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

Abstract

In Brønsted acid catalysis, hydrogen bonds play a crucial role for reactivity and selectivity. However, the contribution of weak hydrogen bonds or multiple acceptors has been unclear so far since it is extremely difficult to collect experimental evidence for weak hydrogen bonds. Here, our hydrogen bond and structural access to Brønsted acid/imine complexes was used to analyze BINOL-derived chiral disulfonimide (DSI)/imine complexes. 1H and 15N chemical shifts as well as 1JNH coupling constants revealed for DSI/imine complexes ion pairs with very weak hydrogen bonds. The high acidity of the DSIs leads to a significant weakening of the hydrogen bond as structural anchor. In addition, the five hydrogen bond acceptors of DSI allow an enormous mobility of the imine in the binary DSI complexes. Theoretical calculations predict the hydrogen bonds to oxygen to be energetically less favored; however, their considerable population is corroborated experimentally by NOE and exchange data. Furthermore, an N-alkylimine, which shows excellent reactivity and selectivity in reactions with DSI, reveals an enlarged structural space in complexes with the chiral phosphoric acid TRIP as potential explanation of its reduced reactivity and selectivity. Thus, considering factors such as flexibility and possible hydrogen bond sites is essential for catalyst development in Brønsted acid catalysis.

Ternary complexes of chiral disulfonimides in transfer-hydrogenation of imines: the relevance of late intermediates in ion pair catalysis

“…Out of these ve binary complexes, the previous computational approaches always favoured the E N -complex while the experiments revealed a signicant population of the Zcomplexes. 28 This large energetic overestimation of the E-imine binary complexes in Brønsted acid catalysis by the applied theoretical methods has been a general problem so far. 10 Since hydrogen bonds and p,p-interactions dominate the interaction mode revealed by NMR, we tested the B97-D functional, as it was developed to adequately describe hydrogen-bonded DNA base pairs.…”

“…This evidence is supported by the correlation of hydrogen bond strength and NMR parameters such as 1 H and chiral DSIs form rather weak hydrogen bonds with imines and the complexes have more ion-pair character, allowing some additional structural exibility. 28,29 The reducing agent then approaches the binary complex. The reaction utilizes a sacricial hydrogen donor, such as Hantzsch ester, to transfer a hydride ion to the LUMO-activated imine.…”

“…36 Moreover, we have shown that a very acidic bis(sulfonic) acid catalyst gave the slowest reaction most likely due to the intramolecular Hbond stabilization of the resulting monosulfonate. 28 However, the bifunctional binding of the substrate and reducing agent to the classical CPAs has not been experimentally disclosed so far. This topic is even more pressing with the far more complex situations in multiple H-bond acceptors present in the recently developed, highly-acidic ion pair catalysts.…”

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

“…[38] Detailed NMR studies revealed that in case of the ATH of N-alkylimines, several different complexation modes occur when using CPAs, meanwhile only the (E) and (Z) binary complexes are formed in those reactions relying of DSI-catalysis, which explains the superiority of the latter catalyst class for these transformations. [39] Relying on chiral DSI-catalysis, the List group reported the synthesis of C 2 -symmetric secondary amines via reductive condensation of NÀ H imines (Scheme 13). Using the acid (R)-DSI 2 and Hantzsch ester HE 4, a series of optically active aromatic C 2 -symmetric secondary amines (28 a-n) were prepared in moderate to good yields and in excellent stereocontrol.…”

Sulfonimide-Based Dendrimers: Progress in Synthesis, Characterization, and Potential Applications

“…The compounds are structurally similar to acid anhydrides. The basic nitrogen atom of a sulfonimide with a lone pair can participate in hydrogen bonding [ 23 ]. The sulfonyl group is a relatively inactive functional group characterized as weak basic, where a sulfur atom is doubly bonded to two oxygen atoms [ 24 ].…”