NH Tautomerism of N-Confused Porphyrin: Solvent/Substituent Effects and Isomerization Mechanism

Tomoya Ishizuka, Ryuichi Sakashita, Osamu Iwanaga, Tatsuki Morimoto, Shigeki Mori, Masatoshi Ishida, Motoki Toganoh, Kiyonori Takegoshi, Atsuhiro Osuka, Hiroyuki Furuta

Research output: Contribution to journalArticle

Abstract

The effects of substituents and solvents on the NH tautomerism of N-confused porphyrin (2) were investigated. The structures, electronic states, and aromaticity of NH tautomers (2-2H and 2-3H) were studied by absorption and nuclear magnetic resonance (1H, 13C, and 15N) spectroscopies, single-crystal X-ray diffraction analysis, and theoretical calculations. The relative stability of the tautomers is highly affected by solvents, with the 3H-type tautomer being more stable in nonpolar solvents, while the 2H-type tautomer being highly stabilized in polar solvents with high donor numbers such as N,N-dimethylformamide (DMF), pyridine, and acetone. Electron-withdrawing groups on the meso-aryl substituents as well as the methyl group at the ortho position also stabilize the 2H-type tautomer. Kinetically, the tautomerism rate is significantly influenced by solvent and concentration, and a particularly large activation entropy (ΔS⧧) is obtained in pyridine. The first-order deuterium isotope effect on the reaction rates of NH tautomerism (kH/kD) is determined to be 2.4 at 298 K. On the basis of kinetic data, the mechanism of isomerization is identified as an intramolecular process, including the rotation of the confused pyrrole in pyridine/chloroform and DMF/chloroform mixed solvent systems, and as a pyridine-mediated process in pyridine alone.

Original languageEnglish
Pages (from-to)5756-5769
Number of pages14
JournalJournal of Physical Chemistry A
Volume124
Issue number28
DOIs
Publication statusPublished - Jul 16 2020

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

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