Research Projects

Prediction of Enantioselectivities Based on Ion Pair Intermediate Structures

The prediction of structures or even enantioselectivities in In the field of Br?nsted acid catalysis is still a major challenge, since only? a few experimental studies exist in contrast to a multitude of theoretical calculations. Therefore, in this project experimental descriptors for the stereoinduction in ion pairing catalysis shall be developed on the example of chiral phosphoric acids.

Elucidation and Control of Transition State Combinations in Ion Pair Catalysis

The highest level of complexity in ion pair catalysis is to analyse and control reactions based on transition states and active transition state combinations. In ion pair catalysis, the typical challenges of ion pairs in terms of structures, aggregation, and solvent effects are aggravated by e.g. kinetic effects of pre-equilibria and solubility issues. In this project the active transition state combination of selected ion pair reactions in Br?nsted acid catalysis shall be investigated based on experimental data.

Identifying the Governing Effects on Reaction Path Bifurcations (BIFs) in the Frag-mentation of Terpenoid Diazoniumions by Experimental Means

The reactivity of secondary carbocations in bicyclic frameworks have been shown to not only be governed by their delocalized electronic structure (non-classical) but also by the special topology of their potential energy surface (PES). The latter can feature valley ridge inflection points that enable off-MEP processes (MEP = minimum energy pathway) and hence pose fundamental questions of how to predict selectivity. We found, for the first time, experimental evidence of such bifurcations (BIFs) in the dissociation and structural rearrangements of photochemically generated diazoniumions (Figure P3). To gain deeper understanding of the governing principles of how product selectivities are achieved one needs to go the extra mile of including counterion and specific solvent interactions. One seminal paper did show that reactions taking place on bifurcating PES are hypersensitive to changes of ‘external’ conditions , however these important factors are usually ignored in published data.

Off-MEP Scenarios in Pseudo-Pericyclic Reactions of Open-Shell Systems: Impact of Ion-Pairing and Solvation on Reaction Dynamics Effects

In closed shell-pericyclic processes, the concerted reaction pathway can be satisfyingly rationalized by FMO concepts (Woodward-Hoffmann-rules). If, however, the closed shell species is turned into an open-shell ion (here: radical cation), what happens in terms of its electronic nature and the FMO-based reactivity concepts? The challenges associated to the prediction and description of mechanisms are two-fold: The occurrence of non-MEP behavior questions the established models of predictive power and as such constitute a research field on its own. And secondly, the mechanistic complexity increases exponentially by the so far scarcely studied effects on MEP and non-MEP situations arising from the mutual interaction of the radical cations and its assocaited counterion.

Control of Reactivity and Selectivity through Ion Pairs in Pericyclic Reactions

The central question of this project is whether one can alter the reaction path?way pericyclic reactions through ion pair interactions in dipols or radical cations, this way changing both sterics as well as electronics of the intermediates.1 For this purpose, we will investigate monocyclopropanated pyrroles and furans, for which we could demonstrate a 6π-electrocyclic ring-opening / [3+2]-cycloaddition cascade under thermal conditions.

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Carbonylates, Structures, Intermediates and Reactivities

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Ion pairs can be generated in liquid ammonia by direct reduction of neutral species with solutions of the alkali metals. The relative endpoint of the reactions will be determined by the low-temperature single crystal X-ray analysis of the ammoniates which crystallize. In solution, the reactions will be monitored via 13C-NMR in collaboration with R. Gschwind and via vibrational spectroscopy with P. Nürnberger. In collaboration with the Bauer group the chemistry of silylium carbonylates will be explored.

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Lewis Base Catalysis with Pyridinamide Ion Pairs

The Lewis base-catalyzed condensation of electrophiles is highly useful syntheti- cally, but encounters formidable obstacles for electrophiles of low reactivity. The ion pair catalysts developed in this project on the basis of pyridinamide anions are expected to pro- vide significantly higher reaction rates and thus an equally expanded scope of these reac- tions.

Website Prof. Dr. Hendrik Zipse

Unveiling Ion Pair Effects in Ultrafast Photochemical Reaction Dynamics

We will investigate how counterion effects influence photochemical processes and want to identify mechanisms for steering the reaction outcome in this way, in collaboration with groups from synthetic chemistry. Topics to address comprise spectroscopic signatures of ion pairing in liquid ammonia, how photoswitching can be utilized in ion-pair catalysis, and the temporal evolution of short-lived ion pairs formed in a photoinduced charge-transfer process.