Fullerene C₇₀as Photoredox Catalyst for the Synthesis of Pyrrolo[2,1‐a]isoquinolines by 1,3‐Dipolar Cycloaddition‐Aromatization Sequence

Abstract

Herein, fullerene (C 70 ) is introduced as an effective photoredox catalyst for the construction of a highly functionalised pyrrolo[2,1-a]isoquinoline framework by 1,3-dipolar cycloaddition-aromatisation reaction sequence. The ability of C 70 to efficiently harvest visible light, its long-triplet state lifetime, good photostability, and strong singlet oxygen generation potential (Φ ~� 1), make it an efficient photoredox catalyst. Upon photoirradiation, C 70 promotes the formation of singlet oxygen and superoxide radical by energy transfer (EnT) and single electron transfer (SET) mechanism. The superoxide radical acts as a potential oxidant in the formation of azomethine ylide through the oxidation-deprotonation tandem process. Azomethine ylide further participates in [3 + 2]-cycloaddition reaction protocol with alkene/alkyne to give the corresponding pyrrolo[2,1-a]isoquinolines. Interestingly, this protocol allows the activation of a wide range of substrates giving access to a diverse library of 48 welldecorated pyrrolo[2,1-a]isoquinolines with good functional group tolerance.

Photochemical Carbocatalysis: Fullerene‐, Carbon Nanotube‐ or Graphene‐Based Metal‐Free Photocatalysts for Organic Transformations

“…In this frame, the development of novel methods for the large production of fullerene members, beyond the well-established C 60 , could trigger the research on the field of photocatalysis through the supply of new potential catalysts. This aspect has been supported by the recent observed increasing number of studies focused on the photocatalytic ability of C 70 , [63,65] since such kind of fullerenes are not still exotic academic materials, but can be obtained by the market.…”

“…In a recent study, pyrrolo[2,1-a]isoquinoline derivatives were synthesized through the use of C 70 as the photocatalyst under blue light irradiation. [65] The reaction performed via a [3 + 2] cycloaddition between N-substituted tetrahydroisoquinolines and maleimides, followed by aromatization with N-bromosuccinimide (NBS) to produce the desired products. As an alternative approach towards this synthetic goal, electron-deficient alkynes were used, instead of maleimides (Scheme 5a).…”