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

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 1mol% in methanol upon 365nm irradiation with a low-pressure mercury lamp (type TLC=thin layer chromatography, 6W) 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 ((BP)-B-3*) is via its reaction with the methanol solvent. We propose that the generated radicals, in particular (CH2OH)-C-., play a key role in the initiation step forming R-f(.) by reacting with RfI, R-f(.) then entering a radical chain cycle. (HNMR)-H-1 studies provided evidence that a substantial amount (approximate to 7% NMR yield) of the hemiacetal CH3OCH2OH is formed, i.e., the possible by-product of the reaction between (CH2OH)-C-. and RfI. Finally, DFT calculations indicate that a triplet-triplet energy transfer (TTET) process from (BP)-B-3* to perfluorooctyl iodide (C8F17I) is unlikely or should be rather slow under the reaction conditions, consistent with the transient absorption studies.