Heterogeneous Photocatalysis for Selective Oxidations with Molecular Oxygen

Photocatalysis, in its broader meaning, indicates catalytic processes initiated by light. The term is now largely used with reference to heterogeneous catalysis on semiconductor materials due to the enormous development of this discipline in wastewater decontamination or water splitting for the generation of hydrogen as a fuel. Related applications in organic syntheses have equally attracted large interest [1-14] already from the infancy of the studies on photoelectrocatalysis, thirty years ago [15]. Photocatalysis for synthetic purposes concerns the light-induced chemical transformations of organic or inorganic substrates that are transparent in the wavelength range examined. Radiation absorbed by a photocatalyst leads to electronically excited states that, if enough long-lived, can cause chemical reactions to take place. The overall process can be considered photocatalytic when i) the photoactive species is regenerated in its initial state at the end of a reaction cycle, as happens in thermal catalysis; ii) the photocatalyst is consumed less than in stoichiometric amounts, while light is a stoichiometric reagent. Albini et al. report that the reaction path in photocatalysis involves the lowest potential energy surface at any configuration of the reagent, as it occurs in thermal reactions and contrary to direct irradiation or photoinduced energy transfer processes where part of the path occurs through excited-state surface [8]. Photocatalysis is particularly relevant to “sustainable and green chemistry” by virtue of the possibility to obtain fine chemicals with a low environmental impact. In fact, photochemical reactions require milder conditions than thermal processes as they involve sunlight as a completely renewable energy source. Additionally, side processes are often minimal because of mechanistic pathways involving short and efficient reaction sequences. In the first part of this contribution, starting with the very basic principles that govern the primary photochemical steps, we proceed to examine the mechanisms of subsequent activation processes of both the substrate and molecular oxygen. The second part mainly aims to present representative synthetic applications of photocatalysis. Some of the articles considered for this purpose are also mentioned in the first part that focuses on mechanistic aspects. We show that organized assemblies can drive the photochemical processes as well as the subsequent thermal reactions in order to control efficiency and selectivity of the oxidation process. The more significant data on conversion and productivity are reported in Tables or mentioned in the text.