Syntheses and Reactivity of meso-Unsubstituted Azuliporphyrins Derived from 6-tert-Butyl- and 6-Phenylazulene

Abstract

A series of six azuliporphyrins with substituents on the seven-membered ring were prepared by two different "3 + 1" routes from 6-tert-butyl- and 6-phenylazulene. The substituted azulenes can be converted into dialdehydes under Vilsmeier-Haack conditions, and these react with tripyrranes in the presence of TFA in CH2Cl2 to give azuliporphyrins in excellent yields. Alternatively, tripyrrane analogues can be prepared by reacting the substituted azulenes with an acetoxymethylpyrrole in the presence of acetic acid, and following a deprotection step, these condensed with a pyrrole dialdehyde to give the related azuliporphyrins in 45-51% yield. Five of the azuliporphyrins were sufficiently soluble in CDCl3 to afford high-quality proton and carbon-13 NMR data. The internal CH and NH resonances were observed near 3 ppm, although the precise values were dependent upon substituent effects. The presence of a tert-butyl group on the azulene moiety slightly enhanced the diatropicity of the macrocycle compared to the phenyl-substituted azuliporphyrins. Polar solvents also increased the downfield shifts to the external protons by stabilizing the dipolar resonance contributors that are responsible for the carbaporphyrinoid aromatic character. A tert-butyl-substituted azuliporphyrin also gave X-ray quality crystals, and this allowed the first structural analysis of a free base azuliporphyrin to be conducted. The macrocycle is near planar, and the azulene unit was only tilted out of the plane by 7.4 degrees. An analysis of the bond lengths suggests that a 17 atom delocalization pathway significantly contributes to the aromatic properties of this system. Protonation of azuliporphyrins affords dications with enhanced diamagnetic ring currents where the internal CH shifts to ca. -3 ppm. Again, the chemical shifts are influenced by the substituents and the presence of an electron-donating tert-butyl group on the azulene subunit increases the macrocyclic diatropicity. Two of the substituted azuliporphyrins were reacted with nickel(II) acetate or palladium(II) acetate in DMF to give the corresponding organometallic derivatives, and these stable complexes were isolated in excellent yields. Addition of pyrrolidine to NMR solutions of 23-substituted azuliporphyrins 19 demonstrated that nucleophilic addition products were present in equilibrium with the parent porphyrinoids, but these adducts are less favored than for azuliporphyrins lacking the 23-substituents. Although nucleophilic attack of a peroxide anion is believed to be the first step in the conversion of azuliporphyrins to benzocarbaporphyrins with t-BuOOH and KOH, the tert-butyl or phenyl substituents in azuliporphyrins 19a and 19b did not inhibit this chemistry. Two benzocarbaporphyrin products were isolated and characterized in each case, and mechanisms are proposed to explain the origins of these oxidative ring contraction products.

Rhodium(III) Azuliporphyrins

“…This strategy has been quite successful in synthesizing tert-butyl and phenyl-substituted azuliporphyrins 5b and 5c. 16 An alternative "3 + 1" strategy was also developed, 17 and this method was applied to the synthesis of azuliporphyrins, 15−17 their heteroanalogues, 15,18 and related benzoazuliporphyrins. 19 In addition, a "2 + 2" method for preparing azuliporphyrins has been reported, 20 and a direct route to tetraaryl azuliporphyrins has been described.…”

Metalation of Carbaporphyrinoid Systems

“…[43,44] Azulene undergoes electrophilic substitution at the 1 and 3 positions, and this allows the formation of azulitripyrranes 30 when azulenes 31 are reacted with two equivalents of an acetoxymethylpyrrole 32 in the presence of acetic acid. [39,45,46] These tripyrrane analogues were generated with terminal tert-butyl ester protective groups. Following deprotection with TFA, the azulitripyrranes were condensed with pyrrole, furan, thiophene or selenophene dialdehydes to give azuliporphyrins and their heteroanalogues 33 in good yields.…”

“…Following deprotection with TFA, the azulitripyrranes were condensed with pyrrole, furan, thiophene or selenophene dialdehydes to give azuliporphyrins and their heteroanalogues 33 in good yields. [39,46] Resorcinol ( 34a) and 2-methylresorcinol (34b) were also shown to react with acetoxymethylpyrroles 35 in the presence of p-toluenesulfonic acid and calcium chloride to give tripyrrane analogues with terminal benzyl ester protective groups. [47] Following hydrogenolysis over 10% palladium-charcoal, the related dicarboxylic acids 36 were reacted with dialdehydes 37 (X = NH, O, S or Se) to give a series of rather insoluble hydroxyoxybenziporphyrins 38.…”

“…[38,53] Surprisingly, azuliporphyrins 6 with phenyl or tert-butyl substituents on position 2 3 also readily underwent ring contractions to benzocarbaporphyrins 55 (Scheme 4). [46] The results indicated that nucleophilic attack could still occur at positions 2 2 , and to a lesser extent 2 1 , and even the presence of a tert-butyl substituent could not block this process. Tetraphenylazuliporphyrins 44 derived from 6-phenyl or 6-tert-butylazulene also gave benzocarbaporphyrin products 56 upon treatment with KOHt BuOOH, although not unexpectedly these appear to arise solely from nucleophilic attack at position 2 2 of the azuliporphyrin.…”

Late transition metal complexes of oxypyriporphyrin and the platinum(II) complex of oxybenziporphyrin

“…Azuliporphyrins (e.g. 3) are dianionic ligands forming nickel(II) [16,17], palladium(II) [16][17][18][19], platinum(II) [17], ruthenium(II) [20], rhodium(III) [21] and iridium(III) [22] complexes. Oxybenziporphyrins can act as trianionic or dianionic ligands, readily affording silver(III) [10,23] and palladium(II) [24] complexes 5 and 6, respectively (Scheme 1).…”