Photocatalytic transformations catalyzed by inorganic semiconductors and iridium complexes

Chapter 1 provides an overview on the organic synthesis mediated by heterogeneous photocatalysts reviewing recent advances and discussing the underlying photocatalytic mechanisms. Heterogeneous photocatalysis is applicable for a wide range of organic transformations from oxidations and reductions to carbon-heteroatom and carbon-carbon bond forming reactions including enantioselective examples. The wide variety of accessible heterogeneous semiconductor photocatalysts with different redox potentials, photostability, availability and recyclability makes them suitable for synthetic applications. The available data of valence band and conduction band energies in different solvents are still limited, as well as the understanding of the detailed photocatalytic mechanisms. It is expected that with continuing progress in these different aspects of heterogeneous photoredox catalysis more applications for organic synthesis will be develop. Chapters 2 and 3 describe examples of applications of heterogeneous photocatalysis in organic synthesis. Chapter 2 reports the stereoselective bond formation combining heterogeneous photocatalysis and organocatalysis in α-alkylation of aldehydes and Aza-Henry reactions. Heterogeneous photocatalysis could definitely serve as an alternative to the previously reported homogeneous reactions using transition metal complexes or organic dyes giving the comparable yields and stereoselectivity. The conduction band energies of excited TiO2 and PbBiO2Br semiconductors are essential and sufficient for α-alkylation of aldehydes whereas excited CdS provides the required valence band energy for the Aza-Henry reaction. Chapter 3 discusses photocatalytic [4+2] cycloaddition reactions utilizing TiO2 and PbBiO2Br semiconductors as heterogeneous photocatalysts. The readily availability of the photocatalysts and their possible recyclability and easy separation from reactants are advantageous. However, the reaction requires long reaction times in aqueous media and the formation of anisaldehyde as a by-product limits the yield of the target cycloaddition product to 50%. Chapter 4 reports the dehalogenation of benzyl bromides under photocatalytic conditions. There are two possible mechanistic pathways of the photocatalytic reaction utilizing an Ir(III) complex as a photocatalyst leading to the same products: Either the photoinduced electron transfer to the bezyl bromide occurs directly from the excited state of the iridium complex or the photoreduced complex in its ground state acts as the reducing reagent. The short lifetime of the complex excited state disfavors the electron transfer from the excited state, although sufficient redox potential difference. Our investigations have shown that the excited state redox potential of iridium complexes can be engineered by suitable ligand design, but their difficult to predict lifetime is of equal importance for the use as photocatalyst