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Size-induced rate accelerations in organocatalysis
Size-induced rate accelerations in organocatalysis
Steric effects through sizeable moieties are commonly attributed to be mainly repulsive in organic chemistry. However, the size of molecules also significantly influences the strength of dispersion forces. Transition state theory implicates that reaction rates should be accelerated if the transition state is stabilized, e.g. through dispersion energy. Thus, the influence of large aromatic moieties on the reaction rates of several organocatalysed protecting group reactions was investigated. For the Lewis base-catalysed silylation of secondary alcohols as well as Lewis acid-catalysed hydrosilylation reactions of ketones size-dependent rate accelerations were found by competition experiments and computational studies. The size of these effects depends crucially on the solvent. The kinetic resolution of secondary alcohols by fluxionally chiral pyridine catalysts is increasingly selective with increasingly bulky substrates. Detailed kinetic and computational studies show that this is mainly provoked by a selective reaction rate acceleration for the major isomer through non-covalent interactions. More bulky catalysts further increase reaction rate AND selectivity. In a second part empirical studies on an online video library for the organic chemistry laboratory are reported. The online video library was used intensively by bachelor-level students, but preferences and utilization depends on individual student characteristics. The study indicates benefits of the online video library on students’ cognitive, affective, and psychomotor learning in a laboratory course.
Organocatalysis, Dispersion, Kinetic Resolution, Non-covalent interactions, Silylation
Pölloth, Benjamin
2020
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Pölloth, Benjamin (2020): Size-induced rate accelerations in organocatalysis. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Steric effects through sizeable moieties are commonly attributed to be mainly repulsive in organic chemistry. However, the size of molecules also significantly influences the strength of dispersion forces. Transition state theory implicates that reaction rates should be accelerated if the transition state is stabilized, e.g. through dispersion energy. Thus, the influence of large aromatic moieties on the reaction rates of several organocatalysed protecting group reactions was investigated. For the Lewis base-catalysed silylation of secondary alcohols as well as Lewis acid-catalysed hydrosilylation reactions of ketones size-dependent rate accelerations were found by competition experiments and computational studies. The size of these effects depends crucially on the solvent. The kinetic resolution of secondary alcohols by fluxionally chiral pyridine catalysts is increasingly selective with increasingly bulky substrates. Detailed kinetic and computational studies show that this is mainly provoked by a selective reaction rate acceleration for the major isomer through non-covalent interactions. More bulky catalysts further increase reaction rate AND selectivity. In a second part empirical studies on an online video library for the organic chemistry laboratory are reported. The online video library was used intensively by bachelor-level students, but preferences and utilization depends on individual student characteristics. The study indicates benefits of the online video library on students’ cognitive, affective, and psychomotor learning in a laboratory course.