Preparation of Carbazoles Involving 6π‐Electrocyclization, Photoredox‐, Electrochemical‐, and Thermal Cyclization Reactions: Mechanistic Insights

Abstract

Carbazole is a heterocyclic motif that can be found in a diverse array of natural and unnatural products displaying a wide range of biological and physiological properties. Furthermore, this heterocycle is part of electronic materials like photoconducting polymers and organic optoelectronic materials owing to its excellent photophysical characteristics. Consequently, the development of synthetic strategies for carbazole scaffolds holds potential significance in biological and material fields. In this regard, a variety of preparation methods has been developed to exploit their efficient and distinct formation of new C−C and C‐heteroatom bonds under mild conditions and enabling broad substrate diversity and functional group tolerance. Therefore, this review focuses on the synthesis of a set of carbazole derivatives describing a variety of methodologies that involve direct irradiation, photosensitization, photoredox, electrochemical and thermal cyclization reactions.

Synthesis of pyrazino[1,2-b]indazoles via cascade cyclization of indazole aldehydes with propargylic amines

“…Based on previous reports 16,18 and control experiments, a plausible reaction mechanism was proposed, as shown in Scheme 4. According to the proposed mechanism, at first, the reaction of 1a with 2a yields the imine intermediate A .…”

“…The 6π electrocyclization, which involves the closure of conjugated π–π or p–π systems through a cyclic electron shift, represents a classical and versatile approach for synthesizing heterocycles with diverse functional groups. 16 We envisioned that the utilization of 6π electrocyclization, and the cascade strategy for one-pot, tandem cyclization to access pyrazino[1,2- b ]indazole derivatives would be feasible. To the best of our knowledge, there has been no report on a 6π electrocyclization-based tandem cyclization strategy for the synthesis of pyrazino[1,2- b ]indazoles.…”

Perfluoroalkylation of Triarylamines by EDA Complexes and Ulterior Sensitized [6π]‐Electrocyclization to Perfluoroalkylated Endo‐Carbazoles. Mechanistic and Photophysical Studies

“…Having failed in the perfluororalkylation reaction of 9-phenylCBz either with the EDA complex or with Eosin Y as photocatalyst, we proceeded to attempt the [6π]-electrocyclization of the perfluoroalkylated TPAs (i. e.: R F À TPA) in order to obtain the perfluoroalkylated carbazole derivatives. [40] A series of optimization reactions were carried out by direct irradiation, such as those illustrated in Table 4.…”

“…Photocatalytic methods (as shown in Scheme 3A, left), [37] direct UV-irradiation protocols (Scheme 3B, right), [38] and electrochemical techniques [39] are among the available strategies. [40] In the photocatalytic methods, complexes of copper (e. g., Cu(Xantphos)(dmp) and propylene oxide) or iron (e. g., Fe(phen) 3 (NTf 2 ) 2 /O 2 ) serve as photocatalysts under visible light irradiation. [37] These reactions often require extended reaction times, typically exceeding 20 hours, even when performed under flow conditions.…”

“…However, the use of low-intensity light sources like 254 nm may raise environmental concerns, and the excited state behavior of TPA can deactivate through various channels other than photocyclization through the known triplet manifold. [38,40] While perfluoroalkylated carbazoles have been synthesized through coupling reactions of carbazoles or carbazole precursors with perfluoroalkylated aryl rings, [42] direct perfluoroalkylation of the carbazole nucleus remains relatively unexplored.…”