Isotope Effects, Dynamic Effects and Mechanisms of [2+2] Cycloadditions
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In this dissertation, experimental probes and theoretical calculations have been applied to delineate mechanisms of various [2+2] cycloadditions. Besides common experimental observations and transition state theories, this dissertation focuses on the application of kinetic isotope effects (KIEs) and dynamic effects for a better understanding of the mechanisms. The studies of dimerization of allene and [2+2] cycloaddition between 1,1-dimethyl allene and dimethyl maleate showed significant intramolecular KIEs and important reaction intermediates. These experimental observations strongly support a stepwise mechanism via a diradical intermediate. Based on this proposal, theoretical calculations gave excellent predictions of experimental observations. The controversy from previous literatures was well resolved by carefully analyzing the experimental observations. Research on Lewis acid catalyzed [2+2] cycloaddition between allene and alkenes were conducted via a combination of product studies, experimental kinetic isotope effects, common theoretical calculations and quasiclassical trajectory simulations. The results identified two important dynamic effects in these reactions, the bifurcating energy surface and the non-statistical recrossing. These dynamic effects explained the inverse KIEs in the reaction between allene and tetramethylethylene and the regioselectivity in the reaction between allene and isopropylidenecyclohexane. In stabilized Wittig olefination, the betaine was proposed to be an intermediate in the formation of the oxaphophetane as opposed to modern undertandings of a concerted mechanism. Experimental KIEs were consisted with a two-step mechanism and theoretical calculations located an intermediate along the reaction pathway. Trajectory simulations also showed significant amount of recrossing at the transition state and a possible hidden entropic intermediate in the reaction. These results provided a unique angle to understand the mechanism and the selectivity in the stabilized Wittig olefinations. Dynamic effects not only play important roles in common organic reactions, but also in complicated enzynamic reactions, such as the transannular Diels-Alder reaction catalyzed by the corresponding “Diels-Alderase” SpnF. This reaction includes the role of a [6+4] cycloaddition, a bispericyclic transition state, a bifurcating energy surface, a dynamically stepwise cycloaddition, an entropically-delineated intermediate, and transition state recrossing in the mechanism. The reaction is not its caricature from classical mechanistic analysis and it is not well described by either concerted or stepwise labels. Instead, the mechanism is richer and can only be understood by consideration of dynamics.
Chen, Zhuo (2013). Isotope Effects, Dynamic Effects and Mechanisms of [2+2] Cycloadditions. Doctoral dissertation, Texas A & M University. Available electronically from