| Summary: | <p>The Diels-Alder (DA) reaction is among the most important and versatile methods in creating ring molecules, and factors governing stereoselectivity and rate have been widely studied. However, the chemoselectivity in products are often restricted, as electronic natures of the reactants (typically electron-rich diene and electron-poor dienophile) have to be matched. </p>
<p>On the other hand, in radical chemistry, the removal of a free election helps promote an entirely different chemical environment within the reactants. This has been experimentally shown to be imperative in forming a different set of products not predicted in neutral DA reactions using photochemistry or redox chemistry. Chapter 1 will encompass an overview of such reactions and also the different computational techniques applied in understanding this chemistry. </p>
<p>Using an emerging technique known as quasi-classical dynamics (QCD), coupled with Density Functional Theory (DFT), this thesis investigated the radical cation DA reaction of a small sample system (Chapter 2) to provide quantitative understanding to this observed phenomena, and discovered the existence of reactive intermediates that could not be predicted with Transition State Theory (TST). As these intermediates have exceptionally long lifetimes, they also describe the dynamical nature of forming different products via the same transition state, which provides a strong contrast against concerted reactions (Figure A.1). This has never been applied to the study of radical cycloadditions before. In Chapter 3, the same techniques were applied to a larger DA system to predict product stereoselectivity, and kinetics rate theory provided evidence of reversibility from the neutral products. </p>
<p>The different chemical environment promoted from the removal of a single electron is revisited in Chapter 4 in the field of atropisomerism. In the radical cation systems chosen, we observed an unprecedented lowering of the energy barriers towards racemization. This effect also created a two-step mechanism, which was subsequently explained using Frontier Molecular Orbital (FMO) theory, and could allow better methods in isolating atropisomers. </p>
<p>Once again, and lastly in Chapter 5, the aforementioned computational techniques (DFT and QCD) are utilized in understanding different reaction pathways in non-heme enzymes, this time with the addition of hybrid quantum mechanics/molecular mechanics (QM:MM). The selectivity between the radical rebound (RR) pathway and the desaturation pathway was revealed and these different pathways were observed under different timescales, which would facilitate better catalytic design that enables controlled selectivity.</p>
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