Reaction Pathway and Free Energy Profile for Conversion of π?Conjugation Modes in Porphyrin Isomer

摘要

Porphycene is a structural isomer of porphyrinnwith 18π-conjugated aromatic character. Porphycene modifiednwith trifluoromethyl (CF3) groups in the periphery of thenframework readily affords the isolable 20π-conjugatednantiaromatic form through a reaction with a proton-donatingnreductant. The 20π-conjugated form can be characterized bynnot only a variety of spectroscopies in solutions but also X-rayncrystallography. This paper focuses on the free energy profilenin the conversion of the 18π-conjugated porphycene into the 20π-conjugated form. From the results of kinetics, electrochemicalnmeasurements, and acid/base titrations, the 20π-conjugated CF3 porphycene is formed by a concerted proton−electron transfern(CPET) from a hydroquinone reagent to the 18π-conjugated form. The hydrogen-atom affinity of the 18π-conjugated CF3nporphycene (for two hydrogen atoms) was calculated to be −490 kJ mol−1, indicating that the N−H bonds in the 20π-nconjugated form are rather easily cleaved. This reflects the antiaromatic characteristics of the 20π-conjugated porphycene. Wenpropose that the kinetic and thermochemical analysis using redox potentials and pKa data is applicable for determining thenreaction pathway in conversion of aromatic/antiaromatic mode of π-conjugated macrocycles as well as popular investigations fornoxidations of organic molecules.