An Oxidant- and Catalyst-Free Synthesis of Dibenzo[a,c]carbazoles via UV Light Irradiation of 2,3-Diphenyl-1H-indoles

Abstract

An efficient methodology for the synthesis of dibenzo[a,c]carbazoles via annulation of 2,3-diphenyl-1H-indoles in EtOH under UV-light irradiation (λ = 365 nm) along with hydrogen evolution is described. This method exhibits the advantages of mild reaction conditions, no requirement of any oxidants and catalysts, and release of H2 as the only byproduct. Notably, the mechanism investigation confirms that the trans-4b,8a-dihydro-9H-dibenzo[a,c]carbazole intermediate could convert to cis-4b,8a-dihydro-9H-dibenzo[a,c]carbazole, which relies on the nitrogen atom of the indole ring. Followed by the intramolecular dehydrogenation yielded the dibenzo[a,c]carbazoles.

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

“…A new and interesting photocyclization method for the preparation of a set of dibenzo[a,c]carbazole derivatives has been achieved successfully upon irradiation of substituted 2,3diphenyl-1H-indoles in ethanol with light of 366 nm at room temperature under inert atmosphere along with evolution of hydrogen. [12] This methodology was found to be completely different from previously reported methods and did not require the use of any oxidants and expensive transition-metal catalysts. [13] The photoreaction proceeded smoothly tolerating different electron-donor and electron-withdrawing substituents attached to the phenyl groups as well as heterocyclic compounds (thiophenyl and furanyl rings) replacing one of the Ivan E. Romero obtained his degree in chemistry in 2015 at the Universidad Central de Venezuela.…”

“…A new and interesting photocyclization method for the preparation of a set of dibenzo[ a,c ]carbazole derivatives has been achieved successfully upon irradiation of substituted 2,3‐diphenyl‐1 H ‐indoles in ethanol with light of 366 nm at room temperature under inert atmosphere along with evolution of hydrogen [12] . This methodology was found to be completely different from previously reported methods and did not require the use of any oxidants and expensive transition‐metal catalysts [13] .…”