Extreme Projection of a Proton into the -Cloud of an Aromatic Ring: Record Shielding of an Aromatic Proton in trans-10b-M
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- 作者：Yuhua Ting and Yee-Hing Lai
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：January 28, 2004
- 年：2004
- 卷：126
- 期：3
- 页码：909 - 914
- 全文大小：143K
- 年卷期：v.126,no.3(January 28, 2004)
- ISSN：1520-5126
文摘
A synthetic sequence involving dithiametacyclophane 
metacyclophanediene dihydropyrenewas 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 theintroduction of an internal bulky substituent within the 
-electron cloud. Their electronic spectra suggestinteraction between the two near-perpendicular naphthyl and dihydropyrenyl systems, resulting in redshift and band broadening. All naphthyl protons are well resolved in their 1H NMR spectra due to a strongshielding effect of the dihydropyrene ring. The most shielded protons in 5 and 6 are H1' and H2' at 
2.47and 1.42, respectively, being 5.25 and 5.95 ppm shifted from those of reference protons. There is evidencefor 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 -cloud,thus accounting for the most shielded aromatic proton (H2' in 6) reported to date.
metacyclophanediene dihydropyrenewas 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 theintroduction of an internal bulky substituent within the -electron cloud. Their electronic spectra suggestinteraction between the two near-perpendicular naphthyl and dihydropyrenyl systems, resulting in redshift and band broadening. All naphthyl protons are well resolved in their 1H NMR spectra due to a strongshielding effect of the dihydropyrene ring. The most shielded protons in 5 and 6 are H1' and H2' at 2.47and 1.42, respectively, being 5.25 and 5.95 ppm shifted from those of reference protons. There is evidencefor 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 -cloud,thus accounting for the most shielded aromatic proton (H2' in 6) reported to date.