The Photochemical Magnetic Field Effect of Isoquinoline N-Oxides

Abstract

Abstract The irradiation of isoquinoline N-oxide in ethanol resulted in the formation of lactam in a ca. 67% 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. 66% 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).