Visible-Light-Induced Oxazoline Formations from N-Vinyl Amides Catalyzed by an Ion-Pair Charge-Transfer Complex

Abstract

Visible-light photoredox catalysis plays an important role in various reactions which are inaccessible under typical thermal conditions. Distinctly different from common visible-light photoredox catalysis which often involves transition-metal complexes, conjugated organic dyes, or electron donor−acceptor complexes, herein, the use of ion-pair charge-transfer (IPCT) complex-induced visible-light photoredox catalytic reactions are described, wherein the cyclization−methoxylation of N-vinyl amides in methanol was achieved under irradiation with blue LEDs. The reaction employs a heteroarenium iodide as the photocatalyst and can be extended to cyclization−alkoxylation, −acyloxylation, and −hydroxylation. This protocol provides an eco-friendly synthetic route to a wide range of oxazoline derivatives. Mechanistic investigations with UV−visible spectroscopy and control experiments confirm the existence of the IPCT absorption band in the visible region for the heteroarenium iodide, which is responsible for the observed reactivity.

Visible light-induced synthesis of (Z)-β-iodoenamides from N-vinyl amides mediated by the ion pair charge transfer state

“…According to the above results and previous literature reports, 16 a plausible mechanism of the visible-light-induced synthesis of ( Z )-β-iodoenamide from N -vinyl amide mediated by the IPCT state was proposed (Scheme 6). First, the contact ion pair of PI-7 was formed in DCE, whose absorption band tailed into the visible region (approximately 450 nm) (see the ESI† for details).…”

“…In our previous work on photocatalytic oxazoline synthesis, the formation of a small amount of ( Z )-β-iodoenamides was observed when acetic anhydride (Ac 2 O) was added to provide 4-acetyloxyoxazoline. 16 Therefore, our investigation was started by using 2-fluoro- N -vinyl benzamide ( 1a ) as the model substrate and Ac 2 O as the additive in 1,2-dichloroethane (DCE) under irradiation with blue LEDs (435–445 nm). 1-Methylpyridin-1-ium iodide ( PI-1 ) was first selected as the photosensitizer and iodine source of the reaction.…”

“…14 For example, iodination with iodide salts without using an external oxidant has been successfully achieved via a photochemical approach by employing transition-metal (Ru or Ir) complexes, organic dyes or EDA complexes. 15 Recently, we found that N-heteroarenium iodides could serve as photocatalysts to achieve visible-light-induced oxazoline formation from N-vinyl amides via the ion pair charge transfer (IPCT) state, 16 which is distinctly different from the previously reported photocatalytic system. 17 With the appropriate modification of heteroarenium scaffolds, the absorption bands of iodide salts could be tuned into the visible region, which offers the possibility of electron transfer from iodide anions to N-heteroarenium cations.…”

“…), conjugated organics (10-phenyl-10H-phenothiazine), and organic dyes (eosin B, etc.) proved to be inefficient photocatalysts for this reaction, with the raw material 1a being fully recovered (Table 1,entries 15,(16)(17)(18)(19)(20)(21). Although Ru (bpy) 3 Cl 2 could provide 28% and 4% yields of 2a and 2a′, respectively, messy byproducts were also formed (Table 1, entry 17).…”

Selective N‐Monovinylation of Primary Aromatic Amides Using Calcium Carbide as an Alkyne Source

“…The effects of different temperature were tested, and it was found that 120 °C was an appropriate temperature for the formation of 2 a (Table 1, entry 6), and lowering and raising temperature were unfavorable to the reaction (Table 1, entries 15, 16). Increasing the amount of base and water, and increasing or decreasing the reaction time could not effectively improve the yield of 2 a (Table 1, entries [17][18][19][20]. The amount of calcium carbide also has an effect on the yield of 2 a.…”