Visible-light-promoted stereoselective alkylation by combining heterogeneous photocatalysis with organocatalysis.

The application of sensitizers to utilize visible light for chemical reactions is an established method. Several recent publications have impressively demonstrated the versatile use of visible light for various transformations, such as the conversion of alcohols to alkyl halides, and [2+2], [3+2], and [4+2] cycloadditions as well as carbon– carbon and carbon–heteroatom bond formations. The combination of organocatalysis with visible-light photoredox catalysis using ruthenium or iridium complexes or organic dyes as photocatalysts allows for an expansion to enantioselective reactions. Although inorganic semiconductors, such as titanium dioxide, have been widely used for the photocatalytic degradation of organic waste, the number of examples in which they photocatalyze bond formation in organic synthesis is still limited. Kisch and co-workers explored CdS-mediated bond formations, and oxidative C C coupling reactions with titanium dioxide are known. However, bond formations on heterogeneous photocatalysts typically proceed without control of the stereochemistry and mixtures of isomers are obtained. We demonstrate herein that the combination of stereoselective organocatalysis with visible-light heterogeneous photoredox catalysis promotes the stereoselective formation of carbon–carbon bonds in good selectivity and yield. The approach combines the advantages of heterogeneous catalysis (robust, simple, and easy-toseparate catalyst material) with the stereoselectivity achieved in homogeneous organocatalysis. The enantioselective a-alkylation of aldehydes developed by MacMillan et al. was selected as a test reaction to apply inorganic heterogeneous photocatalysts (Table 1). Five semiconductors were used: commercially available white TiO2 (1), the same material surface-modified covalently with a Phos-Texas Red dye increasing the absorption of visible light (Phos-Texas-Red-TiO2, 2), yellow PbBiO2Br, which absorbs blue light, and PbBiO2Br as bulk material (3) and in nanocrystalline form (4). TiO2 (1) with an average particle size of 21 nm is a stable and inexpensive semiconductor with a band gap of 3.2 eV, but the unmodified powder absorbs only weakly up to 405 nm as a result of to defects and surface deposits. Its absorption range can be extended into the visible range by structure modification or surface modification with dyes. The Texas Red derived dye 10 (Scheme 1) was covalently anchored on TiO2 yielding 2, which absorbs at 560 nm (see the Supporting Information for the synthesis of 10 and the characterization of 2). PbBiO2Br 3 and 4 were prepared by different synthetic routes leading to different particle sizes of the semiconductors: PbBiO2Br bulk material 3 with a band gap of 2.47 eV was prepared by hightemperature solid-phase synthesis, while the nanocrystalline material 4 was obtained from synthesis in aqueous solution leading to an average calculated particle size of (28 6) nm and an optical band gap of 2.56 eV. Yellow CdS (5) has a band gap of 2.4 eV and was prepared as previously reported. Table 1: Enantioselective alkylations using MacMillan’s chiral secondary amine and inorganic semiconductors as photocatalysts.