Photochemistry in synthesis: Where, when, and why

A series of photochemical reactions are assessed under the environmental aspect by using Eissen and Metzger's EATOS (environmental assessment tool for organic syntheses) method and are compared with strictly analogous thermal processes. These include C-C bond-forming reactions (arylation and alkylation) and selective oxidation and reduction reactions. In most cases, the photochemical method is experimentally simpler and less expensive than the thermal alternative. A disadvantage is that photochemical reactions are carried out in rather dilute solution, and this factor gives by far the main contribution to the assessment. However, if the solvent is recovered, the photochemical reaction is more environment-friendly.