Rothermel, Kerstin and Melikian, Maxime and Hioe, Johnny and Greindl, Julian and Gramueller, Johannes and Zabka, Matej and Sorgenfrei, Nils and Hausler, Thomas and Morana, Fabio and Gschwind, Ruth M. (2019) Internal acidity scale and reactivity evaluation of chiral phosphoric acids with different 3,3 '-substituents in BrOnsted acid catalysis. CHEMICAL SCIENCE, 10 (43). pp. 10025-10034. ISSN 2041-6520, 2041-6539
Full text not available from this repository. (Request a copy)Abstract
The concept of hydrogen bonding for enhancing substrate binding and controlling selectivity and reactivity is central in catalysis. However, the properties of these key hydrogen bonds and their catalyst-dependent variations are extremely difficult to determine directly by experiments. Here, for the first time the hydrogen bond properties of a whole series of BINOL-derived chiral phosphoric acid (CPA) catalysts in their substrate complexes with various imines were investigated to derive the influence of different 3,3 '-substituents on the acidity and reactivity. NMR H-1 and N-15 chemical shifts and (1)J(NH) coupling constants of these hydrogen bonds were used to establish an internal acidity scale corroborated by calculations. Deviations from calculated external acidities reveal the importance of intermolecular interactions for this key feature of CPAs. For CPAs with similarly sized binding pockets, a correlation of reactivity and hydrogen bond strengths of the catalyst was found. A catalyst with a very small binding pocket showed significantly reduced reactivities. Therefore, NMR isomerization kinetics, population and chemical shift analyses of binary and ternary complexes as well as reaction kinetics were performed to address the steps of the transfer hydrogenation influencing the overall reaction rate. The results of CPAs with different 3,3 '-substituents show a delicate balance between the isomerization and the ternary complex formation to be rate-determining. For CPAs with an identical acidic motif and similar sterics, reactivity and internal acidity correlated inversely. In cases where higher sterical demand within the binary complex hinders the binding of the second substrate, the correlation between acidity and reactivity breaks down.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | ASYMMETRIC TRANSFER HYDROGENATION; MANNICH-TYPE REACTION; NONCOVALENT INTERACTIONS; CONFORMATIONAL-ANALYSIS; TRANSITION-STATES; STRECKER REACTION; MODEL SYSTEMS; NMR; ACTIVATION; DISPERSION; |
| Subjects: | 500 Science > 540 Chemistry & allied sciences |
| Divisions: | Chemistry and Pharmacy > Institut für Organische Chemie Chemistry and Pharmacy > Institut für Organische Chemie > Arbeitskreis Prof. Dr. Ruth Gschwind |
| Depositing User: | Dr. Gernot Deinzer |
| Date Deposited: | 24 Mar 2020 09:06 |
| Last Modified: | 24 Mar 2020 09:06 |
| URI: | https://pred.uni-regensburg.de/id/eprint/25816 |
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