NMR Spectroscopic Characterization of Charge Assisted Strong Hydrogen Bonds in Bronsted Acid Catalysis
Hydrogen bonding plays a crucial role in Bronsted 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.
Journal of the American Chemical Society