Conjugated Macrocycles Related to the Porphyrins. 12.1 Oxybenzi- and Oxypyriporphyrins: Aromaticity and Conjugation in Highly Modified Porphyrinoid Structures

Abstract

The “3 + 1” route for porphyrinoid synthesis is an excellent methodology for preparing aromatic porphyrin analogues with six-membered ring subunits. Condensation of 5-formylsalicylaldehyde with tripyrranes 15, 24, and 25 in the presence of 5% TFA-dichloromethane, followed by neutralization and oxidation with DDQ, afforded a series of semiquinone-containing porphyrinoids 12, 26, and 27 in 35−52% yield. In these novel systems, the macrocycle achieves aromatization by undergoing a keto−enol tautomerization whereby the phenolic subunit is transformed so that the inner three carbon atoms become part of the 18 π-electron aromatic core, whereas the outer carbons generate an enone unit. The aromatic nature of these “oxybenziporphyrins” is evident from their porphyrin-like electronic spectra and the presence of powerful diamagnetic ring currents in their proton NMR spectra, where the inner CH is shifted upfield to δ = −7 ppm, whereas the external meso-protons appear downfield between 8.8 and 10.3 ppm. The presence of a carbonyl unit is confirmed by IR and proton NMR spectroscopy. Addition of trace amounts of TFA give an aromatic monocation, but further protonation to the dication leads to the loss of macrocyclic aromaticity. Modified tripyrranes 15b, 26, and 27 were used to prepare oxybenziporphyrins with fused benzene, phenanthrene, and acenaphthylene ring systems, the former via a tetrahydrobenzo intermediate; and these compounds showed many of the same characteristics, including aromatic free bases and nonaromatic dications. The ring fusion gave rise to gradual bathochromic shifts from benzo- (23) to phenanthro- (26) to acenaphtho- (27) oxybenziporphyrins, which qualitatively followed the same trends observed for true porphyrins, although the influence of the acenaphthylene ring system was somewhat muted in this series. Condensation of isophthalaldehyde with tripyrrane 15a gave the nonaromatic analogue “benziporphyrin” in 28% yield, and this species was thoroughly characterized and contrasted to oxybenziporphyrin 12a. Reaction of 3-hydroxypyridine-2,6-dicarboxaldehyde with tripyrranes under the “3 + 1” conditions afforded exceptionally high yields of the corresponding azaoxybenziporphyrins (named “oxypyriporphyrins”; 35, 47−49), and these structures also exhibited porphyrinoid aromaticity. For this series, the dications retained their aromatic character as did the related nickel(II), copper(II), and zinc chelates. Ring fusion effects were investigated for the oxypyriporphyrins and their metal complexes, and again the major absorptions shifted to higher wavelength from benzo- to phenanthro- to acenaphthooxypyriporphyrins. However the effect of metalation on the oxypyriporphyrin chromophore differed considerably from the trends seen for metalloporphyrins. These results show that novel aromatic porphyrinoids are easily accessible via the “3 + 1” approach, and this work will facilitate in-depth studies on the chemical and physical properties of these exciting new bridged annulene structures.