Electronic effects of peripheral substituents at porphyrin meso positions.
Porphyrins are stable molecules with a macrocyclic conjugated system and often peripheral substituents. This unique structure makes the electronic properties of the four meso-carbons (the methine bridges) nearly identical. Replacement of the weakly electron-polarizing 2,4-vinyl groups of protoporphyrin IX with strongly electron-polarizing acetyl groups not only leads to much lower meso-carbon reactivities toward electrophilic aromatic substitution but also results in a significant meso-selectivity (the beta- and gamma-meso-positions become much more nucleophilic (basic) than the alpha- and delta-meso-positions). To further investigate the relationship between the porphyrin meso-carbon reactivities and the peripheral substituents, two monoacetylporphyrin analogues also were synthesized. This investigation not only leads to empirical rules for predicting porphyrin meso-carbon selectivities but also provides important models for theoretical calculations of porphyrin aromaticity.
Zhu
Yaoqiu
Y
Department of Chemistry, Center for Drug Discovery and Chemical Biology, Northwestern University, Evanston, Illinois 60208-3113, USA.
Silverman
Richard B
RB
D004583:Electrons; D006207:Half-Life; D006418:Heme / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*
D011166:Porphyrins; D006418:Heme