The complexity of the drug structure poses a great challenge to modern catalysis because the modification of the structure seeks to preserve the efficacy of the drug while preventing the development of side effects. More and more new methods and reactions are being explored which, with good yields in the reaction, will allow the synthesis of new drug candidates with certain biological activity. Photoredox catalysis is a process in which transition metal complexes and corresponding ligands have the ability to convert visible light into chemical energy by electron transfer reaction with organic substrates. It has been described that visible light-mediated photoredox catalysis facilitates the formation of radical intermediates and thus enables rapid and easy late-stage functionalization of drug candidate synthesis. In this thesis, photoredox catalysis using different methods and catalysts such as iridium complexes with different ligands, has formed new C-C bonds and synthesized molecules that have already been described in the literature, but also new molecules that have not yet been synthesized that way. An expensive iridium catalyst has been successfully synthesized from inexpensive chemicals and its use in the alkylation of coumarins, compounds that are important substances in the pharmaceutical industry, has been described. The technologies used include reactions in a conventional reactor using a powerful LED lamp (blue visible spectrum), a liquid chromatography device UPLC-DAD-MS to monitor the progress of the reaction, and NMR to analyze the structure of the synthesized molecules. The photoredox catalysis methods described in this thesis can be applied on other heterocycles to synthesize and modify intermediates to obtain complex drug analogues.