Nitration of Porphyrin Systems: A Toolbox of Synthetic Methods

Abstract

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.

Studies on Synthesis of Porphyrins – β‐Substituted with Multiple Nitrogen‐containing Groups

“…The calculated value (m/z = 848. 19) was in good agreement with the observed one; and theoretical and experimental isotope patterns for this ion ([M + H] + , C 48 H 33 N 8 O 4 Cu) were found to be identical.…”

“…[19]). In contrast, nitration of the respective porphyrin complexes, under similar conditions, led to β ‐nitro‐derivatives [19,20–22] …”

“…In contrast, nitration of the respective porphyrin complexes, under similar conditions, led to β-nitro-derivatives. [19,[20][21][22] Herein, the investigations were targeted at the preparation of β,β-dinitro-substituted porphyrinates. In our previous studies on the synthesis of higher nitro-substituted polynitro-porphyr-ins we have observed formation of even five possible dinitroisomers.…”

“…It should be mentioned that electrophilic nitration of free-base porphyrins, carried out in HNO 3 /CHCl 3 system, usually results in the formation of products bearing NO 2 groups in meso-aryl rings [11][12][13][14][15][16][17][18] (for review -see ref. [19]). In contrast, nitration of the respective porphyrin complexes, under similar conditions, led to β-nitro-derivatives.…”

Organic Nitrating Reagents

“…16 In addition, several reviews cover the synthesis of nitro compounds of distinct structures, such as aliphatic 17 and aromatic 18 19 compounds, alkenes, 20 alkynes, 21 and porphyrins. 22 These reports provide a comprehensive understanding of synthetic tools currently available in the field. However, despite the almost 200-year history of nitration reactions, 23 there is still room for improvement.…”