EPR/Spin-trapping study of free radical intermediates in the photolysis of trifluoromethyl ketones with initiators

• Published in: Magnetic resonance in chemistry 2010-03, Vol.48 (3), p.198-204
• Main Authors: Rosa, Esmeralda, Guerrero, Angel, Bosch, Mª Pilar, Julià, Luis
• Format: Article
• Language: English
• Subjects: Electron paramagnetic resonance; Electron Spin Resonance Spectroscopy; EPR; Free radicals; Free Radicals - analysis; Free Radicals - chemistry; Initiators; irradiation; Ketones; Ketones - chemistry; Molecular Structure; Phenyls; Photolysis; Radicals; Reference Standards; spin adduct; spin trap; Trapping; trifluoromethyl ketones
• ISSN: 0749-1581; 1097-458X; 1097-458X
• DOI: 10.1002/mrc.2566

Photolysis of trifluoromethyl ketones (TFMKs) 1a–1e versus the non‐fluorinated ketones 2a–2b in the presence of radical initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals generated after irradiation of the ketones were identified by trapping with 2‐methyl‐2‐nitrosopropane (MNP) and 2,4,6‐tri‐tert‐butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of t‐butylperoxide, short‐chain TFMKs, such as 1,1,1‐trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the elusive trifluoromethyl radical. In contrast, long‐chain TFMKs did not provide clues to prove formation of the trifluoromethyl radical but instead to radicals derived by ion of one α‐methylene proton to the carbonyl. Although TFMKs are quite stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from ion of a γ‐hydrogen to the carbonyl group. Copyright © 2010 John Wiley & Sons, Ltd. The elusive trifluoromethyl radical has been detected upon irradiation of 1,1,1‐trifluoroacetone and hexafluoroacetone in the presence of hydrogen peroxide and 2‐methyl‐2‐nitrosopropane (MNP) or 2,4,6‐tri‐tert‐butylnitrosobenzene (TTBNB). However, photolysis of long‐chain 1,1,1‐trifluoromethylketones yields α‐carbonylmethylene radicals instead of trifluoromethyl radical.