Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes, gem‐Difluoroalkenes, and Trifluoromethylalkenes

Abstract

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value‐added fluorinated organic compounds. Recent progress has been mainly focused on the transition‐metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B−H activation of N‐heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom‐economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem‐difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition‐metal free, and the regioselectivity obtained is complementary to the reported transition‐metal‐catalysis in many cases.

Chemoselective catalytic hydrodefluorination of trifluoromethylalkenes towards mono-/gem-di-fluoroalkenes under metal-free conditions

“…However, these reactions usually involve the preparation of complicated fluorinated precursors and are usually conducted under harsh conditions, e.g., in strongly basic solutions, thus leading to a quite limited substrate scope. One other good alternative could be defluorination of polyfluoroalkenes via transition-metal catalysis 40 – 45 , photocatalysis 46 – 49 , or the classical S N 2’ reactions 50 – 53 , delivering functionalized fluoroalkenes via alkenylation 54 , arylation 42 , borylation 55 , 56 , or hydrodefluorination (HDF) (Fig. 2b ) 45 .…”

Metal‐Free Photoredox‐Catalyzed Hydrodefluorination of Fluoroarenes Utilizing Amide Solvent as Reductant

“…We also investigated multiple HDF of polyfluorobenzenes to obtain benzene 3 a without addition of i Pr 2 NEt (Table 3), since complete HDF of polyfluoroarenes by photoredox catalysis has not been achieved yet due to the difficulty of reduction of less fluoro-substituted arenes. [10] Double HDF of difluorobenzenes 2 p-r proceeded well to give 3 a in 56-76 % yields. 1,2,4-Trifluorobenzene (2 s) underwent triple HDF in moderate yield.…”

“…To initiate our study, HDF of fluorobenzene (2 a) to benzene (3 a) was investigated as a model system under photoirradiation conditions (Table 1, see Table S1 for details). Common photoredox catalysts such as [Ir(ppy) 2 (dtbbpy)]PF 6 and fac-Ir(ppy) 3 with i Pr 2 NEt, which are effective for hydrodehalogenation of aryl halides and partial HDFs of polyfluoroarenes, [8,10] did not afford 3 a at all under irradiation with blue LEDs (entries 1, 2). In contrast, the use of stoichiometric amounts of TMPD 1 or paraphenylenediamine (4) led to formation of 3 a in 73 and 63 % yield, respectively, under irradiation with a mercury (Hg) lamp with heating in DMF (entries 3, 4), displaying the highly reducing ability of diaminobenzene derivatives in their excited states.…”

“…Concerning reduction of fluoroarenes, photoredoxcatalyzed partial HDF reactions and CÀ F functionalizations of polyfluoroarenes have been reported (Scheme 1b). [10] However, HDF of monofluoroarene derivatives with photoredox systems remains undeveloped due to their inertness towards singleelectron-transfer (SET), [11] while several methods utilizing photoirradiation have recently been developed, which involve direct photoexcitation of fluoroarenes [12a,b] and the use of stoichiometric amounts of strong photoreductants. [12c,d] To achieve photoredox-catalyzed HDF of monofluoroarenes of large negative reduction potentials, we have focused on electron-rich aromatic amines as strongly reducing photocatalysts.…”

Photocatalytic C−F Bond Activation of Fluoroarenes, gem‐Difluoroalkenes and Trifluoromethylarenes

“…In 2020, the photocatalytic defluoroborylation of gem ‐difluoroalkenes with NHC‐BH 3 30 was reported by Wu and coworkers, with a combination of [Ir(dF(CF 3 )ppy) 2 (5,5’‐dFbpy)]PF 6 , S2 and DIPEA at room temperature under blue LED irradiation (Scheme 30). [46] The scope covered the symmetric and asymmetric gem ‐difluoroarylalkenes in good yields for the monofluoroalkenylborane products 44 albeit poor stereoselectivity for later substrates.…”