This review addresses nitration reactions of porphyrin derivatives. Simple porphyrins modifications afford valuable intermediates in this area of chemistry. They are useful materials for further transformations, as the NO2 group introduced into parent porphyrin system increases its electrophilic character, thus allowing a broad spectrum of subsequent reactions, e. g. reduction of NO2 to NH2 , subsequent diazotisation, nucleophilic substitution of hydrogen (in ortho-position to NO2 ), a variety of cyclizations, etc. Such reactions are often utilized in the first steps of the designed syntheses, leading to attractive and useful target porphyrin molecules. This approach (via nitro-derivatives) allows synthesizing numerous porphyrin-like compounds of a high degree of complexity, and has thus become one of the methods choice. The substitution by NO2 group can take place at all positions of the porphyrin systems: four meso-positions (5, 10, 15, 20) and eight positions β (2, 3, 7, 8, 12, 13, 17, 18). The third possibility includes the nitration in meso-aryl rings attached to positions 5, 10, 15, and 20. The latter derivatives (meso-aryl substituted ones) are a large, well-known group of synthetic porphyrins. The nitration reactions described herein follow three various mechanisms: (a) radical, (b) via π-cation radicals and π-dications, and (c) electrophilic. All the above cases are discussed in detail. According to our knowledge, this is the first such systematic account concerning these reactions.
Keywords: Porphyrins; electrophilic species; nitration; radicals; π-cation radicals.
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