efficient asymmetric alpha-alkylation of aldehydes. To the best of our knowledge, this is the first case to make a systematic survey on semiconductors in visible light driven asymmetric catalysis and will become an indispensable reference for the scientists in this area.Nanosized particles possess large specific surface area and could be well dispersed in solvent, thus being able to effectively promote the reaction process. [8] As proved by scanning electron microscope (SEM) imaging, all the purchased semiconductor powders are in nanosize to assure high efficiency, except for a contrast sample of bulk WO 3 sheets (Figure 1). It should be pointed out that all the semiconductors in Figure 1 are carefully selected. According to the only four published results, TiO 2 , [9] PbBiO 2 Br, [9,10] Bi 2 O 3 , [11] and Bi 2 S 3 [11] are recognized as the meaningful photocatalysts in asymmetric catalysis with the help of chiral ligand. All the reported semiconductors have a band gap of around 2.5 eV, characteristic of light absorption around 500 nm. Furthermore, we suppose that not only the band gap but also the redox potentials of semiconductors greatly influence the photocatalytic efficiency. Learning from homogeneous catalysis, when ruthenium (II), iridium (III) complexes or organic dyes [4] are used as photosensitizers in asymmetric photocatalysis, the oxidation potential is around 1 V while the reduction potential is around −1 V. Therefore, all the photosensitizers are chosen based on the above criteria. As shown in Figure 2, Co 3 O 4 exhibits a wide light absorption in the UV and visible region (Figure 2), which is well reflected by its black color (inset in Figure 1d). Both TiO 2 and ZnO only have absorption below 400 nm, implying their poor exploitation of visible light. In addition, CuO, Fe 2 O 3 , and WO 3 have good absorption capability toward visible light, and their centered peaks are 444, 408, and 352 nm, respectively (Figure 2). Owing to their different light absorption properties, these powders are different in colors seen by the naked eye (insets in Figure 1).The enantioselective alpha-alkylation of aldehydes is examined with a combination of the selected semiconductors and the second-generation Macmillan organocatalyst ((2R,5S)-2-tert-butyl-3,5-dimethylimidazolidin-4-one). Table 1 summarizes the experimental results, from which several conclusions are drawn.(1) Not all the semiconductors can effectively trigger the asymmetric reaction. ZnO (band gap is 3.2 eV, while the conduction band (CB) and valence band (VB) are −0.31 V and 2.89 V vs normal hydrogen electrode (NHE), respectively) [12] as the photosensitizer affords only 5% conversion likely because of its poor visible light absorption (Entry 1). Fe 2 O 3 (band gap is 2.2 eV, while CB and VB are 0.28 and 2.48 V vs NHE, respectively) [12] obtains 20% conversion similar to the reported To find cheap and commercially available materials that are efficient in the catalytic process is of paramount importance for practical application in both chemical and...