Comparative Analysis of the Conformations of Symmetrically and Asymmetrically Deca- and Undecasubstituted Porphyrins Bearing Meso-Alkyl or -Aryl Groups

Abstract

Conformational analysis of highly substituted porphyrins has potential implications for modeling the behavior of macrocycles in tetrapyrrole-containing protein complexes and during catalytic reactions. In order to study the influence of different substituent patterns on the conformation of the porphyrin macrocycle, a series of metal free and nickel(II) decasubstituted porphyrins bearing aryl or ethyl groups at opposite meso positions and alkyl groups at the pyrrole positions have been synthesized and characterized by X-ray crystallography. Crystal structures of the free-base porphyrins with 5,15-diaryl substituents showed negligible out-of-plane distortion but a large amount of in-plane distortion along the 5,15-axis accompanied by large bond angle changes similar to those previously seen for related porphyrins with 5,15-dialkyl substituents. Nickel(II) complexes of the 5,15-diaryl-substituted porphyrins show planar or modestly nonplanar conformations, suggesting that these complexes are not intrinsically nonplanar, whereas a complex with 5,15-diethyl substituents has a very ruffled conformation similar to those observed for related complexes with other metals. The nickel(II) complexes are also elongated along the 5,15-axis in a qualitatively similar but less dramatic fashion than are the free-base porphyrins. Spectrosopic studies ((1)H NMR, optical, and resonance Raman spectroscopy) suggest that conformations similar to those determined by X-ray crystallography are present in solution for the 5,15-dialkyl- and 5,15-diaryl-substituted porphyrins. Several asymmetric nickel(II) and metal-free deca- and undecasubstituted porphyrins containing both aryl and alkyl meso-substituents were also investigated. Metal-free 5,15-disubstituted porphyrins with one aryl and one alkyl group showed considerably elongated porphyrin cores, whereas nickel(II) complexes of porphyrins with 5,10- or 5,10,15-substitution patterns showed very nonplanar structures consisting mainly of ruffle and saddle type distortions.

Synthesis of Porphyrins from α-Unsubstituted Dipyrromethanes

“…[1] Easily available meso-tetraarylporphyrins 1 and more related to natural β-octaalkylporphyrins 2 are the most common in practice. Some of them, like 5,10,15,20-tetraphenylporphyrin 1 (Ar = Ph) and 2,3,7,8,12,13,17,18-octaethylporphyrin 2 (Alk = Et) have symmetrically arranged substituents and are sufficiently soluble in organic solvents, unlike a more simple 2,3,7,8,12,13,17,18-octamethyporphyrin 2 (Alk = Ме) which is almost not soluble in the majority of organic solvents.…”

“…[2][3][4][5][6][7] This reaction is assumed to proceed via decarboxylation of the original dipyrromethane 3 with the subsequent formylation to give an α-formyl derivative 4 which undergoes cyclocondensation unto porphodimethene 5 with the…”

“…Easily available meso-tetraarylporphyrins 1 and more related to natural β-octaalkylporphyrins 2 are the most common in practice. Some of them, like 5,10,15,20-tetraphenylporphyrin 1 (Ar = Ph) and 2,3,7,8,12,13,17,18-octaethylporphyrin 2 (Alk = Et) have symmetrically arranged substituents and are sufficiently soluble in organic solvents, unlike a more simple 2, 3,7,8,12,13,17,18-octamethyporphyrin 2 (Alk = Ме) which is almost not soluble in the majority of organic solvents.Of interest is a simple method of synthesis of β-substituted alkylporphyrins with the general structure of 6 by condensation of α-carboxydipyrromethanes 3 (Х = СООН) with triethyl-ortho-formate in the presence of an acid (Scheme 1). …”

Hemoglobina extracelular gigante de Glossoscolex paulistus: um extraordinário sistema supramolecular hemoproteico

“…Devese, igualmente, fazer alusão ao fato de que a distorção não é única, podendo se apresentar de formas variadas e, por consequência, com propriedades peculiares correspondentes a cada uma delas. 184 Em complexos modelos, uma configuração distorcida ocorre se uma das condições seguintes é satisfeita: os ligantes axiais serem, ao mesmo tempo, fracos s-doadores e fortes p-receptores, tais como tert-butil isocianeto (BuNC) e 4-cianopiridina (4-CNPy); 163,185 os anéis porfirínicos serem fortemente S 4 -distorcidos, tais como meso-tetraisopropilporfirina, meso-tetrametilquiroporfirina e mesotetraciclo-hexilporfirina. 163 Considerando-se que o bolsão do heme e a significativa influência mecânica das cadeias polipeptídicas podem funcionar nas hemoproteínas como os impedimentos estéricos nos complexos metálicos, as mudanças conformacionais das cadeias polipeptídicas, ocasionadas pela alteração de pH, poderiam ser um fator desencadeador da mudança conformacional do anel porfirínico.…”

Distortions and Deformations of Metaled Meso-Substituted and Unsubstituted Porphyrins and Derivatives in Crystal Structures

“…However, most of attempts in order to determine characteristics have been focused on the planarity of the cycles [14] [15]. Thus, few researches had considered the substitution approaches [16] [17]. In addition, most of studies we can find have performed by considering the substituent effects on specific experimental phenomena and generally electronic structure [2] [8] [18] [19].…”