Benzophenone vs. Copper/Benzophenone in Light‐Promoted Atom Transfer Radical Additions (ATRAs): Highly Effective Iodoperfluoroalkylation of Alkenes/Alkynes and Mechanistic Studies

Abstract Iodoperfluooralkylation of terminal alkenes and alkynes is effectively photo‐promoted by benzophenone 2 (BP) or the photoreducible copper(II) complex 1 . In particular, BP at 1 mol% in methanol upon 365 nm irradiation with a low‐pressure mercury lamp (type TLC=thin layer chromatography, 6 W) results in a fast reaction with excellent reaction yields. Complex 1 and BP 2 exhibited very similar reactivity, suggesting that the reactions involving 1 are likely to be governed by the benzophenone photoactivation processes, rather than copper(I)/(II) redox processes. Mechanistic investigations using transient absorption spectroscopy revealed that a deactivation pathway of the benzophenone triplet ( 3 BP*) is via its reaction with the methanol solvent. We propose that the generated radicals, in particular . CH 2 OH, play a key role in the initiation step forming R f . by reacting with R f I, R f . then entering a radical chain cycle. 1 H NMR studies provided evidence that a substantial amount (∼7% NMR yield) of the hemiacetal CH 3 OCH 2 OH is formed, i.e., the possible by‐product of the reaction between . CH 2 OH and R f I. Finally, DFT calculations indicate that a triplet‐triplet energy transfer (TTET) process from 3 BP* to perfluorooctyl iodide (C 8 F 17 I) is unlikely or should be rather slow under the reaction conditions, consistent with the transient absorption studies. magnified image