Unveiling Potent Photooxidation Behavior of Catalytic Photoreductants

Abstract

We describe a photocatalytic system that reveals latent photooxidant behavior from one of the most reducing conventional photoredox catalysts, <i>N</i>-phenylphenothiazine<i> </i>(<b>PTH</b>). This aerobic photochemical reaction engages difficult to oxidize feedstocks, such as benzene, in C(sp<sup>2</sup>)–N coupling reactions through direct oxidation. Mechanistic studies are consistent with activation of <b>PTH</b> via photooxidation and that Lewis acid co-catalysts scavenge inhibitors formed upon catalyst activation.

Non-Innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides

“…anions, including those derived from commonly employed photoredox catalysts, can serve as potent photocatalytic reductants upon cathodic reduction. These data fit into a growing body of literature from our group 59 and others 40,60 that suggest photocatalyst-based redox events can engender more potent activity from conventional photocatalysts. Taken together, these data led us to consider whether we could redesign a photocatalytic system to favor formation of photoactive radical anion intermediates to elicit deeply reducing potentials and expand the repertoire of coupling reactions available from aryl chlorides under operationally simple conditions (Figure 1C).…”

Isolated Neutral [4]Helicene Radical Provides Insight into Consecutive Two-Photon Excitation Photocatalysis

“…A second successive photoexcitation generates the radical excited state PC • * that can act as super photoreducing agent (E1/2 red * = -2.3 to -3.4 V vs SCE) (Figure 1a). 6 This concept of two-photon excitation process has been reported with numerous notable photocatalysts such as PDI, 7 DCA, 8 anthraquinone, 9 Rhodamine 6G, 10 benzo[ghi]perylene (BPI), 11 4-DPAIPN, 12 3-CzEPAIPN, 13 Mes-Acr, 14 and Deazaflavin 15 (Figure 1b). As a common benchmark reaction, photoredox C(sp 2 )-X bond activation in aryl bromides and chlorides, Birch reduction, and sulfonamide cleavage showcased the extreme photoreducing ability of radical photocatalysts in most cases.…”

Unveiling Extreme Photoreduction Potentials of Donor-Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides

“…With the optimized conditions in hand, we examined the scope of borylation of aryl chlorides through the ConPET process. We found that this protocol was effective with not only activated aryl chlorides (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) but also challenging nonactivated aryl chlorides (14) and electron-rich aryl chlorides (3-13) that possess very negative reduction potentials (up to -2.94 V, Table 2A). A number of functional groups were well tolerated, including ethers (8)(9)(10)(11), phenols (12), sulfides (13), trifluoromethyl (24), and borates (25), as well as those potentially sensitive to strongly reducing conditions, such as aryl fluorides (16)(17)(18)(19), nitriles (20)(21)(22), esters (23), indoles (26), and epoxides (27).…”

“…We found that this protocol was effective with not only activated aryl chlorides (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) but also challenging nonactivated aryl chlorides (14) and electron-rich aryl chlorides (3-13) that possess very negative reduction potentials (up to -2.94 V, Table 2A). A number of functional groups were well tolerated, including ethers (8)(9)(10)(11), phenols (12), sulfides (13), trifluoromethyl (24), and borates (25), as well as those potentially sensitive to strongly reducing conditions, such as aryl fluorides (16)(17)(18)(19), nitriles (20)(21)(22), esters (23), indoles (26), and epoxides (27). Diborylation products (25,28,29), which have found wide application in materials science, were obtained good to excellent yields when dihaloarenes were employed, probably because the generated monoborylated chloride intermediates were more reducible than dihaloarenes.…”

“…18,19 The ConPET strategy relies on a photocatalyst and a sacrificial electron donor that, upon visible light irradiation, results in a longlived radical anion, which can be excited by a second photon to induce a much more powerful reducing species. 20 The König group pioneered this field by developing a perylene bisimide (PDI) photocatalyst whose photoreduced form can undergo consecutive absorption of a second photon to achieve a much stronger reducing species for aryl halide reduction. 21 Subsequently, several other organic dyes, including rhodamine 6G, 22 9,10-dicyanoanthracene (DCA), 23 anthraquinone, 24 acridinium salts (Mes-Acr + ), 25 and benzo[ghi]perylene (BPI), 26 were disclosed to be competent ConPET catalysts for aryl halide reduction, Birch reduction, 26 and sulfonamide cleavage.…”