The synthesis of the first naphthannelated dimethyldihydropyrene and chemical shift - bond order correlations
Date
1989
Authors
Venugopalan, Santhanagopalan
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Abstract
The synthesis of the title compound, trans-14c,14d-dimethyl-14c,14d-dihydrodibenzo[de,qr]naphthacene, 25, has been achieved in 9 steps from 2,3-dibromonaphthalene in an overall yield of 3.9%. This represents the first synthesis of a naphthannelated DMDHP, with 25 being the more stable isomer compared with the corresponding cyclophanediene 28. An improved correlation has been devised for predicting the chemical shift(s) of the internal methyl protons of various DMDHP's based on the change in RE when a delocalized aromatic moiety is fused onto a DMDHP with the change in shielding of the DMDHP's methyl protons. The anisotropy introduced by the mode of annelation can also be taken into account.
The X-ray structural determination of the teraryl 31 clearly demonstrates for the first time the existence of syn- and anti-conformers in the teraryl series, which had been postulated from solution studies. The barrier to rotation in the teraryls 31 and 43 have been determined by 13 c NMR and 1H NMR, respectively, and found to be slightly higher than that for the corresponding benzo-analogues. Naphtho[e]DMDHP, 25, is found to be photochromic like its benzo-analogue 15. Though not overly sensitive to oxygen, 25 undergoes decomposition during prolonged exposure to UV light. The effect that (eJ-annelation has on the photochromic behaviour (as opposed to (a]-annelation) has been noted, with implications for use in switch-type devices. The photoisomerization of 28 to 25 has been studied by UV-VIS spectroscopy and the activation barrier determined to be very similar to that observed in the case of the benzo-analogue 15.