UC Davis

This dissertation is a series of investigations into reaction mechanisms of organic or organometallic transformations through the application of Density Functional Theory and high-level quantum chemical methods to generate models useful for understanding chemical reactivity, often arriving at unusual mechanistic proposals. The underlying quantum chemical methods and relevant topics are discussed in Chapter 1.  Chapter 2 is about an unusual Post Transition-State bifurcation between a diradical and zwitterion intermediate. This post-transition state surface intersection (PTSSI) between diradical and zwitterionic states that causes a bifurcation in the reaction pathway was discovered through density functional theory calculations on potential energy surfaces and ab initio molecular dynamics simulations of cycloadditions between a bicyclobutane and a triazolinedione (BCB-TAD). It was predicted that changes to the solvent polarity would enable control over the dynamic selectivity in this system. In collaboration with the Schomaker group at University of Wisconsin-Madison, we obtained experimental evidence that supported this prediction. This work not only provides new insights into an unusual type of post-transition state bifurcation but also demonstrates how the nonstatistical dynamic effects that control selectivity for such reactions can be manipulated rationally to increase the yields of synthetically useful reactions. Chapter 3 is about a formal dyotropic rearrangement. A new reaction mechanism for the construction of dioxabicyclo[4.2.1]nonanone skeletons via a cation cascade has been proposed and examined by DFT and ab initio computations. This mechanism features the following steps: (1) intramolecular Friedel–Crafts-type cyclization with a methyl oxocarbenium cation formed by carboxylate disconnection, (2) electron-rich aromatic ring assisted methoxide loss followed by lactone formation, and (3) stepwise dyotropic rearrangement resulting in skeletal isomerization from a dioxabicyclo[3.2.2]nonanone to the dioxabicyclo[4.2.1]nonanone product observed experimentally. The high regioselectivity and driving force for the overall rearrangement were rationalized, and Lewis and Brønsted acid-mediated reactivities were compared. Chapter 4 is a collaboration with the Pitts group at University of California, Davis regarding reductive elimination from Te centre. Reaction Mechanism of both sp2 and sp3 C−F bond formation through formal reductive elimination from organotellurium(VI) compounds in superacidic media are investigated using DFT calculations. The results suggest that SbF5 plays an important role beyond fluoride abstraction. Chapter 5 is a series of collaborations with the Pitts group at University of California, Davis on SF5 and SF4CF3 radical addition across bicyclobutanes, propellane and azabicyclobutanes. Reaction mechanisms of Radical Chain reactions of addition of SF5Cl and SF4CF3Cl to bicyclobutanes, propellane and azabicyclobutanes had been investigated using DFT calculations. Reactivity indices and molecular properties were also predicted to explain unusual reactivity. Chapter 6 is a series of collaborations with the Panda Research Group at IIT Kharagpur on an extension of the Zweifel Olefination. Different stereoselectivity in formation of vinyl heteroarenes was observed for different heteroarenes under identical conditions. DFT calculations provide evidence for diverging reaction pathways leading to different stereochemistry. An unusual reversal of stereoselectivity with DDQ was found to lead to a stereoconvergent mechanism. Chapter 7 is a collection of organometallic catalysis involving a Ru/Cu dual metal mediated transformation collaborating with Manmohan Kapur Group at IISERB and two Co-mediated transformations collaborating with the Ponneri Ravikumar Group at NISER Bhubaneswar. These organometallic systems all exhibit interesting behaviors such as two-state reactivity and unusual reaction mechanisms.