AbstractUsing a spin‐trap reagent 3‐nitrosodurene (1,2,4,5‐tetramethyl‐3‐nitrosobenzene), the short‐lived free radicals generated by ultraviolet irradiation of fenvalerate (RS)‐α‐cyano‐3‐phenoxybenzyl (RS)‐2‐(4‐chlorophenyl)‐3‐methylbutyrate in degassed benzene or dichloromethane, were scavenged as the more stable nitroxide radicals. These radicals were separated by high‐performance liquid chromatography and identified individually by electron‐spin resonance spectroscopy, as well as by gas chromatography/mass spectrometry. As a result, they were found to be the spin adduct mixtures of the 4‐chloro‐α‐isopropylbenzyl and α‐cyano‐3‐phenoxybenzyl radicals, which are involved in the photo‐induced decarboxylation process of fenvalerate. Discrimination of the radicals was also performed by the isotope‐labelling method whereby the benzylic proton in the acid moiety was deuteriated. The spin numbers of the nitroxides decreased by about five‐fold when photolysis was carried out in oxygenated benzene solution.N‐Benzylidene‐tert‐butylamineN‐oxide trapped both radicals but much less efficiently. The nitroxide of the 4‐chloro‐α‐isopropylbenzyl radical was predominant at 25°C or –40°C, but the proportion of the α
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