The nature of the excited state renders the development of chiral catalysts for enantioselective photochemical reactions a considerable challenge. The absorption of a 400 nm photon corresponds to an energy uptake of approximately 300 kJ mol(-1) . Given the large distance to the ground state, innovative concepts are required to open reaction pathways that selectively lead to a single enantiomer of the desired product. This Review outlines the two major concepts of homogenously catalyzed enantioselective processes. The first part deals with chiral photocatalysts, which intervene in the photochemical key step and induce an asymmetric induction in this step. In the second part, reactions are presented in which the photochemical excitation is mediated by an achiral photocatalyst and the transfer of chirality is ensured by a second chiral catalyst (dual catalysis).

Six 2-quinolones, which bear a terminal alkene linked by a three- or four-membered tether to carbon atom C4 of the quinolone, were synthesized and subjected to an intramolecular [2 + 2]-photocycloaddition. The reaction delivered the respective products in high yields (78-99%) and with good regioselectivity in favor of the straight isomer. If conducted in the presence of a chiral hydrogen-bonding template (2.5 equiv) at low temperature in toluene as the solvent, the reaction proceeded enantioselectively (83-94% ee). An organocatalytic reaction was achieved when employing a chiral hydrogen-bonding template with an attached sensitizing unit (benzophenone or xanthone). The xanthone-based organocatalyst proved to be superior as compared to the respective benzophenone. Closer inspection revealed that the reaction of 4-(pent-4-enyloxy)quinolone leading to a six-membered ring, annelated to the cyclobutane, was less enantioselective (up to 41% ee with 30 mol % catalyst) than the reaction of 4-(but-3-enyloxy)quinolone leading to a five-membered ring (90% ee with 5 mol % and 94% ee with 20 mol % catalyst). Photophysical data (emission spectra, laser flash photolysis experiments) proved that the latter photocycloaddition was significantly faster, supporting the idea that the dissociation of the substrate from the catalyst prior to the photocycloaddition is responsible for the decreased enantioselectivity. Under optimized conditions, employing 10 mol % of the xanthone-based organocatalyst at -25 °C in trifluorotoluene as the solvent, three of the other four substrates gave the intramolecular [2 + 2]-photocycloaddition products with high enantioselectivities (72-87% ee). In all catalyzed reactions, the yields based on conversion were moderate to good (40-93%).

Intermolecular [2+2] photocycloadditions represent the most versatile and widely applicable of photochemical reactions. For the first time, such intermolecular reactions have been carried out in a catalytic fashion using a chiral triplet sensitizer, with high enantioselectivity (up to 92% ee). The low catalyst loading (2.5-5 mol%) underlines the high efficiency of the process both in terms of reaction acceleration and differentiation of the enantiotopic faces of the substrate. The substrate is anchored to the chiral catalyst through noncovalent interactions (hydrogen bonds), thus providing a chiral environment in which the enantioselective photocycloaddition proceeds. The densely functionalized products present numerous possibilities for further synthetic transformations.

A proof of principle is presented for the rational design of metal clusters functionalized with hydrophilic, chiral ligands. A colloidal method is used to prepare "unprotected" metal clusters of well-defined size that are subsequently functionalized in a separate step with hydrophilic, chiral ligands. As clusters from the same batch are functionalized with different organic molecules while the cluster size is maintained, the approach allows for systematic investigations and the differences in the observed properties to be related to the influence of the functionalizing ligand. Within this work cysteine and two cysteine derivatives (glutathione and N-acetyl-cysteine) are used as functionalizing ligands for Pt clusters. The materials are characterized using various methods allowing for the determination of ligand coverage, binding mode and chiro-optical properties. Finally, 2-butanone hydrogenation is used as a simple model reaction to demonstrate that these systems exhibit the potential to be used as asymmetric, heterogeneous catalysts. The observed differences in selectivity and reactivity are discussed based on the knowledge gained from the characterization.

The intramolecular [2+2] photocycloaddition of four 4-(but-3-enyl)oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH2, Z-CHEt, E-CHEt, CMe2) and two 3-(but-3-enyl)oxyquinolones (substitution pattern: CH2, CMe2) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83-95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z- or E-substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio- and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which-in contrast to simple alkoxyquinolones such as 3-butyloxyquinolone and 4-methoxyquinolone-decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4-diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3-(but-3-enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at -25 °C, presumably because a significant racemic background reaction occurred. The 4-substituted quinolones showed higher enantioselectivities (92-96 % ee at -25 °C) and, for the terminally Z- and E-substituted substrates, an improved regio- and diastereoselectivity.