Catalyst Substrate Interaction of Organo Phosphate Brønsted Acid Catalysts with Imines
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Abstract
In this thesis, I study the interaction of Brønsted acid organocatalysts with imines. I answer the question, which species are formed and quantify their association constants. In my work, I investigate the solvent dependence of those association constants and compare the results for two different imines in order to generalize my findings. My results show that the formed aggregates are based on very strong doubly ionic hydrogen bonds in which the proton vibrations are strongly coupled. Apart from this, I find the proton residing in a very anharmonic energy potential, which strongly affects the energies of the vibrational transitions. I use linear and non-linear infrared (IR) spectroscopy as well as nuclear magnetic resonance (NMR) and dielectric relaxation spectroscopy (DRS).
In order to address the question which species are formed in solutions of a Brønsted acid organocatalyst and an imine, I first present the results for a model system. I choose diphenyl phosphate (DPP) as the catalyst and quinaldine (Qu) as the imine for my study. My concentration-dependent NMR and DRS experiments in dichloromethane (DCM) provide evidence for not only the formation of ion-pairs, but also of multimers, such as trimers consisting of one Qu and two DPP molecules. The presence of multimers in solution and their possible contribution to the catalytic process could explain the high catalyst loadings, which are needed in organocatalysis.
A major challenge in the field of organocatalysis remains the development of rational catalyst design. I address this topic in my thesis by quantifying the ion-pair and multimer formation of DPP with two imines (quinaldine and 2-phenylquinoline) in three different solvents (DCM, chloroform, and tetrahydrofuran). I find a correlation of the solvents hydrogen bonding affinity with the yield of the reaction and the strength of its ion-pair formation.
Finally, I use optical spectroscopy to take a detailed look at the interaction motif within the ion-pair. Here, I apply linear and non-linear IR spectroscopy to study the vibrations related to the acidic proton, which governs the catalyst-substrate interaction. The concentration-dependent linear IR data show that doubly ionic hydrogen bonds are formed between the catalyst and the substrate. By means of non-linear IR spectroscopy, I observe very fast dynamics related to the acidic proton vibrations. Further, the 2D-IR data suggest that this vibration is strongly coupled to lower-frequency modes and that the proton resides within a very anharmonic energy potential, which is supported by ab initio calculations. The strong coupling may allow for efficient excess energy dissipation, which is a potential pathway for energy transport during the catalytic transfer hydrogenation reaction.