Harnessing Visible/UV Light for the Activation and/or Functionalization of C-H Bonds: Metal- and Photocatalyst-Free Approach.

Photosynthesis in plants has inspired photochemical reactions in organic chemistry. Synthetic organic chemists always seek cost-effective, operationally simple, averting the use of toxic and difficult-to-remove metallic catalysts, atom economical, and high product purity in organic reactions. In the last few decades, the use of light as a catalyst in organic reactions has increased exponentially as literature has exploded with examples, particularly by using toxic and expensive metal complexes, photosensitizers like organic dyes, hypervalent iodine, or by using inorganic semiconductors. In this report, we have selected a few interesting examples of photochemical reactions performed without using any metallic catalyst or photosensitizers. These examples use the inherent potential of reactants to utilize light energy to initiate chemical reactions. Our main emphasis is to highlight the structural features in the reactants that can absorb light energy or form an electron donor-acceptor (EDA) complex during the reaction to initiate the photochemical reaction. Considering the high degree of variability in the photochemical reactions, the utmost care has been taken to present the most accurate reaction conditions. A short introductory section on photochemical reactions will act as an anchor that will revolve around the examples discussed and explain the underlying principle of the photochemical reaction mechanism.