Photocatalyst-free aryl halides reduction using visible light


Photochemistry concerns the chemical effects of light; more so, the chemical reaction caused by absorption of ultraviolet, visible light and infrared radiation. In particular, visible light-driven photochemical methods have proven to be valuable tools that enable achieve various challenging organic transformations. In fact, it is well documented that a variety of metal complexes, organic dyes, and inorganic semiconductors exhibit efficiency in photoredox reactions through visible light excitation. However, a single visible photon (≤300 kJ mol-1) does not provide sufficient energy for the activation or dissociation of many strong chemical bonds such as aryl-Br, aryl-Cl, C-O, and C-H bonds.

Pyrrole and its derivatives are an important class of heterocyclic compounds, containing pyrrole rings, which are widely distributed in many biologically active natural products and pharmaceuticals. Literature has it that besides the extensive biological and organic synthetic interest in pyrrole derivatives, polymers derived from pyrrole and its derivatives are capable of photochemical polymerization via photoinduced disproportionation processes under visible light illumination. Despite extensive research studies in photochemical polymerization of pyrrole and its derivatives, the application of radical-anion species of pyrrole during C-H activation using visible light has less unexplored.

To bridge this gap, researchers from the Department of Chemistry at Syracuse University in New York: Dr. Zhi-Jun Li, Elan Hofman, Andrew Hunter Davis and Professor Weiwei Zheng, in collaboration with Shuya Li and Professor Gyu Leem from the Department of Chemistry at the State University of New York College of Environmental Science and Forestry investigated that pyrrole derivatives are possibly responsible for the initiation of aryl bromide and chloride reduction, specifically, the photoreduction of aryl halides through a detailed study of alkyl halide arylation with pyrrole derivatives in the absence of any photocatalysts or additives. Their work is currently published in the research journal, Green Chemistry.

The research team proposed a photoinduced disproportionation approach without the addition of any photocatalysts or additives to afford radical anions of pyrrole derivatives, which have enough reduction power to transfer an electron to aryl halide, giving rise to the corresponding aryl radical to afford the desired C–H arylated heterocyclic product. They demonstrated that the photoinduced disproportionation process of pyrrole derivatives led to radical anion pyrrole derivatives, which acted as strong reduction species to undergo single electron transfer (SET) to an aryl halide, producing an aryl radical after carbon–halogen bond cleavage. Subsequently, the capture of aryl radicals by pyrrole derivatives produced C–H arylated heterocyclic products. The team further noted that products generated in situ open up the possibility to follow the photoinduced disproportionation pathway as well, to transfer electrons to aryl halides, thereby accelerating the conversion.

“This new approach provides a new radical initiation mode using aryl halides through a photochemical approach that is highly desirable in organic synthesis, materials, and life sciences.” Zheng told Advances in Engineering.

In summary, the study by Professor Weiwei Zheng and colleagues presented a novel radical initiation mechanism via photoinduced disproportionation approach for the photoreduction of relatively inactive aryl bromides and chlorides. The proposed photoinduced disproportionation pathway offers new photocatalytic insights to show that the energy absorbed from visible light allows to drive transformations that were traditionally believed to be challenging, while also enriching the understanding of the photoreduction mechanism of many relevant reactions.

Photocatalyst-free aryl halides reduction using visible light - Advances in Engineering

About the author

Dr. Zhi-Jun Li obtained a Ph.D. degree in 2013 from the Technical Institute of Physics and Chemistry, CAS (China) under the direction of Prof. Li-Zhu Wu and Prof. Chen-Ho Tung. After he completed his Ph.D., he worked as a Research Scientist in Prof. Li-Zhu Wu and Prof. Chen-Ho Tung’s Lab from 2013 to 2016. His research was mainly focused on the synthesis and assembly methodologies for nanocrystal-based hybrid catalysts and their application for solar energy conversion. In 2017, he jointed Prof. Weiwei Zheng group in the Department of Chemistry at Syracuse University (USA) as a postdoctoral fellow.

His research focuses on the functional one-dimensional and two-dimensional semiconductor nanocrystals, and their application in optoelectronics and photocatalysis.

About the author

Dr. Gyu Leem earned a B.S. degree in Chemical Engineering from Hanyang University, Seoul, Korea and a Ph. D. degree in Chemistry from University of Houston, Houston, Texas in the US. Currently, he is an assistant professor in the Department of Chemistry at State University of New York College of Environmental Science and Forestry (SUNY ESF) starting Fall 2018. Prior to join the SUNY ESF, he worked as a PostDoc and research assistant professor at the University of North Carolina (UNC) Energy Frontier Research Center (EFRC) for Solar Fuels and the University of Texas at San Antonio.

His current research interests are in designing and synthesizing macromolecular-based chromophore-catalyst assemblies capable of light harvesting, charge-separation, and charge storage for solar energy and biomass conversion. Previously, he was a previous editorial board member in Frontiers in Energy Research, a newsletter published by the U.S. Department of Energy. He is now invited to serve as a topic editor in Frontiers in Chemistry and member on the editorial advisory board of ACS Applied Nano Materials.

About the author

Dr. Weiwei Zheng is an Assistant Professor in the Department of Chemistry at Syracuse University. He received his doctorate degree in Inorganic Chemistry from the Florida State University in 2011 under the supervision of Geoffrey F. Strouse. His graduate work focused on developing a fundamental understanding of the magnetic properties of transition metal ion-doped semiconductor quantum dots. In 2012, he joined the Emory University as a postdoctoral fellow to work with Khalid Salaita on the properties of hybrid organic/inorganic interfaces and biological applications of semiconductor nanocrystals. Zheng began his current appointment at Syracuse University in 2015.

His research interests include the synthesis and properties of novel functional nanomaterials for emerging applications in renewable energy and biological systems. He is particularly interested in the optical, electronic and magnetic properties of transition metal ion doped nanomaterials, selective surface functionalization of nanoparticles, hybrid nanocomposites, and the assembly of nanoparticle superlattices.


Zhi-Jun Li, Shuya Li, Elan Hofman, Andrew Hunter Davis, Gyu Leem, Weiwei Zheng. Visible-light induced disproportionation of pyrrole derivatives for photocatalyst-free aryl halides reduction. Green Chemistry, 2020, volume 22,1911.

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