Metal- and photocatalyst-free synthesis of 3-selenylindoles and asymmetric diarylselenides promoted by visible light

Abstract

A novel and sustainable procedure was developed for the synthesis of 3-selenylindoles employing diorganyl diselenides and indoles or electron-rich arenes as starting materials. Visible blue light was used to promote the reaction without employing transition metal complexes or organic photocatalysts as sensitizers. Additives such as strong oxidants or bases were not required. Moreover, ethanol was employed as a benign solvent under mild reaction conditions. Through this easy and eco-friendly approach, several 3-selenylindoles and a number of asymmetric diarylselenides were obtained in good to excellent isolated yields.

Scheme 1. Synthesis of 3-chalcogenylindole employing visible light.

Scheme 2. Synthesis of 3-selenylindoles using visible light.^{a,b a}Reaction conditions: 1 (0.15 mmol) and 2 (0.5 equiv.) in ethanol (2 mL) irradiated for 18 h with 3 W blue LED (467 nm) at room temperature under air atmosphere. ^bIsolated yields. ^cReaction irradiated for 36 h.

Scheme 3. Photoinduced arylselenation of electron-rich arenes.^{a,b a}Reaction conditions: 4 (0.15 mmol) and 2a (0.5 equiv.) in ethanol (2 mL) irradiated for 18 h with 3 W blue LED (467 nm) at room temperature under air atmosphere. ^bIsolated yields. ^cReaction performed with 1 equiv. of 2a, irradiated for 36 h.

Fig. 1. Absorption spectra of diselenides (0.06 mM) and indole (0.06 mM) in ethanol, and blue LED emission. Inset: zoom from 360 to 440 nm spectrum zone.

Fig. 2. Profile of switching on–off experiment by following indole conversion along light on–off periods of time.

Scheme 4. Control reactions.

Scheme 5. Proposed mechanisms for the synthesis of selenium compounds.