Extreme Projection of a Proton into the π-Cloud of an Aromatic Ring: Record Shielding of an Aromatic Proton in trans-10b-Methyl-10c-(1-naphthyl)-10b,10c-dihydropyrene

Abstract

A synthetic sequence involving dithiametacyclophane → metacyclophanediene → dihydropyrene was employed to prepare trans-10b-methyl-10c-(2-naphthyl)- and trans-10b-methyl-10c-(1-naphthyl)-10b, 10c-dihydropyrene 5 and 6, respectively. Both exhibit a strong diamagnetic ring current despite the introduction of an internal bulky substituent within the π-electron cloud. Their electronic spectra suggest interaction between the two near-perpendicular naphthyl and dihydropyrenyl π systems, resulting in red shift and band broadening. All naphthyl protons are well resolved in their 1H NMR spectra due to a strong shielding effect of the dihydropyrene ring. The most shielded protons in 5 and 6 are H1′ and H2′ at δ 2.47 and 1.42, respectively, being 5.25 and 5.95 ppm shifted from those of reference protons. There is evidence for free rotation on the NMR time scale of the 2-naphthyl ring in 5 with a preference for a particular conformer, whereas the 1-naphthyl ring in 6 is conformationally rigid with its H2′ projecting deeply into the π-cloud, thus accounting for the most shielded aromatic proton (H2′ in 6) reported to date.