Visible-Light-Induced Dearomatization of Indoles/Pyrroles with Vinylcyclopropanes: Expedient Synthesis of Structurally Diverse Polycyclic Indolines/Pyrrolines

Abstract

Visible-light-induced cycloaddition reactions initiated via energy-transfer processes have recently evolved as powerful methods for the construction of strained cyclic molecules that are not easily accessed using known ground-state synthetic methods. Particularly, the reactions initiated by the excitation of aromatic rings provide an alternative solution to the direct transformations of aromatic feedstocks under the scheme of dearomatization. Vinylcyclopropanes (VCPs) are well-known reagents in radical clock experiments, working as a probe to detect transient radical intermediates. However, the synthetic applications in this regard still remain limited due to uncontrollable selectivities. Herein, we report visible-light-induced dearomatization of indole-or pyrrole-tethered VCPs, in which several competitive reaction pathways, including [5 + 2], [2 + 2], interrupted [5 + 2], and [5 + 4] cycloadditions, can be well regulated by engineering substrate structures and tuning reaction conditions. The reaction mechanism has been explored by combined experimental and computational investigations. These reactions provide a convenient method to synthesize structurally diverse polycyclic molecules with high efficiency and good selectivity.

Visible-light induced dearomatization reactions

“…In addition, You and co-workers employed O-methyl oxime, alkyne, naphthalene and vinylcyclopropane as the biradical acceptors to react with the indole or pyrrole derivatives (Scheme 63). [111][112][113][114] Interestingly, structurally diverse polycyclic compounds were accessible via [5+2], [2+2] and [5+4] cycloadditions which were well regulated by engineering substrate structures and tuning reaction conditions in their latest work. In 2020, the Koenig group disclosed that a highly efficient, cheap and organic photosensitizer 1,2-bis(carbazol-9-yl)-4,5dicyanobenzene (2CzPN), which is alternative to the commonly used iridium complexes, could be applied in dearomative [2+2] cycloaddition of naphthols and indoles.…”

Visible-light-mediated defluorinative cyclization of α-fluoro-β-enamino esters catalyzed by 4-CzIPN

“…Moreover, a series of inhibition experiments were conducted to confirm the EnT processes for this reaction. The reaction of 1a in the presence of a known triplet quencher, 2,5-dimethylhexa-2,4-diene, was partly inhibited, indicating the involvement of triplet intermediates 23 (Scheme 2a). On the other hand, when the electron sacrificial agent i Pr 2 NEt or radical scavenger 2,2,6,6-tetramethyl piperidin-1-oxyl (TEMPO) was added into the reaction system, the reaction is likely to be initiated by the SET process from i Pr 2 NEt or TEMPO to 4CzIPN* rather than the EnT process between 1a and 4CzIPN* (Schemes 2b and c).…”

Post-spin crossing dynamics determine the regioselectivity in open-shell singlet biradical recombination

“…1b). 12 In this reaction, the indole-derived substrates can be excited via an energy transfer mechanism 13 under the irradiation of visible light with a properly chosen photosensitizer. The resulting triplet indole derivatives undergo intramolecular cyclization at the C2 position with the VCP moiety.…”