Senge Mathias O., Kalisch Werner W.
Institut für Organische Chemie (WE02), Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany.
Inorg Chem. 1997 Dec 17;36(26):6103-6116. doi: 10.1021/ic970765g.
Different porphyrin conformations are believed to play a role in controlling the cofactor properties in natural tetrapyrrole-protein complexes. In order to study the correlation between macrocycle nonplanarity and physicochemical properties in detail, a series of six porphyrins with graded degrees of macrocycle distortion were synthesized via mixed condensation of pyrrole, diethylpyrrole, and benzaldehyde. The formal introduction of successively more beta-ethyl groups into the tetraphenylporphyrin parent macrocycle gave access to diethyltetraphenylporphyrin (H(2)DETPP), two regioisomers of tetraethyltetraphenylporphyrin (H(2)tTETPP, H(2)cTETPP), and hexaethyltetraphenylporphyrin (H(2)HETPP). These conformationally designed compounds bridge the gap between the well-known tetraphenylporphyrin (H(2)TPP) and the very nonplanar octaethyltetraphenylporphyrin (H(2)OETPP), which are also formed during the reaction. Crystallographic studies showed that the macrocycle distortion in the solid state increases gradually in the order TPP < DETPP < tTETPP < cTETPP < HETPP < OETPP, i.e. with increasing degree of beta-ethyl substitution and the number and localization of potential beta-ethyl meso-phenyl interactions. This correlates well with increasing bathochromic shifts of the absorption bands in solution. Depending on the substituent pattern, different saddle-shaped macrocycle conformations were observed. While the conformation of tTETPP was symmetric, DETPP, cTETPP, and HETPP showed asymmetric distortion modes with individual beta-pyrrole displacements reaching those described for dodecasubstituted porphyrins. Overall, higher displacements from planarity were found close to beta-ethyl-meso-phenyl groups whereas smaller displacements were observed in parts of the molecules bearing beta-hydrogen atoms. Nevertheless, a certain amount of redistribution of steric strain occurs as evidenced by significant displacements for pyrrole carbon atoms with beta-hydrogens. Synthesis and structural investigation of the respective metal complexes with M = Cu(II), Ni(II), and Zn(II) showed similar correlations between beta-ethyl substitution, bathochromic shift of absorption bands and nonplanarity as described for the free bases. The only exception was found for Ni(II)tTETPP, which exhibited a highly nonplanar ruffled conformation. Additionally, the metal complexes allowed a study of the conformational effects of different metals at each level of macrocycle distortion. As observed for symmetric, nonplanar porphyrins larger metals led to a decrease in conformational distortion with associated changes in bond lengths and bond angles.
不同的卟啉构象被认为在控制天然四吡咯 - 蛋白质复合物中的辅因子性质方面发挥作用。为了详细研究大环非平面性与物理化学性质之间的相关性，通过吡咯、二乙基吡咯和苯甲醛的混合缩合反应合成了一系列六种具有不同程度大环扭曲的卟啉。在四苯基卟啉母体大环中依次引入更多的β - 乙基，得到了二乙基四苯基卟啉（H₂DETPP）、四乙基四苯基卟啉的两种区域异构体（H₂tTETPP、H₂cTETPP）和六乙基四苯基卟啉（H₂HETPP）。这些构象设计的化合物填补了著名的四苯基卟啉（H₂TPP）和非常非平面的八乙基四苯基卟啉（H₂OETPP）之间的差距，这两种卟啉在反应过程中也会形成。晶体学研究表明，固态下大环扭曲按TPP < DETPP < tTETPP < cTETPP < HETPP < OETPP的顺序逐渐增加，即随着β - 乙基取代程度、潜在的β - 乙基 - 间位苯基相互作用的数量和位置增加而增加。这与溶液中吸收带的红移增加密切相关。根据取代基模式，观察到不同的马鞍形大环构象。虽然tTETPP的构象是对称的，但DETPP、cTETPP和HETPP表现出不对称的扭曲模式，单个β - 吡咯的位移达到了十二取代卟啉所描述的程度。总体而言，在靠近β - 乙基 - 间位苯基的位置发现了更大程度的偏离平面，而在带有β - 氢原子的分子部分观察到较小的位移。然而，由于带有β - 氢的吡咯碳原子有明显位移，表明存在一定程度的空间应变重新分布。对M = Cu(II)、Ni(II)和Zn(II)的相应金属配合物的合成和结构研究表明，β - 乙基取代、吸收带红移和非平面性之间的相关性与游离碱类似。唯一的例外是Ni(II)tTETPP，它呈现出高度非平面的褶皱构象。此外，金属配合物还可以研究不同金属在每个大环扭曲水平上的构象效应。正如在对称的非平面卟啉中观察到的那样，较大的金属会导致构象扭曲程度降低，同时键长和键角也会发生相关变化。
