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).

Enantioselective catalysis of photochemical reactions.

Die Natur des angeregten Zustands macht die Entwicklung chiraler Katalysatoren fur enantioselektive photochemische Reaktionen zu einer enormen Herausforderung. Die Absorption eines 400-nm-Photons entspricht einer Energieaufnahme von etwa 300 kJ mol−1. Es bedarf innovativer Konzepte, um in einem derart grosen Abstand vom Grundzustand Reaktionspfade zu eroffnen, die gezielt zu einem Enantiomer einer chiralen Verbindung fuhren. Hier werden die beiden wesentlichen Vorgehensweisen fur homogen katalysierte, enantioselektive Prozesse diskutiert. Im ersten Teil werden chirale Photokatalysatoren besprochen, die in den photochemischen Schlusselschritt eingreifen und in diesem Schritt eine asymmetrische Induktion bewirken. Im zweiten Teil werden Reaktionen vorgestellt, in denen die photochemische Anregung durch einen achiralen Katalysator erfolgt und die asymmetrische Induktion durch einen zweiten chiralen Katalysator gewahrleistet wird (duale Katalyse).

Enantioselektive Katalyse photochemischer Reaktionen

We have developed a metal-free process for the aerobic photooxygenation of sulfides to sulfoxides mediated by riboflavin tetraacetate or riboflavin (vitamin B2) photocatalysts and visible light (450 nm) in an acetonitrile-water (85:15 v/v) mixture. The optimised solvent system leads to both singlet-oxygen and electron-transfer pathways in photooxygenation, thus allowing oxidation of electron-poor and electron-rich thioanisoles, dialkyl sulfides and sterically hindered sulfides. Besides having a broad substrate scope, the method has very short reaction times and requires low catalyst loading (down to 0.1 mol%). These properties are due to the high photocatalyst stability and the extremely high quantum yields (1.3 for thioanisole oxygenation). Moreover, the method is chemoselective, producing only sulfoxides without overoxidation to sulfones. Taking into account the broad substrate scope, high selectivity and high efficiency, this method distinguishes itself from those previously reported. Other advantages include easy work-up of the reaction mixture, the availability and biodegradability of the photocatalysts and mild reaction conditions. We demonstrated, on a preparative scale, its practical application in the synthesis of the psychostimulant modafinil, in the selective oxidation of methionine derivatives, and in the detoxification of mustard gas.

Efficient Metal-Free Aerobic Photooxidation of Sulfides to Sulfoxides Mediated by a Vitamin B2 Derivative and Visible Light

Smectites or swelling clay minerals are naturally occurring nanomaterials that can be fully delaminated to elementary clay mineral platelets in dilute aqueous dispersion. This review article gives an overview of the recent progress on how the elementary clay mineral platelets can be reorganized in monolayered or multilayered hybrid nanofilms by layer-by-layer assembly or the Langmuir-Blodgett technique. In the latter case one hybrid layer consists of one layer of elementary clay mineral platelets with a theoretical thickness of 0.96 nm, covered on one side by amphiphilic cations. The organization of the elementary clay mineral platelets and that of the adsorbed amphiphilic cations in the nanofilms has been studied in great detail by ATR-FTIR, UV-Vis and fluorescence spectroscopy, XRD and AFM. The nanofilms carry functional properties, such as chirality, optical nonlinearity and magnetism, which are due to the nature of the amphiphilic cations and to the organization of both the amphiphilic molecules and the elementary clay mineral platelets.

Ultrathin hybrid films of clay minerals

Chemoselective photooxidation: Tetra-O-acetylriboflavin and blue light mediate the selective oxidation of sulfides to sulfoxides without overoxidation to sulfones. Reaction is very effective in ethanol/water (95:5,v/v) mixture reaching a quantum yield of Φ=0.60. The photooxidation reaction scope includes aliphatic and aromatic sulfides including sterically hindered substrates.

Photooxidation of Sulfides to Sulfoxides Mediated by Tetra-O-Acetylriboflavin and Visible Light

The present work pursued a possibility that enantioselectivity was achieved through weak intermolecular interactions between a catalyst and a substrate. For that purpose, we studied the photooxidation of α-ethylbenzyl phenyl sulfide catalyzed by a polypyridyl ruthenium(II) complex as a chiral photosensitizer. No covalent bonding was formed between a catalyst and a substrate, because the complexes used ([Ru(phen)3]2+ or [Ru(bpy3)2+]) were coordinatively saturated. Enantiomer excess (ee) was attained to be 30% when a chiral photosensitizer was immobilized on montmorillonite clay. It was even improved to 43% in the presence of an additional chiral auxiliary, dibenzoyl-d(+)-tartaric acid. Notably, no enantioselectivity was achieved when the reaction took place in homogeneous solutions. The ab initio calculations were performed on the stability of an associate composed of a catalyst (metal complex) and a product (sulfoxide) to obtain a clue to reaction mechanisms. The calculations suggest that chiral discrimin...

Enantioselective Photooxidation of a Sulfide by a Chiral Ruthenium(II) Complex Immobilized on a Montmorillonite Clay Surface: The Role of Weak Interactions in Asymmetric Induction

Norishige Kakegawa

Papers

Enantioselective Photooxidation of a Sulfide by a Chiral Ruthenium(II) Complex Immobilized on a Montmorillonite Clay Surface: The Role of Weak Interactions in Asymmetric Induction

Racemic adsorption of tris(1,10-phenanthroline)ruthenium(ii) onto a mica surface

Nanometer-scale ordering in cast films of columnar metallomesogen as revealed by STM observations