Aerobic Transition-Metal-Free Visible-Light Photoredox Indole C-3 Formylation Reaction

Abstract

An aerobic visible-light-promoted indole C-3 formylation reaction catalyzed by Rose Bengal has been developed. This transition-metal-free process employs molecular oxygen as the terminal oxidant and uses TMEDA as the one-carbon source through C–N bond cleavage. The reaction is compatible with a variety of functional groups.

Visible‐Light‐Induced Pyridylation of Remote C(sp3)−H Bonds by Radical Translocation of N‐Alkoxypyridinium Salts

“…From the viewpoint of green chemistry,t he development of organic photocatalysts has attracted great interest. [9][10][11] Recently,w er eported that 3-phosphonated quinolinone (Q 1 ; Scheme 1) can form reactive ethoxy radicals from Nethoxypyridinium salts [12] upon photoexcitation under mild reaction conditions. [12b] Drawing inspiration from these studies,w es peculated that an appropriately designed N-alkoxypyridinium substrate may serve not only as the alkoxy radical precursor but also as the heteroaryl source for pyridylation.…”

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

“…Rose Bengal (RB) was widely used as a visible light-activated photocatalyst [32][33][34][35][36][37][38][39][40][41][42][43][44]. Tan's group studied the photoredox catalysis using RB [32][33][34][35][36].…”

“…This cascade transformation is the one-pot reaction which begins from furan substrates (Figure 10) [37]. Despite the extraordinary complexity of reaction cascade, the reaction between 2-methylfuran 23 and L-serine ethyl ester 24 led to the formation of bicy- The aerobic visible light-promoted indole C3 formylation reaction was achieved by using RB as a photocatalyst and N,N,N′,N′-tetramethylenediamine (TMEDA) as a one-carbon source (Figure 11) [40]. Upon the irradiation of visible light, the reaction of N-methylindole 26 with TMEDA in the presence of RB (5 mol%) and KI as an additive under air afforded 3-formyl-N-methylindole 27 in 70% yield.…”

Selective and Efficient Formylation of Indoles (C3) and Pyrroles (C2) Using 2,4,6‐Trichloro‐1,3,5‐Triazine/Dimethylformamide (TCT/DMF) Mixed Reagent

“…Moreover, during the multistep synthesis of many natural products, formylation reaction is often necessary, and the selectivity, simplicity, and efficiency of this step are highly critical . Some of the widely used formylating agents that have been reported up to now are POCl 3 / N , N ‐dimethylformamide (DMF) , HCO 2 H/ N,N′ ‐Dicyclohexylcarbodiimide (DCC) , 2,2,2‐trifluoroethyl formate , formic acid/1‐Ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide (EDAC) , acetic formic anhydride , formyl‐pivaloyl anhydride , cyanomethylformate , pentafluorophenyl formate , imidazole/DMF , MeOH/Ru , CO 2 /PhMe 2 SiH , AgOTf/Cl 2 CHOMe , tetramethylethylenediamine/O 2 /visible light , FeCl 3 /DMSO , CO/acid , 2‐benzotriazolyl‐1,3‐dioxolane , N ‐methyl aniline/Ru , Pd/CO/H 2 , tert ‐butyl isocyanide , Pd/CH 3 NO 2 , etc. In spite of the diversity of formylating agents, there are serious restrictions for the preparation of some of them, such as harsh experimental conditions, the use of unusual reagents, formation of unwanted or toxic byproducts, use of expensive procedures for their synthesis, and thermal instability of the reagents.…”