Hydrogen Bonds and Ion Pairs in Enantioselective Photochemistry

Introduction. The field of enantioselective photochemistry has rapidly grown in recent years and many contributions deal with reactions in which a hydrogen bonding or ion pairing interaction is involved. The distinction whether a hydrogen bond or an ion pair is operative is not always trivial and the classification shown below is somewhat arbitrary. Brønsted acids act via a hydrogen-bonded ion pair, but charge separation has frequently been observed. In the current project we rely mainly on two non-covalent motifs that operate to form a given assembly, (1) a two-point hydrogen bond interaction between chiral lactams and a given substrate, and (2) the protonation of a substrate with a chiral Brønsted acid leading to a hydrogen-bonded or a strict ion pair. In both scenarios the species that undergoes an enantioselective transformation is in its excited singlet or triplet state, which is an important distinction to most reactions reported so far.

Summary of the Project. The project aims to employ hydrogen bonding and ion pairing interactions in enantioselective photochemical reactions. The focus is on the generation of excited species within a substrate-catalyst assembly as opposed to the formation of radical intermediates by photoredox processes. The goal is approached (a) by the use of chiral lactams or phosphoric acids which display an internal, light-collecting sensitizing unit to transfer energy exclusively to a bound substrate and (b) by devising substrates which are photochemically transparent but are activated towards excitation by a chiral Brønsted acid. In the former scenario, emphasis is placed on catalytic deracemization reactions in their purest sense converting a racemic mixture into a single enantiomer by sensitization. Suitable catalysts for this transformation are based on the 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton and new substrate classes will be explored. Mechanistic insights are expected from transient absorption spectroscopy experiments and from titration studies. The work with chiral acids will rely largely on the binding motif of carboxylic acids and imines to these acids. Both motifs can be interrogated by NMR spectroscopic techniques and the assemblies can be studied prior to, and possibly also during excitation. In terms of scope, the exclusive focus will be on reactions that are unique to photochemistry and cannot be achieved thermally. The repertoire thus includes [2+2] photocycloaddition, photochemical rearrangement, and photocyclization reactions of synthetically relevant substrate classes, which have so far remained unexplored.
 

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