TY - JOUR
T1 - NH Tautomerism of N-Confused Porphyrin
T2 - Solvent/Substituent Effects and Isomerization Mechanism
AU - Ishizuka, Tomoya
AU - Sakashita, Ryuichi
AU - Iwanaga, Osamu
AU - Morimoto, Tatsuki
AU - Mori, Shigeki
AU - Ishida, Masatoshi
AU - Toganoh, Motoki
AU - Takegoshi, Kiyonori
AU - Osuka, Atsuhiro
AU - Furuta, Hiroyuki
N1 - Funding Information:
The work was supported by the JSPS KAKENHI grant numbers (JP15K13646, JP20H00406) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
Publisher Copyright:
Copyright © 2020 American Chemical Society.
PY - 2020/7/16
Y1 - 2020/7/16
N2 - 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.
AB - 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.
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U2 - 10.1021/acs.jpca.0c04779
DO - 10.1021/acs.jpca.0c04779
M3 - Article
C2 - 32559101
AN - SCOPUS:85088266537
VL - 124
SP - 5756
EP - 5769
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
SN - 1089-5639
IS - 28
ER -