A rational synthesis of 23-methylcarbaporphyrin has been developed. 3,4-Diethyl-1-methylpyrrole reacted with acetoxymethylpyrrole under acidic conditions to give N-methyltripyrrane, and following cleavage of the ester protective groups, the tripyrrolic intermediate condensed with an indene dialdehyde in the presence of trifluoroacetic acid to afford the required N-methylcarbaporphyrin. Reaction with palladium(II) acetate in refluxing acetonitrile for short time periods gave a 23-methyl palladium(II) complex but prolonged reaction times afforded a rearranged 21-methyl product. The 23methyl complex can be isolated but gradually converts into the 21-methyl derivative even in the solid state. 21-Methyl-and 22-methylcarbaporphyrins reacted with [Rh(CO) 2 Cl] 2 to give rhodium(I) complexes. However, when the 23-methylcarbaporphyrin was reacted under the same conditions, a rhodium-(III) derivative was isolated. This complex incorporates a bridging methylene unit between the Rh(III) and the 21-carbon and must therefore be formed by a methyl group migration−cyclization process.
MacDonald-type "3 + 1" condensations of an N-methyltripyrrane with a series of dialdehydes afforded a matched set of N-methylporphyrins, Nmethylheteroporphyrins, N-methyloxybenziporphyrin, N-methyloxypyriporphyrin, N-methyltropiporphyrin, and a N-methylcarbaporphyrin aldehyde. meso-Unsubstituted heteroporphyrins have been little explored previously, and this strategy was also used to prepare N-unsubstituted 21-oxa-, 21-thia-, and 21-selenaporphyrins. In every case, the N-methylporphyrinoids exhibited weaker, bathochromically shifted UV−Vis absorptions compared to their core unsubstituted congeners. However, proton NMR spectroscopy demonstrated that these derivatives retained strong diamagnetic ring currents and the presence of the internal alkyl substituents had little effect on the global aromatic characteristics. Nevertheless, the UV−Vis spectra of N-methyl-oxybenzi-and N-methyl-oxypyriporphyrins were dramatically altered and gave greatly weakened absorptions. N-Methyl-oxybenzi-and N-methyltropiporphyrins reacted with palladium(II) acetate to give stable palladium(II) complexes, demonstrating that N-alkylation alters the metalation properties for these carbaporphyrinoids. The organometallic derivatives also retained strongly aromatic properties, and the proton NMR spectra showed the N-methyl resonances near −3 ppm. N-Methylcarbaporphyrin-2-carbaldehyde also gave a palladium(II) complex, but this gradually rearranged at higher temperatures to afford a C-methyl complex. The results demonstrate that core alkylation of porphyrinoids greatly alters the reactivity and spectroscopic properties for these systems.
The macrocyclic cavities in carbaporphyrins are well suited for the formation of metalated derivatives. A carbaporphyrin diester and a naphthocarbaporphyrin reacted with [Rh(CO) 2 Cl] 2 to give good-to-excellent yields of rhodium(I) complexes, and these were fully characterized by X-ray crystallography. Both rhodium(I) derivatives were converted into rhodium-(III) complexes in refluxing pyridine, albeit in moderate yields. Carbachlorins also formed rhodium(I) complexes, but these could not be further transformed into rhodium(III) products. The rhodium(III) complexes incorporate two axial pyridine ligands, which exhibit strongly shielded resonances in their 1 H NMR spectra, and the rhodium(III) carbaporphyrin diester was further characterized by X-ray crystallography. adj-Dicarbaporphyrins also formed rhodium(I) complexes, but these reactions involved the relocation of a proton to generate an internal methylene unit. The environments associated with the two faces of the resulting macrocycles are very different from one another, and this results in the 1 H NMR chemical shifts for the two internal methylene protons being separated by well over 3 ppm. Although the diatropicities of rhodium(I) complexes for monocarbaporphyrins and carbachlorins are comparable to those of the parent ligands, the chemical shifts for rhodium(I) dicarbaporphyrins are consistent with a significant reduction in the porphyrinoid aromaticity. A dicarbachlorin also gave a rhodium(I) complex, but this species fully retained the diatropic characteristics of the parent ligand. Nevertheless, the internal CH 2 unit still gave two widely separated doublets indicative of radically differing environments for the two faces of the macrocycle. Rhodium(I) dicarbaporphyrin and dicarbachlorin complexes were further characterized by X-ray crystallography.
Examples of internally alkylated azuliporphyrins were prepared by MacDonald-type “3 + 1” condensations. 2-Methyl- and 2-ethylazulene reacted with an acetoxymethylpyrrole in the presence of an acid catalyst to give azulitripyrranes. Following cleavage of the terminal protective groups, condensation with a diformylpyrrole in the presence of hydrochloric acid and oxidation with ferric chloride afforded 21-alkylazuliporphyrins. An azulene dialdehyde similarly reacted with an [Formula: see text]-methyltripyrrane to generate a 23-methylazuliporphyrin. The products could only be isolated in protonated form and the free-base internally alkylated azuliporphyrins proved to be unstable. Nevertheless, the dications are highly diatropic and the internal alkyl group resonances were shifted upfield to beyond -3 ppm. Reaction of a 23-methylazuliporphyrin with palladium(II) acetate primarily afforded a palladium(II) complex with loss of the internal methyl substituent. However, two palladium(II) benzocarbaporphyrins were also identified that were formed by sequential oxidative ring contraction and methyl group migration. Internally alkylated azuliporphyrins provide new insights into the reactivity of the system and the results show that the introduction of alkyl substituents within porphyrinoid cavities greatly modifies the properties of these structures.
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