Semiconductor Photocatalysis for Chemoselective Radical Coupling Reactions

Abstract

Photocatalysis at semiconductor surfaces is a growing field of general photocatalysis because of its importance for the chemical utilization of solar energy. By analogy with photoelectrochemistry the basic mechanism of semiconductor photocatalysis can be broken down into three steps: photogenerated formation of surface redox centers (electron-hole pairs), interfacial electron transfer from and to substrates (often coupled with proton-transfer), and conversion of primary redox intermediates into the products. Sun driven water cleavage and carbon dioxide fixation are still in the state of basic research whereas aerial degradation reactions of pollutants have reached practical application for the cleaning of air. In addition, a great variety of organic transformations (not syntheses) have been reported. They include cis-trans isomerizations, valence isomerizations, cycloaddition reactions, intramolecular or intermolecular C-N and C-C couplings, partial oxidations, and reductions. In all cases, well-known products were formed but very rarely also isolated. As compared to conventional homogeneous organic synthesis, the photocatalytic reaction mode is of no advantage, although the opposite is quite often claimed in the literature. It is also noted that a high quantum yield does not implicate a high product yield, since it is measured at very low substrate conversion in order to minimize secondary photoreactions. That is especially important in semiconductor photocatalysis since photocorrosion of the photocatalyst often prevents long-time irradiation, as is the case for colloidal metal sulfide semiconductors, which in general are photochemically too unstable to be used in synthesis. In this Account, we first classify the numerous organic photoreactions catalyzed by semiconductor powders. The classification is based on easily obtainable experimental facts, namely the nature of the light absorbing reaction component and the reaction stoichiometry. Next we discuss the problem of quantitative comparisons of photocatalytic activities or apparent quantum yields and propose a basic three-step mechanistic model. Finally, we address the question whether or not the unique photoredox properties of simple inorganic semiconductor powders may lead to previously unknown visible light induced organic syntheses. For that, we summarize novel radical C-C- and C-N- couplings photocatalyzed by self-prepared cadmium sulfide powders. Electron acceptor and donor substrates like imines or 1,2-diazenes, and cyclic olefins or unsaturated ethers, respectively, undergo a linear addition reaction. The hitherto unknown products have all been isolated in good to moderate yields and may be of pharmaceutical interest. In the first reaction step photogenerated electron-hole pairs produce through proton-coupled electron transfer the corresponding radicals. Their subsequent chemoselective heterocoupling affords the products, correlating with an insertion of the imine or 1,2-diazene into an allylic C(sp)-H bond of the donor substrate. In the absence of an i...

Metal Sulfide Photocatalysis: Visible‐Light‐Induced Organic Transformations

“…The conduction band minimum potential of CdS is À 0.9 V (vs. normal hydrogen electrode, NHE), [23] more negative than the reduction potential for H 2 evolution. Therefore, without the presence of O 2 as e cb À acceptors, homo-coupling of benzyl alcohols (Scheme 4a) and benzyl amines (Scheme 4b), heterocoupling between benzyl alcohols and benzyl amines (Scheme 4c) occurs on polycrystalline CdS, accompanied with the release of H 2 .…”

Synthesis, Characterization, and Photocatalytic Application of Type‐II CdS/Bi2W2O9 Heterojunction Nanomaterials towards Aerobic Oxidation of Amines to Imines

“…However, the Palmisano and Bahnemann groups have successfully conducted the partial oxidation of alcohol to aldehyde, with excellent selectivity, over the surface of TiO 2 and Bi 2 WO 6 photocatalysts . Recently, many improved heterogeneous photocatalytic routes have been developed for the selective oxidation of alcohol, reduction of nitro compounds, olefin–imine addition, radical coupling reactions, Suzuki coupling reactions, and the synthesis of polymers and fine chemicals . The oxidation of amines to imines is one of the most important reactions, since imines are used as building blocks for many pharmaceutical and organic‐dye molecules .…”

Organic semiconductor photocatalyst can bifunctionalize arenes and heteroarenes

“…Organic semiconductor materials are photoand chemically stable toward otherwise reactive radicals and nucleophiles and have a suitable bandgap between valence band maxima and conduction band minima (12,13) for controlled oxidation and reduction of many practical substrates. Light absorption by heterogeneous semiconductor photocatalysts generates surface redox centers as electron-hole pairs (14)(15)(16). As such, a semiconductor photocatalyst, upon photoexcitation, accomplishes two aligned redox transformations on the same particle surface (14)(15)(16), whereas a molecular photocatalyst, after electron transfer to one reaction partner, completes the overall redox process through a subsequent redox reaction of the oxidized or reduced catalyst.…”

“…Light absorption by heterogeneous semiconductor photocatalysts generates surface redox centers as electron-hole pairs (14)(15)(16). As such, a semiconductor photocatalyst, upon photoexcitation, accomplishes two aligned redox transformations on the same particle surface (14)(15)(16), whereas a molecular photocatalyst, after electron transfer to one reaction partner, completes the overall redox process through a subsequent redox reaction of the oxidized or reduced catalyst. In the latter case, the reactive catalyst intermediate may also engage in unwanted chemical reactions, leading to catalyst decomposition (4,17).…”