Green-light-driven thioxanthylium-based organophotoredox catalysts: Organophotoredox promoted radical cation Diels-Alder reaction

Friedel–Crafts approach to the one-pot synthesis of methoxy-substituted thioxanthylium salts

“…2, almost all measured UV–vis spectra are nearly identical in spite of different substituents on the benzene ring at the 9-position of the thioxanthylium core. Moreover, when the solvent effects were examined using MeCN, CH 3 NO 2 , DMSO and MeOH, no substantial shifts of the main peak at around 460 nm in the UV–vis absorption spectra were observed [24], indicating that the main absorption of these catalysts would be due to π–π* transition, which is supported by DFT calculations (TD-DFT B3LYP method) (Fig. 3,b).…”

“…Analogously as described in [24] thioxanthylium salts 3a – q were prepared according to the following procedure.…”

“…We have developed the synthesis of multisubstituted condensed heterocyclic compounds in the presence of an acid catalyst [15–23]. More recently, we have reported the design and synthesis of thioxanthylium organophotoredox catalysts, which can work under green light irradiation [24–25]. In the course of this study, we found that these thioxanthylium photocatalysts efficiently oxidized styrene derivatives such as trans -anethole, and promoted radical cation Diels–Alder reactions.…”

Radical Cation Diels‐Alder Reactions of Non‐Conjugated Alkenes as Dienophiles by Electrocatalysis

“…Reductive SET to aryl radical cations (C˙+) provided the desired cycloadducts (C). However, reactions of the alkenes (6)(7)(8) in combination with 2,3-dimethyl-1,3-butadiene (2) were unsuccessful and no significant amounts of cycloadducts (10)(11)(12) were obtained, even after a stoichiometric amount of electricity was passed (Scheme 4), likely due to the electron density of the alkenes. Since di-and tri-substituted alkenes are not electron-rich enough, anodic oxidative SET generated aryl radical cations (D˙+) rather than alkenyl radical cations (A˙+).…”

“…[4] A typical example of this class is reaction of trans-anethole (1) as the electron-rich dienophile (Scheme 1), [5] which is possible using various photocatalysts, including Ru, [6] Cr, [7] and Fe [8] complexes, graphitic carbon nitride, [9] and organic dye. [10] Although radical cation Diels-Alder reactions by photocatalysis can be complementary to normal and inverse electron-demand versions, most recent examples have been limited to β-methylstyrenes as dienophiles, except for aryl vinyl ethers (3) [6] and aryl vinyl sulfides (4) (Scheme 2). [11] Scheme 1 Radical cation Diels-Alder reaction of trans-anethole (1) We have been developing radical cation Diels-Alder reactions by electrocatalysis [12] in lithium perchlorate (LPC)/nitromethane (NM) electrolyte solution, [13] which facilitates the reaction be-tween the carbon-centered radical cation and carbon nucleophile.…”