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

Abstract

NMR H-bond analysis reveals an offset of internal and external acidities of catalysts and allows for a detailed reactivity analysis.

What is the role of acid–acid interactions in asymmetric phosphoric acid organocatalysis? A detailed mechanistic study using interlocked and non-interlocked catalysts

“…7B), which is a typical chemical shi for protons in strong hydrogen bonds and is similar to the hydrogen bond signals in CPA$imine complexes. 13,29 The detection of magnetization transfer between the H-bond proton and both quinoline and CPA further corroborated the assignment of this hydrogen bond signal (ESI Fig. S30 ‡).…”

“…For CPA$imine systems, a correlation between hydrogen bond strength and reactivity has been observed previously, giving lower reactivities for weaker hydrogen bonds. 13 This trend is also re-ected for the monomeric and dimeric reaction pathways in the investigated quinoline systems, as the dimeric reaction pathway featuring a weaker PO À /H$N + hydrogen bond shows lower reaction rates than the monomeric pathway (Fig. 8).…”

“…10 Phosphoric acid aggregation has also been proven by spectroscopic means: dimers and trimers of dimethylphosphoric acid were identied by NMR 11 and Hunger could show the presence of multimers for complexes of diphenyl phosphoric acid and a quinoline. 12 For chiral phosphoric acids (CPA) with a BINOL-backbone, extended aromatic surfaces allow additional weak non-covalent interactions, which further stabilize hydrogen-bonded catalyst$substrate complexes, 13,14 and can also enable the formation of higher aggregates, such as dimers of CPA$imine complexes. 15 We recently found that integration of two BINOL-phosphoric acids in a catenane structure 16 leads to drastic changes both in reaction rates and stereoselectivities for the transfer-hydrogenation of quinolines.…”

Disulfonimides versus Phosphoric Acids in Brønsted Acid Catalysis: The Effect of Weak Hydrogen Bonds and Multiple Acceptors on Complex Structures and Reactivity

“…1 Extensive NMR studies of the binary complexes between different CPAs and aromatic N -arylimines proved the formation of strong, charge-assisted hydrogen bonds in these catalyst/substrate complexes, supported by a network of CH−π and π–π interactions. 3−5 Thus, the induced stereoselectivity stems from the noncovalent interactions. 6,7 However, for some transformations, even stronger Brønsted acids are required or at least show higher activities.…”

“…All results were compared to the previously investigated less acidic CPAs. 3−5,20 Finally, the internal acidity of these different classes of catalysts was correlated to the reactivity in the transfer hydrogenation. (c) In contrast to the CPAs, not only two but five possible hydrogen bond acceptors exist for the DSIs.…”