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Internal acidity scale and reactivity evaluation of chiral phosphoric acids with different 3,3′-substituents in Brønsted acid catalysis

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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 1H and 15N chemical shifts and 1JNH 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.

Graphical abstract: Internal acidity scale and reactivity evaluation of chiral phosphoric acids with different 3,3′-substituents in Brønsted acid catalysis

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Publication details

The article was received on 14 May 2019, accepted on 02 Sep 2019 and first published on 06 Sep 2019


Article type: Edge Article
DOI: 10.1039/C9SC02342A
Chem. Sci., 2019, Advance Article
  • Open access: Creative Commons BY-NC license
    All publication charges for this article have been paid for by the Royal Society of Chemistry

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    Internal acidity scale and reactivity evaluation of chiral phosphoric acids with different 3,3′-substituents in Brønsted acid catalysis

    K. Rothermel, M. Melikian, J. Hioe, J. Greindl, J. Gramüller, M. Žabka, N. Sorgenfrei, T. Hausler, F. Morana and R. M. Gschwind, Chem. Sci., 2019, Advance Article , DOI: 10.1039/C9SC02342A

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