Heterogeneous graphitic carbon nitrides in visible-light-initiated organic transformations

In recent years, g-C3N4 photocatalyst-mediated organic reactions have represented an important mode of chemical transformations and are expected to become a crucial field at the forefront of organic chemistry. Polymeric g-C3N4 as a heterogeneous catalyst has attracted great attention compared to other metal-based catalysts in photocatalytic applications because of its cost-effectiveness, good chemical stability, no heavy metal pollution, and eco-friendliness. In recent years, aerobic oxidation under visible-light has been identified as an attractive and fast-developing field in synthetic chemistry due to the simple operating procedure, easy work up and high efficiency. This review highlights carbon nitride-catalyzed visible-light-promoted strategies for the oxidation of alcohols, synthesis of esters, phenols, and sulfoxides, reduction of nitro compounds, the formation of carbonyl compounds, and several coupling reactions. However, g-C3N4 demands high temperature and O2 pressure in addition to visible-light in the system to deliver the desired organic transformations. An alternative strategy may be to anchor organic dyes on the surface of heterogeneous catalysts for mild reaction conditions. The more challenging reaction is the reduction, since investigations related to the selective photoreduction of organic substrates are limited in comparison with photocatalytic selective oxidation; specifically, g-C3N4 is restricted to nitro compounds predominantly. Attempts can be made to enhance the selectivity in reduction by modifying surfaces, changing external conditions, forming composites, etc.