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Design Principles of Ion Pairs in Organocatalysis – Elucidation of Structures, Intermediates and Stereoselection Modes as well as Assessment of Individual Interaction Contributions

Final Report Summary - IONPAIRSATCATALYSIS (Design Principles of Ion Pairs in Organocatalysis – Elucidation of Structures, Intermediates and Stereoselection Modes as well as Assessment of Individual Interaction Contributions)

Ion pairs are nearly omnipresent in chemistry and biochemistry and represent the key to numerous molecular functions. Their importance and extreme impact on molecular structures are based on their intrinsic physical properties, since counter ions provide the highest energies of all intermolecular interactions and are able to bridge the longest distances. However, ion pairs also challenge scientists in many aspects. Thus, structures of small contact ion pairs based on experimental data in solution are very rare and even for the highly developed theoretical chemistry it is difficult to reproduce the subtle interplay of interactions in small organic ion pairs. As a result, in catalysis, which is the ultimate goal of synthetic chemistry, ion pair complexes are highly effective but new catalysts must be screened in a black box mode with huge effort.
In this project for the first time detailed structures of ion pair catalyst complexes in Brønsted acid catalysis were provided based on a multitude of experimental data. Especially, hydrogen bonds were found to be extremely valuable sensors, which act as structural anchor in Brønsted acid catalysts and explain their broad substrate scope. Also the strength of these hydrogen bonds were correlated to the reactivity within similar structures. With chiral phosphoric acids as catalysts, four core structures were detected, which are highly conserved using different catalysts. The detailed structure investigations revealed these catalyst/substrate complexes to be by far more flexible than expected and with special NMR methods the sensitivity and the time scale could be refined to reveal even the switching of a single hydrogen bond. Furthermore, methods were developed that combine the potential of illumination, structural insight via NMR spectroscopy and UV/Vis spectroscopy and that allow an experimental access to the active transition state combinations. Overall a multitude of so far elusive structures, experimental data and methods were provided to chemists in spectroscopy, theory and synthesis to enable a refinement of concepts in ion pairing catalysis and to suggest future developments in this area.