Browsing by Subject "Isotope Effects"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Intramolecular Isotope Effects for the Study of Reactions with MassTransfer Limitations(2010-01-16) Wagner, Joshua G.The research presented provides a method to use the comparison of intermolecular isotope effects vs. the intramolecular isotope effects for the study of reactions in which study of the rate limiting step is ambiguous due to interfering mass transfer effects. The oxidation of unfunctionalized hydrocarbons at mild conditions developed by Sir Derek Barton, the Gif reaction is the model used. The history is provided to demonstrate the relevance of using this model as one which could show the usefulness of this method. Evidence has been provided and used to theorize that the rate limiting step of the reaction may be diffusion of the reactants, not a chemical change. Starting materials were made which would allow for the measurement for both the intermolecular and intramolecular KIE and those values were compared. The results show that there is little difference between the intermolecular and intramolecular KIE, therefore the reaction is not diffusion controlled.Item Isotope Effects, Dynamic Effects and Mechanisms of [2+2] Cycloadditions(2013-12-09) Chen, ZhuoIn 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.Item The experimental and theoretical determination of combinatorial kinetic isotope effects for mechanistic analysis(2009-05-15) Christian, Chad F.Unfortunately, chemists can never experimentally unravel a full reaction pathway. Even our ability to define key aspects of mechanisms, such as short-lived intermediates and the even more ephemeral transition states, is quite limited, requiring subtle experiments and subtle interpretations. Arguably the most important knowledge to be gained about the mechanism of a reaction is the structure and geometry of the transition state at the rate-limiting step, as this is where a reaction?s rate and selectivity are generally decided. The Singleton group has developed a methodology for predicting the combinatorial kinetic isotope effects (KIEs) at every atomic position, typically carbon or hydrogen, at natural abundance. A combination of experimental isotope effects and density functional theory (DFT) calculations has greatly aided our ability to predict and understand a reaction?s pathway and transition state geometries. Precise application of this method has allowed for the mechanistic investigation of a myriad of bioorganic, organic, and organometallic reactions. The technique has been applied in the analysis of the catalytic borylation of arenes via C-H bond activation, dynamic effects in the enyne allene cyclization, palladium catalyzed allylic alkylation, the nature of proton transfer in orotate decarboxylase, and the epoxidation of enones with t-butyl hydroperoxide.