Aryl-substituted dimethylbenzimidazolines as effective reductants of photoinduced electron transfer reactions

Abstract

a b s t r a c tPhotoinduced electron transfer (PET) reactions promoted by 2-aryl substituted 1,3-dimethylbenzimidazolines (Ar-DMBIH) were investigated. Excited states of Ar-DMBIH, formed by irradiation using light above 360 nm, initiate PET reductions of various organic substrates, including transformations of epoxy ketones to aldols, free radical rearrangements such as the Dowd-Beckwith ringexpansion and 5-exo hexenyl cyclization, deprotection of N-sulfonyl-indols, and allylation of acyl formates. In these processes, Ar-DMBIH possessing 1-naphthyl, 2-naphthyl, 1-pyrenyl and 9-anthryl substituents formally act as two electron and one proton donors while the hydroxynaphthyl substituted derivative serves as a two electron and two proton donor. On the basis of the results of absorption spectroscopy studies, cyclic voltammetry and DFT calculation, a mechanistic sequence for these reduction reactions is proposed that involves initial photoexcitation of the aryl chromophore of the Ar-DMBIH followed by single electron transfer (SET) to the organic substrate to generate the radical cation of benzimidazoline and the radical anion of the substrate.

Supramolecular Photocatalysts for the Reduction of CO2

“…BIH and BI­(OH)H have much stronger reducing power (BIH: E 1/2 ox = 0.33 V; BI­(OH)­H: E 1/2 ox = 0.31 V vs SCE) compared to that of the NADH model compounds. − Owing to this property, both compounds efficiently quenched the excited state of the Ru photosensitizer unit in the supramolecular photocatalysts (for example, k q = 9.7 × 10 8 M –1 s –1 , η q = 99% for RuRe12 with BIH (0.1 M)) (Process II). , Irradiation of a mixed solution of DMF-TEOA containing RuRe12 and BIH (0.1 M) induced the selective catalytic formation of CO with much greater efficiency, durability, and rate of CO formation (entry 23: Φ CO = 0.45, TON CO = 3029, TOF CO = 35.7 min –1 ) compared with those using BNAH (entry 22: Φ CO = 0.15, TON CO = 207, TOF CO = 4.7 min –1 ) . The efficiency and durability of the photocatalysis using RuRe15 were also much improved by using BIH (entry 27: Φ CO = 0.54, TON CO = 2915) instead of BNAH (entry 26: Φ CO = 0.13, TON CO = 233).…”

Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption

“…In addition, since this process competes with the radiative and non-radiative deactivation processes from the excited state of a photosensitizer by itself, the shorter lifetime results in less opportunity of the reductive quenching process to occur. To evaluate whether reductive quenching occurs, the emission intensity from Ru(pic) was compared in the presence of five different concentrations of a sacrificial electron donor, 1,3-dimethyl-2-phenyl-2,3-dihydro-1 H -benzo[ d ]imidazole (BIH) (Tamaki et al, 2013a; Hasegawa et al, 2015) in DMA-triethanoamine (TEOA; 5:1 v/v). As shown in Figure 4, the emission intensities from the 3 MLCT excited state of Ru(pic) decreased at higher concentrations of BIH, which indicated that the excited Ru(pic) was quenched by BIH.…”

Photoinduced Electron Transfer-Promoted Reactions Using Exciplex-Type Organic Photoredox Catalyst Directly Linking Donor and Acceptor Arenes

“…Photoinduced electron transfer (PET) promoted reactions are a powerful and environmentally friendly tool for the construction of complex organic molecules [1][2][3][4][5] and polymers [6,7] that cannot be prepared by other methods. This is due to the highly reactive radical cation and anion intermediates generated via PET, which facilitate oxidation and reduction reactions under mild conditions (e.g., at room temperature), utilizing light as a clean and traceless reagent.…”

“…This is due to the highly reactive radical cation and anion intermediates generated via PET, which facilitate oxidation and reduction reactions under mild conditions (e.g., at room temperature), utilizing light as a clean and traceless reagent. PET-promoted reactions using photoredox catalysts, such as transition-metals (Ru, Ir) [1][2][3][4][5]8] and Fukuzumi photocatalysts [1][2][3][4][5]9], are among the recent trends in organic synthesis (Scheme 1a). The Fukuzumi photocatalyst, in particular an electron donor-acceptor-linked dyad (9-mesityl-10-methylacridinium ion, Acr + -Mes) acts as an efficient organic photoredox catalyst due to the long-lived electron-transfer state formed upon irradiation.…”