The Photochemical Magnetic Field Effect of Isoquinoline N-Oxides

Article abstract of DOI:10.1246/bcsj.58.1088

The irradiation of isoquinoline N-oxide in ethanol resulted in the formation of lactam in a ca.67percent yield via the S₁ state (Process I).When the photochemical isomerization was carried out in the presence of a magnetic field, the chemical yield of lactam was found to show a minimum at approximately 1T.In order to interpret this new phenomenon, Process I was assumed to be initiated by electron transfer from the excited singlet N-oxide to the hydrogen-bonded ethanol (the formation of a singlet hydrogen-bonded radical ion pair).Thus, the new type of magnetic-field effect was excellently explained in terms of an electron-nuclear hyperfine interaction mechanism including the electron-exchange interaction in the radical-ion pair (the magnetic-field effect due to HFI-J mechanism).This implies that the magnetic-field effect is unambiguous evidence that Process I (N-oxide - lactam) does not involve the oxaziridine intermediate, but proceeds via the singlet hydrogen-bonded radical-ion pair (radical-ion-pair mechanism of Process I).The irradiation of 1-cyanoisoquinoline N-oxide in ethanol resulted in the formation of 1,3-oxazepine in a ca. 66percent yield via the S₁ state (Process II).The photochemical isomerization was not affected at all by an external magnetic field.This is consistent with the idea that Process II (N-oxide - oxazepine) does not involve a field-sensitive intermediate, but occurs with the intervention of oxaziridine (the oxaziridine mechanism of Process II).