Dearomative (3+2) Cycloadditions between Indoles and Vinyldiazo Species Enabled by a Red‐Shifted Chromium Photocatalyst

Abstract

A direct dearomative photocatalyzed (3+2) cycloaddition between indoles and vinyldiazo reagents is described. The transformation is enabled by the development of a novel oxidizing CrIII photocatalyst, its specific reactivity attributed to increased absorptive properties over earlier Cr analogs and greater stability than Ru counterparts. A variety of fused indoline compounds are synthesized using this method, including densely functionalized ring systems that are feasible due to base‐free conditions. Experimental insights corroborate a cycloaddition initiated by nucleophilic attack at C3 of the indole radical cation by the vinyldiazo species.

Energy transfer photocatalysis: exciting modes of reactivity

“…Several subsequent works were reported exploiting a similar principle of exciting the core organic molecule followed by olefin trapping, and vice versa . 60–65 Parallelly, You and coworkers also reported a double dearomative intramolecular [4+2]-cycloaddition engaging an indole and a pendant naphthalene moiety. 66…”

Photoredox‐Enabled Self‐(3+2) Cyclization of Vinyldiazo Reagents: Synthesis of Cyclopentenyl α‐Diazo Compounds

“…The reaction initiates by single electron oxidation of electron‐rich alkenes to form radical cation intermediates, [16] while vinyldiazo compound acts as a nucleophile rather than its classical electrophilic behaviour in transition metal‐catalyzed cyclopentannulation reactions [17] . Recently, a dearomative (3+2) cycloaddition of indoles with vinyldiazo compounds using a red‐shifted chromium photocatalyst was also developed by the same group [18] . Although the direct photo‐activation of vinyldiazo reagents can be avoided, the conservation of their diazo functional group in the final products remains a challenge.…”

“…[17] Recently, a dearomative (3+2) cycloaddition of indoles with vinyldiazo compounds using a red-shifted chromium photocatalyst was also developed by the same group. [18] Although the direct photo-activation of vinyldiazo reagents can be avoided, the conservation of their diazo functional group in the final products remains a challenge. Herein, we report on a photocatalytic annulation reaction of vinyldiazo compounds to forge cyclopentenes, which involves an unprecedented single electron oxidation of vinyldiazo compounds (Scheme 1b).…”

Photocatalytic Boryl Radicals Triggered Sequential B─N/C─N Bond Formation to Assemble Boron‐Handled Pyrazoles

“…Later, Ferreira group further expanded the strategy to dearomative photocatalyzed [3+2]cycloaddition between indoles and vinyldiazo reagents using a novel oxidizing [Cr(PMP 2 phen) 3 ](BF 4 ) 3 photocatalyst. [5] Very recently, Zhou, Koenigs et al [6] realized the photocatalytic self-[3+2]-cycloaddition of vinyldiazo reagents leading to a variety of cyclopentenyl 𝛼-diazo compounds in good yields. Remarkably, Zhou group utilized vinyldiazo reagent as radical acceptor to realize its photochemical 1,3-difunctionalization, including 1,3-perfluoroalkyliodization, [7a] 1,3-selenosulfonylation and 1,3-diselenation (Scheme 1a, right).…”

“…Later, Ferreira group further expanded the strategy to dearomative photocatalyzed [3+2]‐cycloaddition between indoles and vinyldiazo reagents using a novel oxidizing [Cr(PMP 2 phen) 3 ](BF 4 ) 3 photocatalyst. [ 5 ] Very recently, Zhou, Koenigs et al. [ 6 ] realized the photocatalytic self‐[3+2]‐cycloaddition of vinyldiazo reagents leading to a variety of cyclopentenyl α‐diazo compounds in good yields.…”