Enantioselective Catalysis of [2+2] Photocycloaddition Reactions

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The [2+2] photocycloaddition is the most important light-induced reaction (photochemical reaction) of organic chemistry. The products of the reaction are cyclobutanes, which possess great potential for possible synthetic applications and which are in general chiral. The selective access to a single enantiomeric form of a chiral compound is called an enantioselective reaction. The goal of the present project is to find chiral catalysts that allow for the enantioselective preparation of cyclobutanes in a [2+2] photocycloaddition reaction. Due to the fact that photochemical reactions proceed upon excitation ¿ even in the absence of a catalyst ¿ very rapidly, novel concepts are required, which are different from those of classical catalysis. It appears particularly promising to combine the process of substrate-catalyst binding with selective light absorption and ¿ by doing so ¿ to ensure that substrate excitation is only possible in the chiral environment of the catalyst. To this end, one proposed approach is based on the fact that [2+2] photocycloaddition substrates exhibit a long-wavelength absorption in complexes with acids (Lewis or Brønsted acids) or in certain intermediates (onium ions or enamines), which invites selective excitation. The other approach aims to develop chiral sensitizers that are catalytically active and that transfer energy selectively within a substrate-sensitizer complex.