title: NMR Spectroscopic Characterization of Charge Assisted Strong Hydrogen Bonds in Brønsted Acid Catalysis.
abstract: 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 investigations1H and15N chemical shifts, a Steiner-Limbach correlation, a deuterium isotope effect as well as quantitative values of1JNH,2hJPHand3hJPNwere 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.
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: Nils
authors.initials: N
authors.lastname: Sorgenfrei
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: Johnny
authors.initials: J
authors.lastname: Hioe
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: Julian
authors.initials: J
authors.lastname: Greindl
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: Kerstin
authors.initials: K
authors.lastname: Rothermel
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: Fabio
authors.initials: F
authors.lastname: Morana
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Lokesh
authors.affiliation: Institut für Organische Chemie, Universität Regensburg , D-93053 Regensburg, Germany.
authors.forename: Ruth M
authors.identifier: 0000-0003-3052-0077
authors.initials: RM
authors.lastname: Gschwind