Chiral organocatalysts for enantioselective photochemical reactions.

In this account the development of chiral organocatalysts for enantioselective photochemical reactions is described from a personal perspective. The need for enantioselective photochemical reactions is immediately linked to the challenge of establishing the first stereogenic centre in a molecule by organic photochemistry. Hydrogen bonds were identified as ideal means to embed a prochiral substrate into a chiral environment. The combination of a binding site with a suitable chiral architecture led to the generally applicable chiral template 12, which has been amply employed in photochemical reactions. Examples of its use are discussed in this report. It was shown that the general working mode of such a template can also be applied to radical reactions, irrespective of whether the radical chain process is initiated photolytically or chemically. While the robustness of template 12 allows for its almost quantitative recovery and its reuse, it has so far been used in a catalytic sense only in a single case. For the development of photochemical organocatalysts it was proposed to prepare modified templates, in which a light-collecting device acts simultaneously as sensitiser and as a steric shield. A prototype 40 for such a template was devised and it was shown that it can indeed induce catalytic turnover in a photoinduced electron transfer reaction.