Global versus Local Aromaticity in Porphyrinoid Macrocycles:Experimental and Theoretical Study of 'Imidacene', an Imidazole-Containing Analogue of Porphycene

摘要

The synthesis spectroscopic properdties and omputational analysis of an imidazole-based analysis of an imidazole-based analogue of porphycene are described The marocycle given the trivial name "imidacene" was prepared by reductive coupling of a diformyl-substituted 2,2'-biimidazole using low-valent titanium followed by treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone Imidacene displays a porphyring-like electronic struc ture as judged by its ~1H NMR ,~13CNMR and UV/Vis spectral characteristics Despite a cyclic 18 pi-electron pathway dichlormethane or ethyl acetate solutions of imidaene were found dto under go rapid gedcomposition even in the abse4nce of light and air A series fo high-level theoretical calcualations per formed to probe the origin of thsi instability revealed that the presence of a delocalized 18 pi-electron pathway in both imoidcene and porphycene provides less aromiatic stabilization energy than locally aromatic 6 pi-electron heterocylcless in their reduced counterparts That reductin of imidacene occurs aon perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds thereby disstininguishing it from porphycen and more generally fromporphycene and more generally from porphyrin Despite the presence of dboth 18 pi- and 22 pi-electron pathways in the planar reduced form of imidacene aromaticity is evident only in the 6pi-electron five-mem-bered rings Our computatinal analyisis predicts that routine ~1H NMR spectroscopy can be used ot distringuish between loal and global aromaticity in planar porphyrinoid maccrocycles the difference in the chemical shift for the internal NH protone is expected to bwe on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.