Synthesis, Reactivity and Structural Chemistry of 5,10,15,20-Tetraalkylporphyrins

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Abstract
The synthesis, reactivity and full characterization of a series of meso-tetraalkyl porphyrins and metalloporphyrins with R ≡ n-butyl (6), 2-methyl-propyl (7), isopropyl (8), l-ethyl-propyl (10) and tert-butyl (11) groups are reported. Derivatives of the last of these show considerably bathochromically shifted absorption bands and the crystal structure of Zn ( II )11(pyr) exhibits a severely ruffled macrocycle conformation. Systematic crystallographic studies of the porphyrins showed that the free base porphyrins with R ≡ n-butyl ( H 26), 2-methyl-propyl ( H 27) and 1-ethyl-propyl ( H 210) are planar. A larger conformational variety was found for the metal complexes. While most Ni ( II ) derivatives and Pd ( II )8 showed a ruffled macrocycle conformation with a degree of ruffling exceeding that of meso-unsubstituted porphyrins, both planar and non-planar forms were found for the related Cu ( II ) derivatives. The Zn ( II ) complexes of porphyrins with isopropyl or 1-ethyl-propyl exhibited conformations with variable degree of distortion. Together with comparative structures from the literature, this study provides experimental evidence that considerable conformational flexibility exists for meso-alkylporphyrins with substituents less bulky than tert-butyl groups.