Anti-Markovnikov Oxidation of β-Alkyl Styrenes with H₂O as the Terminal Oxidant

Abstract

Oxygenation of alkenes is one of the most straightforward routes for the construction of carbonyl compounds. Wacker oxidation provides a broadly useful strategy to convert the mineral oil into higher value-added carbonyl chemicals. However, the conventional Wacker chemistry remains problematic, such as the poor activity for internal alkenes, the lack of anti-Markovnikov regioselectivity, and the high cost and chemical waste resulted from noble metal catalysts and stoichiometric oxidant. Here, we describe an unprecedented dehydrogenative oxygenation of β-alkyl styrenes and their derivatives with water under external-oxidant-free conditions by utilizing the synergistic effect of photocatalysis and proton-reduction catalysis that can address these challenges. This dual catalytic system possesses the single anti-Markovnikov selectivity due to the property of the visible-light-induced alkene radical cation intermediate.

Photocatalytic Oxygenation Reactions Using Water and Dioxygen

“…Catalytic cycles of photocatalytic hydroxylation of benzene to phenol using O 2 as at wo-electron oxidant, H 2 Oa sa no xygen source,D DQ as ar edox photocatalyst, and NO or HNO 3 as ao xidation catalystf or DDQH 2 to DDQ. [79][80][81][82][83][84][85][86] 1,2-Dihydronaphthalene is oxidized by ET to AcrCÀMesC + to produce 1,2-dihydronaphthalener adical cations, affording 2-tetralone (isolated in 83 %y ield) and H 2 in the presence of [Co(dmgH) 2 pyCl] (dmgH = dimethylglyoximate monoanion;p y = pyridine) after photoirradiationb yablue LED light for 24 h. [87] The photocatalytic mechanism is shown in Scheme 11,w hereby ET from an alkene to the MesC + moiety of AcrCÀMesC + is followed by anti-Markovnikov addition of water and deprotonation to afford aC -radical intermediate III,w hich is further oxidizedb y[ Co III (dmgH) 2 pyCl] to produce cationic intermediate IV and Co II species, respectively. [78].Copyright 2018, American Chemical Society.…”

“…Reprinted with permission from Ref. [87] Intermediate IV is then converted into the carbonyl compounds after deprotonation. ChemSusChem 2019, 12,3931 -3940 www.chemsuschem.org the ET state (AcrC ÀMesC + )p roduced by photoinduced ET from the Mes moiety to the Acr + moiety.…”

“…ChemSusChem 2019, 12,3931 -3940 www.chemsuschem.org the ET state (AcrC ÀMesC + )p roduced by photoinduced ET from the Mes moiety to the Acr + moiety. [87] The oxygen atom in the carbonyl compound comes from water and ap rotoni s used as the oxidant for the oxidation of alkenes. [87] Intermediate IV is then converted into the carbonyl compounds after deprotonation.…”

“…[79][80][81][82][83][84][85][86] 1,2-Dihydronaphthalene is oxidized by ET to AcrCÀMesC + to produce 1,2-dihydronaphthalener adical cations, affording 2-tetralone (isolated in 83 %y ield) and H 2 in the presence of [Co(dmgH) 2 pyCl] (dmgH = dimethylglyoximate monoanion;p y = pyridine) after photoirradiationb yablue LED light for 24 h. [87] The photocatalytic mechanism is shown in Scheme 11,w hereby ET from an alkene to the MesC + moiety of AcrCÀMesC + is followed by anti-Markovnikov addition of water and deprotonation to afford aC -radical intermediate III,w hich is further oxidizedb y[ Co III (dmgH) 2 pyCl] to produce cationic intermediate IV and Co II species, respectively. [87] Zhang et al reported an expedient and efficient photocatalytic oxygenationo fi ndoles catalyzed by a dicyanopyrazine-derived chromophore (DPZ) to produce either isatins or formylformanilidesi ng ood yields (Scheme 12). AcrCÀMes reduces the Co II species to regenerate Acr + ÀMes and the Co I speciest hat reacts with H + to produce the Co III ÀH species.…”

“…[87] Zhang et al reported an expedient and efficient photocatalytic oxygenationo fi ndoles catalyzed by a dicyanopyrazine-derived chromophore (DPZ) to produce either isatins or formylformanilidesi ng ood yields (Scheme 12). [87].Copyright 2016, American Chemical Society. [88] Electron transferf rom indoles to the triplet excited state ( 3 DPZ*) occurs to produce indole radical cation because the one-elec-tron reduction potentialo f 3 DPZ* (0.91 Vv s. SCE) is more positive than the one-electron oxidation potentialo fi ndole (À0.04 Vv s. SCE).…”