NMR-Spectroscopic Investigations in Brønsted Acid Catalysis.

Die Arbeit befasst sich hauptsächlich mit der NMR-spektroskopischen Untersuchung von binären Komplexen bestehend aus chiralen Brønstedsäure Katalysatoren und Iminen. Dazu wurden Tieftemperaturmessungen bei bis zu 130 K durchgeführt und die Strukturen der untersuchten Komplexe mit Hilfe von NOE Analysen identifiziert und mit durch DFT berechneten Strukturen verglichen. Des Weiteren wurden die Wasserstoffbrückenbindungen der Komplexe durch Messung der skalaren Kopplungen über die Wasserstoffbrücke und durch Anfertigung einer Steiner-Limbach Kurve beschrieben und die daraus resultierende Geometrie untersucht. Auch hier erfolgte der Vergleich mit theoretischen Rechnungen. Weiterhin wurde der Einfluss der Substituenten der Katalysatorklasse auf die Reaktivität und Selektivität der Transferhydrierung von Iminsubstraten untersucht. Dazu wurde die Struktur eines weiteren chiralen Brønstedsäure Katalysators im binären Komplex mit Iminen aufgedeckt und die Wasserstoffbrücken von insgesamt drei Katalysatoren in Komplexen mit verschiedenen Iminen analysiert und diskutiert. Zusätzlich erfolgte die Untersuchung und Aufklärung des Mechanismus einer solvothermalen Syntheserute für Chinolinderivate mit Hilfe von NMR-Spektroskopie

NMR Spectroscopic Characterization of Charge Assisted Strong Hydrogen Bonds in Brønsted 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 H-1 and N-15 chemical shifts, a Steiner-Limbach correlation, a deuterium isotope effect as well as quantitative values of (1)J(NH), (2h)J(PH) and (3h)J(PN) were used to determine atomic distances (r(OH), r(NH), r(NO)) 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

Brønsted acid catalysis – the effect of 3,3′-substituents on the structural space and the stabilization of imine/phosphoric acid complexes

BINOL derived chiral phosphoric acids (CPAs) are widely known for their high selectivity. Numerous 3,3 0 substituents are used for a variety of stereoselective reactions and theoretical models of their effects are provided. However, experimental data about the structural space of CPA complexes in solution is extremely rare and so far restricted to NMR investigations of binary TRIP/imine complexes featuring two E-and two Z-imine conformations. Therefore, in this paper the structural space of 16 CPA/imine binary complexes is screened and 8 of them are investigated in detail by NMR. For the first time dimers of CPA/ imine complexes in solution were experimentally identified, which show an imine position similar to the transition state in transfer hydrogenations. Furthermore, our experimental and computational data revealed an astonishing invariance of the four core structures regardless of the different steric and electronic properties of the 3,3 0 -substituent. However, a significant variation of E/Z-ratios is observed, demonstrating a strong influence of the 3,3 0 - substituents on the stabilization of the imine in the complexes. These experimental E/Z-ratios cannot be reproduced by calculations commonly applied for mechanistic studies, despite extensive conformational scans and treatment of the electronic structure at a high level of theory with various implicit solvent corrections. Thus, these first detailed experimental data about the structural space and influence of the 3,3 0 -substituent on the energetics of CPA/imine complexes can serve as basis to validate and improve theoretical predictive models

Brønsted Acid CatalysisStructural Preferences and Mobility in Imine/Phosphoric Acid Complexes

Despite the huge success of enantioselective Brønsted acid catalysis, experimental data about structures and activation modes of substrate/catalyst complexes in solution are very rare. Here, for the first time, detailed insights into the structures of imine/Brønsted acid catalyst complexes are presented on the basis of NMR data and underpinned by theoretical calculations. The chiral Brønsted acid catalyst <i>R</i>-TRIP (3,3′-bis­(2,4,6-triisopropylphenyl)-1,1′-binaphthyl-2,2′-diyl hydrogen phosphate) was investigated together with six aromatic imines. For each investigated system, an <i>E</i>-imine/<i>R</i>-TRIP complex and a <i>Z</i>-imine/<i>R</i>-TRIP complex were observed. Each of these complexes consists of two structures, which are in fast exchange on the NMR time scale; i.e., overall four structures were found. Both identified <i>E</i>-imine/<i>R</i>-TRIP structures feature a strong hydrogen bond but differ in the orientation of the imine relative to the catalyst. The exchange occurs by tilting the imine inside the complex and thereby switching the oxygen that constitutes the hydrogen bond. A similar situation is observed for all investigated <i>Z</i>-imine/<i>R</i>-TRIP complexes. Here, an additional exchange pathway is opened via rotation of the imine. For all investigated imine/<i>R</i>-TRIP complexes, the four core structures are highly preserved. Thus, these core structures are independent of electron density and substituent modulations of the aromatic imines. Overall, this study reveals that the absolute structural space of binary imine/TRIP complexes is large and the variations of the four core structures are small. The high mobility is supposed to promote reactivity, while the preservation of the core structures in conjunction with extensive π–π and CH−π interactions leads to high enantioselectivities and tolerance of different substrates

NMR Study of Solvation Effect on the Geometry of Proton-Bound Homodimers of Increasing Size

Hydrogen bond geometries in the proton-bound homodimers of quinoline and acridine derivatives in an aprotic polar solution have been experimentally studied using H-1 NMR at 120 K. The reported results show that an increase of the dielectric permittivity of the medium results in contraction of the N center dot center dot center dot N distance. The degree of contraction depends on the homodimer's size and its substituent-specific solvation features. Neither of these effects can be reproduced using conventional implicit solvent models employed in computational studies. In general, the N center dot center dot center dot N distance in the homodimers of pyridine, quinoline, and acridine derivatives decreases in the sequence gas phase > solid state > polar solvent

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

The concept of hydrogen bonding for enhancing substrate binding and controlling selectivity and reactivity is central in catalysis. However, the properties of these key hydrogen bonds and their catalyst-dependent variations are extremely difficult to determine directly by experiments. Here, for the first time the hydrogen bond properties of a whole series of BINOL-derived chiral phosphoric acid (CPA) catalysts in their substrate complexes with various imines were investigated to derive the influence of different 3,3 '-substituents on the acidity and reactivity. NMR H-1 and N-15 chemical shifts and (1)J(NH) coupling constants of these hydrogen bonds were used to establish an internal acidity scale corroborated by calculations. Deviations from calculated external acidities reveal the importance of intermolecular interactions for this key feature of CPAs. For CPAs with similarly sized binding pockets, a correlation of reactivity and hydrogen bond strengths of the catalyst was found. A catalyst with a very small binding pocket showed significantly reduced reactivities. Therefore, NMR isomerization kinetics, population and chemical shift analyses of binary and ternary complexes as well as reaction kinetics were performed to address the steps of the transfer hydrogenation influencing the overall reaction rate. The results of CPAs with different 3,3 '-substituents show a delicate balance between the isomerization and the ternary complex formation to be rate-determining. For CPAs with an identical acidic motif and similar sterics, reactivity and internal acidity correlated inversely. In cases where higher sterical demand within the binary complex hinders the binding of the second substrate, the correlation between acidity and reactivity breaks down

NMR Spectroscopic Characterization of Charge Assisted Strong Hydrogen Bonds in Brønsted Acid Catalysis

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 (<i>R</i>)-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 ¹<i>J</i>_NH, ^2h<i>J</i>_PH and ^3h<i>J</i>_PN were used to determine atomic distances (r_OH, r_NH, r_NO) 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 (<i>R</i>)-TRIP imine complex