1985 Volume 58 Issue 12 Pages 3423-3430
The irradiation of 4-methyl-2-quinolinecarbonitrile 1 in ethanol or cyclohexane resulted in the formation of 2-(1-hydroxyethyl)-4-methylquinoline 2 or 2-cyclohexyl-4-methylquinoline 3 in the yield of ca. 48 or 65%. The effects of an external magnetic field upon the photosubstitution reaction were investigated in either the absence or presence of 1,3-pentadiene (triplet quencher). In the case of the photosubstitution reaction (1→2) in ethanol, the chemical yield of 2 increased quadratically with an increase in the field strength (magnetic field effect due to Δg mechanism) and it also showed a minimum at approximately 1.1T (magnetic field effect due to HFI-J mechanism). The addition of 1,3-pentadiene caused a complete disappearance of the Δg magnetic field effect. Thus, the formation of 2 at a zero field was concluded to occur from the S1-state via the singlet hydrogen-bonded radical pair. In the case of the photosubstitution reaction (1→3) in cyclohexane, the chemical yield of 3 decreased steeply upon the application of a magnetic field 40m T (magnetic field effect due to HFI mechanism) and a further increase in the field strength resulted in a quadratic increase in the yield (magnetic field effect due to Δg mechanism). Neither an HFI nor a Δg magnetic field effect was observed in the presence of 1,3-pentadiene. The reaction was thus concluded to proceed from the S1 and T1 states via the singlet and triplet radical pairs. Reaction mechanisms deduced from the external magnetic field effects were consistent with the results of Stern-Volmer analyses.
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