Hydrogen
bonding plays a crucial role in Brønsted acid catalysis.
However, the hydrogen bond properties responsible for the activation
of the substrate are still under debate. Here, we report an in depth
study of the properties and geometries of the hydrogen bonds in (R)-TRIP imine complexes (TRIP: 3,3′-Bis(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diylhydrogen
phosphate). From NMR spectroscopic investigations 1H and 15N chemical shifts, a Steiner–Limbach correlation,
a deuterium isotope effect as well as quantitative values of 1JNH,2hJPH and 3hJPN were
used to determine atomic distances (rOH, rNH, rNO) and geometry information. Calculations at SCS-MP2/CBS//TPSS-D3/def2-SVP-level
of theory provided potential surfaces, atomic distances and angles.
In addition, scalar coupling constants were computed at TPSS-D3/IGLO-III.
The combined experimental and theoretical data reveal mainly ion pair
complexes providing strong hydrogen bonds with an asymmetric single
well potential. The geometries of the hydrogen bonds are not affected
by varying the steric or electronic properties of the aromatic imines.
Hence, the strong hydrogen bond reduces the degree of freedom of the
substrate and acts as a structural anchor in the (R)-TRIP imine complex.
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.
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