Abstract
A Photoinduced dearomative nucleophilic addition to N-Boc indoles mediated by two-molecule organic photoredox catalysts such as phenanthrene and 1,4-dicyanobenzene with UV irradiation furnished 2-substituted indolines in moderate to quantitative yields. Hydroxide, alkoxide, and cyanide ions can be used as a nucleophile to provide 2-hydroxy, 2-alkoxy, and 2-cyanoindolines, respectively. Both electron-rich and -deficient indoles, including tryptophan derivatives, can be employed in the photoreaction to provide various indolines. This method provides transition-metal-free access to 2-subsituted indolines from indoles using organic photoredox catalysts under mild conditions.
Highlights
The category of 2-substituted indolines, such as 2-hydroxy- and 2-alkoxy-indolines, are structural components of a myriad of interesting natural compounds and synthetic intermediates (Scheme 1a) [1,2,3,4,5]
Common methods developed for the synthesis of 2-alkoxyindolines generally involve cyclization reactions of tryptophol catalyzed by transition metals, such as Cu, Sc, Pd, Ru, and Ir, under harsh conditions, such as high temperatures [10,11,12,13,14,15], several approaches that utilize arylhydrizines [16,17], anilines [18,19], or spirocyclopropanes [20] have been reported
With the aim of developing synthetic approaches that utilize less toxic and more readily available reagents under mild conditions, we have been investigating photochemical reactions using organic photoredox catalysts, as they constitute environmentally friendly methods wherein light is applied as a traceless reagent, and harsh reaction conditions, such as high temperature and pressure, are not required
Summary
The category of 2-substituted indolines, such as 2-hydroxy- and 2-alkoxy-indolines, are structural components of a myriad of interesting natural compounds and synthetic intermediates (Scheme 1a) [1,2,3,4,5]. Common methods developed for the synthesis of 2-alkoxyindolines generally involve cyclization reactions of tryptophol catalyzed by transition metals, such as Cu, Sc, Pd, Ru, and Ir, under harsh conditions, such as high temperatures [10,11,12,13,14,15], several approaches that utilize arylhydrizines [16,17], anilines [18,19], or spirocyclopropanes [20] have been reported. With the aim of developing synthetic approaches that utilize less toxic and more readily available reagents under mild conditions, we have been investigating photochemical reactions using organic photoredox catalysts, as they constitute environmentally friendly methods wherein light is applied as a traceless reagent, and harsh reaction conditions, such as high temperature and pressure, are not required. We have recently reported photoinduced electron transfer (PET)-promoted reactions of carboxylic acids [21,22,23,24,25,26,27], arylboronic acids [28], indene [29,30], and electron-rich alkenes [31] catalyzed by two-molecule organic photoredox catalysts, such as a combination of phenanthrene (Phen) as an electron donor and 1,4-dicyanobenzene (1,4-DCB) as an electron acceptor, under UV irradiation (313 nm)
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