Porphyrins with Exocyclic Rings. 11.1 Synthesis and Characterization of Phenanthroporphyrins, a New Class of Modified Porphyrin Chromophores

Abstract

To obtain insights into the factors that influence the electronic spectra of conjugated porphyrin systems, a series of porphyrins with fused phenanthrene subunits have been synthesized. 9-Nitrophenanthrene reacted with esters of isocyanoacetic acid in the presence of DBU in THF to give a series of phenanthro[9,10-c]pyrroles 14 in good to excellent yields, and subsequent acid-catalyzed condensation with various (acetoxymethyl)pyrroles 18 gave six examples of dipyrrylmethanes 17 that incorporate a fused phenanthrene ring. Cleavage of the benzyl esters from 17a by hydrogenolysis over 10% Pd/C gave the corresponding dicarboxylic acid 24 and this condensed with diformyldipyrrylmethanes 22 under modified MacDonald “2 + 2” condensation conditions to afford the monophenanthroporphyrins 19 and 20. Dipyrrylmethanes 17d and 17f with mixed benzyl and tert-butyl ester moieties were converted into the related formyl dipyrrylmethanecarboxylic acids 29, and subsequent head-to-tail self-condensation in the presence of p-toluenesulfonic acid yielded two examples of opp-diphenanthroporphyrins 27. Reaction of phenanthropyrroles 14 with dimethoxymethane and p-toluenesulfonic acid in acetic acid afforded the symmetrical dipyrrylmethanes 31, and following cleavage of the ester moieties and MacDonald condensation with dialdehyde 22b, the adj-diphenanthroporphyrin 30 was isolated in moderate yield. Metal chelates of the mono-, opp-di-, and adj-diphenanthroporphyrin systems were also prepared, and the electronic spectra for these modified porphyrin systems and their nickel(II), copper(II), and zinc complexes were examined. Surprisingly, the UV-vis absorptions were only slightly shifted to higher wavelengths than those for octaalkylporphyrins. Reduction of ethyl ester 14a with lithium aluminum hydride gave an unstable carbinol, and subsequent tetramerization in the presence of BF3 etherate and oxidation with DDQ afforded the tetraphenanthroporphyrin 10. The free base porphyrin was virtually insoluble in organic solvents, but protonation with TFA gave a soluble dication 10H22+ with a strong Soret band at 482 nm and two weaker absorptions at 615 and 668 nm. The bathochromic shifts for 10H22+ are far more significant than those observed for the mono- and diphenanthroporphyrin structures, although again somewhat less than might have been expected for this extraordinarily high degree of ring fusion.