We further analyzed the frequency of the substance use in the field of homogeneous catalysts (Figure ). Iodine is the most frequently used catalyst substance. Various studies suggest that iodine catalysis primarily happens by activation of reactant through halogen bonding. For example, iodine is used for oxidative cyclization and oxo-acyloxylation of alkenes and enol ethers. iron, 169 and copper 169 usually catalyze coupling reactions, C-H functionalization, asymmetric hydrogenation, and direct arylation with their metal-complex forms. Eosin, 171-174 iron, 169 and copper 169 usually catalyze coupling reactions, C-H functionalization, asymmetric hydrogenation, and direct arylation with their metal-complex forms. Eosin, tetrabutylammonium iodide, isophthalonitrile perform like iodine by creating radical species to initiate the reaction.