THE SYNTHESES AND PROPERTIES OF A NAPHTHO-ANNELATED AND A PYRENO-ANNELATED DIHYDROPYRENE

Abstract

A general synthetic approach to ortho-diaryl benzenoids employing o-quinone derivatives was designed. lntramolecular couplings of two appropriately functionalized ortho-diaryl benzenoids gave the respective anti-thiacyclophanenes. Two derivatives of trans-10b, 10c-dimethyl-10b, 10c-dihydropyrene, namely trans 14d, 14e-dimethylnaphtho[1,2-e]-14d, 14e-dihydropyrene and trans-16d, 16e dimethylpyreno[4,5-e]-16d, 1 fe-dihydropyrene, were then synthesized from their corresponding anti-thiacyclophanenes via a Wittig rearrangement-Hofmann elimination sequence. The naphtha derivative was unstable and highly reactive thus no pure sample of it could be isolated. The pyreno derivative was relatively more stable and found to be in equilibrium with its cyclophanediene valence isomer, the latter being the major isomer. Both the naphthodihydropyrene and pyrenodihydropyrene prepared are strongly diatropic with their internal methyl protons appearing at o -2.6 and -2.94, respectively in their 1H NMR spectra, retaining about 68 and 75% of ring current of the parent dihydropyrene system. Rotational isomerization between the syn and anti conformers of two 1,2 diarylnaphthalenes was investigated by dynamic 1H NMR spectroscopy. The conformational barriers were estimated at 75-82 kJ mol 1. These values suggest that rotational isomerization in 1,2-diarylnaphthalenes proceeds selectively via rotation of the 2-aryl ring in the transition states. A buttressing effect consistent with an increasing order of steric demand of cylindrical cyano group and spherical chlorine atom was also observed. The rate of thermal isomerization of the corresponding cyclophanediene to the pyrenodihydropyrene was too slow to be estimated accurately. This is attributed to the higher geometric rigidity in the pyrene moiety which results in severe "bay region" steric interactions when the pyrenodihydropyrene achieves planarity. A more readily conversion could be observed photochemically by irradiation of the cyclophanediene with light at 254 nm. The characteristic absorption bands at 503 and 427 nm are bathochromically shifted from those of the parent dihydropyrene consistent with extended conjugation. An empirical method correlating the Jiatropicity of an annulene and its x-SCF bond order deviation was used to predict the methyl proton chemical shifts of the naphthodihydropyrene and pyrenodihydropyrene, respectively. The calculated value for the former agrees well with but that of the latter differs significantly from observed experimental values. The relative diatropicities of 10b, 10c-dimethyl-10b, 10c-dihydropyrene and five of its [e]-annelated derivatives were compared based on a discussion on the effect of benzannelation. A linear relationship was observed between the diatropldty of dihydropyrene and the change in resonance energy of the annelating benzenoid system among five of these dihydropyrenes with the exception of the pyreno derivative exhibiting an unexpectedly low diatropicity. Parameters used for the empirical treatments may not have fully taken into account an effect due to the 14? peripheral character of pyrene, i.e. the effect of annulenoannelation. Secondly, the severe "bay region" steric interactions in the pyreno derivative result in a severe twisting of two adjacent ?-orbitals at the annelating bond thus decreasing the degree of delocalization in the dihydropyrene moiety. The relative diatropicities of another set of six dihydropyrene systems were compared based on a discussion on the effect of conjugation. A linear relationship was observed between the diatropicity of dihydropyrene and resonance energy of the benzenoid system in conjugation among four of these dihydropyrene systems. Both the naphtho and pyreno derivatives deviate significantly from the plot. Such a correlation seems to apply only to annelated dihydropyrene systems in which the annelating bond could participate freely in the delocalization of the dihydropyrene macroring.