rings and 6 cis-dihydropyrene derivatives from benzenoid precursors using either a thiacyclophane route or an electrocyclic addition of a furan to an annulyne followed by deoxygenation is reported. Their ¹H NMR spectra are studied in detail to obtain correlations between ³J_H,H coupling constants and the internal methyl proton chemical shifts and also between the latter and the more distant external annulene ring proton shifts. These linear correlations are then used to obtain a relationship between the relative aromaticity of benzene and the fused ring in question, such that the aromaticity of the fused ring can be estimated relative to that of a benzene ring simply from a measurement of chemical shift in the fused annulene.