3 years ago

Tautomerism-Induced Cis–Trans Isomerization of Pyridylethenyl N-Confused Porphyrin

Tautomerism-Induced Cis–Trans Isomerization of Pyridylethenyl N-Confused Porphyrin
Ryuichi Sakashita, Yasutaka Oka, Praseetha E. Kesavan, Motoki Toganoh, Tomoya Ishizuka, Hisanori Akimaru, Shigeki Mori, Hiroyuki Furuta, Masatoshi Ishida
Pyridylethenyl-substituted N-confused porphyrins (NCPs) were synthesized, and their cis–trans isomerization was studied. Among four possible isomers, trans-3H and cis-2H types of structures, of which aromaticity and absorption/emission properties differ largely, were isolated. The cis-isomer was largely stabilized by the intramolecular hydrogen bonding between the pyrrolic-NH and the pyridinic-N in the vicinity. The thermal cis–trans isomerization proceeded even at 30 °C, which was significantly accelerated by the pyridine added to the system. The kinetic studies revealed that the isomerization reaction was second-order and the activation energy of the thermal isomerization from cis to trans isomer was ΔG0cis→trans = 35.7 kcal/mol at 298 K, which is significantly smaller than that of Ni complex (42.3 kcal/mol). An intermolecular proton transfer induced cis–trans isomerization mechanism was proposed.

Publisher URL: http://dx.doi.org/10.1021/acs.joc.7b01770

DOI: 10.1021/acs.joc.7b01770

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