Trifluoromethylated ketones are useful building blocks for organic compounds with a trifluoromethyl group. A new and simple synthesis of ketones with a trifluoromethyl substituent in the α-position proceeds through a one-pot photoredox-catalyzed trifluoromethylation-oxidation sequence of alkenes. Dimethyl sulfoxide (DMSO) serves as a key and mild oxidant under these photocatalytic conditions. Furthermore, an iridium photocatalyst, fac-[Ir(ppy)₃ ] (ppy=2-phenylpyridine), turned out to be crucial for the present photoredox process.¹

Fig.１　Photocatalytic oxidative trifluoromethylation of alkenes

The reaction of β-methylstyrene with Togni's reagent in the presence of the Ir photocatalyst (2 mol%) in DMSO solvent under visible light irradiation (LED lamp) afforded the CF₃-ketone in 75% isolated yield (2 h) (Fig. 1). The present oxidative trifluoromethylation of alkenes exhibited broad scope (21 examples). In general, electrophilic or radical trifluoromethylation of enolates, which were prepared from the corresponding carbonyl compounds in advance, provides access to valuable α-trifluoromethylated carbonyl compounds. Examples of the "direct" oxidative trifluoromethylation of alkenes, which are abundant and commonly used feedstocks, have been limited so far, but should be promising way (Fig. 2).