Visible Light-Induced Homolytic Cleavage of Perfluoroalkyl Iodides Mediated by Phosphines

Abstract

In an effort to explain the experimentally observed variation of the photocatalytic activity of t Bu 3 P, n Bu 3 P and (MeO) 3 P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine– and phosphite–IC 4 F 9 adducts by means of relativistic density functional theory and multireference configuration interaction methods. Based on the results of these computational and complementary experimental studies, we offer an explanation for the broad tailing of the absorption of t Bu 3 P-IC 4 F 9 and (MeO) 3 P-IC 4 F 9 into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in n Bu 3 P-IC 4 F 9 , in particular with regard to the P–I–C bending angle, reduce the Franck–Condon factors and thus the absorption probability compared to t Bu 3 P-IC 4 F 9 . Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO) 3 P may be the result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed mechanism for the phosphine-catalyzed homolytic C–I cleavage of perfluorobutane iodide involves S1 ← S0 absorption of the adduct followed by intersystem crossing to the photochemically active T 1 state.

Straightforward Synthesis of Fluorinated Enals via Photocatalytic α-Perfluoroalkenylation of Aldehydes

“…As a result of earlier mechanistic studies addressing the photochemical activation of phosphine EDA complexes by visible light, , we propose the intermediate formation of an EDA complex by the interaction of the lone pair of the phosphine and the σ*C-I-orbital of the perfluoroalkyl iodide (Scheme , I). Irradiation of blue light leads to the excitation and formation of a radical-ion pair (II), which forms the perfluoroalkyl radical (III).…”

“…As an alternative for the formation of perfluoroalkyl radicals, a photocatalyst system based on electron donor−acceptor (EDA) complexes with phosphines has been developed for perfluoroalkylations (Scheme 1b). 33,37,38 During our research, by aiming at perfluoroalkylations of enamines with phosphine EDA complexes, we observed the formation of tetrasubstituted, fluorinated α,β-unsaturated aldehydes. This structural motif is only rarely reported in the literature, 39−42 involving mostly the transformation of ketone derivatives.…”

“…Therefore, a more straightforward methodology is desirable. As an alternative for the formation of perfluoroalkyl radicals, a photocatalyst system based on electron donor–acceptor (EDA) complexes with phosphines has been developed for perfluoroalkylations (Scheme b). ,, …”

Recent Advances in Photoinduced Perfluoroalkylation Using Perfluoroalkyl Halides as the Radical Precursors

“…light-induced homolytic C-I bond cleavage. 30 He and coworkers also reported elegant examples of the ATRA reaction of alkenes and ethyl iododifluoroacetate using phosphines as the halogen bond acceptor. 31 Scheme 7 Photoinduced, phosphine-catalyzed ATAR of alkenes with perfluoroalkyl iodides…”

“…To gain insight into the photoredox activation of the phosphine-R f –I·EDA complexes, they also carried out computational chemistry studies, and the computational results support the mechanism of a phosphine-assisted light-induced homolytic C–I bond cleavage. 30 He and co-workers also reported elegant examples of the ATRA reaction of alkenes and ethyl iododifluoroacetate using phosphines as the halogen bond acceptor. 31…”