Conjugated Macrocycles Related to the Porphyrins. Part 16. Synthesis of Hexa- and Heptaalkyl-Substituted Inverted or N-Confused Porphyrins by the “3 + 1” Methodology

Pyrrole-2,4-dicarboxaldehydes 13a − c were condensed with a tripyrrane dicarboxylic acid 12 in 1% TFA−CH2Cl2 to give, following oxidation with 0.1% FeCl3, a series of “etio-type” N-confused porphyrins 14a − c. Excellent yields were obtained when the diformylpyrrole possessed a 5-alkyl substituent, although the products were most easily isolated in the form of their hydrochloride salts. Addition of TFA to the free base inverted porphyrins led to initial protonation onto the external nitrogen atom to produce monocations 19 and further acidification afforded a dications 21. UV−vis spectroscopy showed strong Soret bands for 14, 19, and 21, features that are associated with fully aromatic porphyrinoid systems, and proton NMR spectroscopy confirmed the presence of powerful diamagnetic ring current where the meso-protons resonated near 10 ppm while the interior CH was strongly shielded to −6 ppm for the free base structures and −4 ppm for the dications 21. Taking into account the effect of the introduction of delocalized positive charges within the protonated species, the diatropic character of the macrocyclic system appeared to be slightly decreased upon protonation. Proton NMR spectroscopy demonstrated that a single tautomer was favored for the free base porphyrinoids 14 and furthermore NH exchange was slow on the NMR time scale even at room temperature. While this is not a factor for the dications, deuterium exchange was noted by NMR spectroscopy for the interior CH upon addition of TFA-d, indicating that C-protonated species are present in equilibrium with 21 and/or 19. Addition of a series of mineral acids (HF, HCl, HBr, HI, and HNO3) to solutions of 14 produced dications with significantly different UV−vis and NMR spectra, and these data indicate that strong ion pairing is occurring at the external NH. Reaction of 14 with nickel(II) acetate in DMF at 145 °C afforded a relatively unstable nickel complex 25. Proton NMR spectra for 25 indicated that the macrocyclic ring current was considerably reduced, as would be expected for a cross-conjugated structure of this type. However, addition of TFA resulted in an unprecedented C-protonation that produced a new nickel(II)-containing aromatic species 29 that slowly demetalated to form the dication 21. The new results demonstrate that the alkyl-substituted inverted porphyrin series has many unique properties and the improved synthetic procedures will facilitate future investigations.