The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp2 or sp3 C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Photoredox-Catalyzed C-H Functionalization Reactions.

In industries and academic laboratories, several late transition metal-catalyzed prerequisite reactions are widely performed during single and multistep synthesis. However, besides the desired products, these reactions lead to the generation of numerous chemical waste materials, by-products, hazardous gases, and other poisonous materials, which are discarded in the environment. This is partly responsible for the creation of global warming, resulting in climate adversities. Thus, the development of environmentally benign, cheap, easily accessible, and earth-abundant metal catalysts is desirable to minimize these issues. Certainly, iron is one of the most important metals belonging to this family. The field of iron catalysis has been explored in the last two-three decades out of its rich chemistry depending on its oxidation states and ligand cooperation. Moreover, this field has been enriched by the promising development of iron-catalyzed reactions namely, C–H bond activation, including organometallic C–H activation and C–H functionalization via outer-sphere pathway, cross-dehydrogenative couplings, insertion reactions, cross-coupling reactions, hydrogenations including hydrogen borrowing reactions, hydrosilylation and hydroboration, addition reactions and substitution reactions. Thus, herein an inclusive overview of these reaction have been well documented.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

N,N-Dimethylformamide (DMF) is frequently used as an aprotic solvent in chemical transformations in laboratories of academia as well as in those of chemical industry. In the present review, we will reveal that DMF is actually something much more than a solvent. It is a unique chemical since, as well as being an effective polar aprotic solvent, it can play three other important roles in organic chemistry. It can be used as a reagent, a catalyst, and a stabilizer.

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Beyond a solvent: triple roles of dimethylformamide in organic chemistry

Visible light promoted cross-dehydrogenative coupling: a decade update

Visible light-induced C–H sulfenylation using sulfinic acids

The visible light-mediated C-3 alkoxylation of imidazopyridines with alcohols has been achieved using rose bengal as an organic photoredox catalyst at room temperature. Widely abundant air acts as the terminal oxidant that avoids the use of a stoichiometric amount of peroxo compounds. A wide range of functional groups could be tolerated under the reaction conditions to produce C(sp2)–H alkoxylated products in high yields.

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Visible Light Organic Photoredox-Catalyzed C–H Alkoxylation of Imidazopyridine with Alcohol

An Fe(III)-catalyzed efficient dicarbonylation of imidazoheterocycles has been developed through cross-dehydrogenative coupling between imidazoheterocycles and oxoaldehydes under ambient air in high yields. The present protocol is also applicable to indolizines. Imidazopyridine produced bisimidazopyridine with arylaldehyde. Experimental results suggest that the reactions proceed through the nonradical pathway.

FeCl3-Catalyzed Cross-Dehydrogenative Coupling between Imidazoheterocycles and Oxoaldehydes

(Diacetoxy)iodobenzene (PIDA)-mediated direct oxidative C-H amination for the synthesis of 3-amino substituted imidazopyridines has been achieved under metal-free conditions at room temperature in short reaction times. This methodology is also applicable for the regioselective amination of indolizines. Experimental results suggest that the reaction likely proceeds through a radical pathway.

Sadhanendu Samanta

Papers

Visible Light Organic Photoredox-Catalyzed C–H Alkoxylation of Imidazopyridine with Alcohol

FeCl3-Catalyzed Cross-Dehydrogenative Coupling between Imidazoheterocycles and Oxoaldehydes

(Diacetoxy)iodobenzene-Mediated Oxidative C–H Amination of Imidazopyridines at Ambient Temperature

Aminomethylation of Imidazoheterocycles with Morpholine.

Organophotoredox-Catalyzed Direct C–H Amination of 2H-Indazoles with Amines