Mechanistic Investigations into the Origin of Selectivity in Organic Reactions
Thomas, Jacqueline Besinaiz
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Detailed mechanistic studies were conducted on several organic reactions that exhibit product selectivity (regio-, peri-, or enantioselectivity). The organic reactions studied were electrophilic aromatic substitutions, Diels-Alder cycloadditions of 1,3- dienes with cyclopentadieneone, Lewis acid catalyzed ene reactions with olefins, chlorinations of alkynes, and the enantioselective intramolecular Stetter reaction. Analyses of these systems were conducted by measurement of kinetic isotope effects, standard theoretical calculations, and in some cases dynamic trajectories. Mechanistic studies of electrophilic aromatic substitution, Lewis acid catalyzed ene reaction with olefins, the chlorination of alkynes, and the Diels-Alder cycloadditions of 1,3-dienes with cyclopentadienones, suggest that the origin of selectivity is not always a result of selectivity result from a kinetic competition between two closely related pathways to form distinct products. All of these systems involve one transition state on a potential energy surface that bifurcates and leads to two distinct products. In these systems, experimental kinetic isotope effects measured using natural abundance methodology, theoretical modeling of the potential energy surfaces, and trajectory analyses suggests that selectivites (regio- and periselectivities) are a result of influences by momenta and steepest-descent paths on the energy surface. The work here has shown that in order to understand selectivity on bifurcating surfaces, transition state theory is not applicable. In place of transition state energetics, the guiding principles must be those of Newtonian dynamics. In the mechanistic studies for the enantioselective intramolecular Stetter reaction, the origin of selectivity is a result of multiple transition states and their relative energies. Experimental H/D kinetic isotopes effects had lead to the conclusion that two different mechanisms were operating for reactions where carbenes were generated in situ versus reactions using free carbenes. However, 13C kinetic isotope effects and theoretical modeling of the reaction profile provide evidence for one mechanism operating in both cases.