Photocatalytic Indole Diels–Alder Cycloadditions Mediated by Heterogeneous Platinum-Modified Titanium Dioxide

Abstract

Indole alkaloids represent an important class of molecules, with many naturally occurring derivatives possessing significant biological activity. One area that requires further development in the synthesis of indole derivatives is the Diels–Alder reaction. In this work, we expand on our previously developed heterogeneous protocol for the [4+2] cycloaddition of indoles and electron-rich dienes mediated by platinum nanoparticles supported on titanium dioxide semiconductor particles (Pt(0.2%)@TiO2) with visible-light irradiation. This reaction proceeds with broad scope and is more efficient per incident photon than the previous homogeneous method, and the catalyst can be easily recycled and reused.

Recent Developments in Photo‐Catalyzed/Promoted Synthesis of Indoles and Their Functionalization: Reactions and Mechanisms

“…In addition, some homogeneous photocatalysts easily suffer from photobleaching under light irradiation, resulting in the decrease of catalytic activities. In this context, heterogeneous photocatalysts including semiconductors (TiO 2 , CdS) and conjugate polymers are investigated in recent photocatalysis for the synthesis of indole derivatives [128,130,133,197] . Heterogeneous photocatalysts have the advantages of recyclability, low cost, structural diversity, and high chemical stability, which are promising alternatives to homogenous photocatalysts.…”

“…In this context, heterogeneous photocatalysts including semiconductors (TiO 2 , CdS) and conjugate polymers are investigated in recent photocatalysis for the synthesis of indole derivatives. [128,130,133,197] Heterogeneous photocatalysts have the advantages of recyclability, low cost, structural diversity, and high chemical stability, which are promising alternatives to homogenous photocatalysts. In all, we hope that this review will help to increase the interest and understanding in the photo-catalyzed-/promoted synthesis of indole and its derivatives for the researchers.…”

“…Yoon and co‐workers discovered a heterogeneous photocatalytic method for the Diels‐Alder reaction of indoles and electron‐rich dienes to generate a series of indole alkaloids under mild conditions using Pt(0.2%)@TiO 2 semiconductor nanoparticles as photocatalyst (Scheme 84). [128] In previous reports, these targeted products exhibit extensive chemical diversity and strong biological activity [129] . As for the reaction mechanism, firstly, indole is adsorbed on the surface of TiO 2 to form a complex with an absorption that extends into the visible region.…”

Deal;Photoredox Catalysis for the Cycloaddition Reactions

“…In 2017, Yoon and coworkers developed a heterogeneous photocatalytic [4 + 2] cycloaddition system for the synthesis of indole alkaloids by using titanium dioxide semiconductor particles (TiO 2 ) doped with platinum nanoparticles under the irradiation of visible light. [80] A variety of indoles and cis-1,3dienes performed well under this condition (Scheme 28). In addition, compared to their previous homogeneous photocatalytic system, this reaction was found to be more efficient.…”

19 Heterogeneous Photocatalysis in Organic Synthesis

“…Platinum-doped titanium dioxide (Pt(0.2 %)@TiO 2 ) was shown to double the photon efficiency in comparison to undoped TiO 2 , and a range of protected tetrahydrocarbazoles (20) was synthesized in moderate to good yield. [16] During a recycling study, a decrease in the activity of Pt(0.2 %)@TiO 2 was observed. The authors hypothesized that surface poisoning from organic compounds, most likely derived from indole, is responsible for catalyst deactivation.…”