Authors: Cheng Fang
Source: D-Scholarship@Pitt (University of Pittsburgh) (article, 2019-06-20)
Field: Chemistry — Organic Chemistry

Computational Studies of Olefin Polymerization and Hydroboration with N-Hetereocyclic Carbene Boranes Alkenes and alkynes are two commonly staring materials to produce various valuable chemicals and bioactive organic compounds. Several chemical transformations with alkenes and alkynes will be discussed in this thesis, including copper-catalyzed and photoredox-mediated atom transfer radical polymerization (ATRP), selectivity-enhancement entropy-driven ring opening metathesis polymerization (SEED-ROMP) and hydroboration of alkynes using N-hetereocyclic carbene (NHC) boranes. Although there are tremendous advances in the development of ATRP, the detailed mechanisms for the key steps in ATRP are not fully explored. Also, it remains challenging to investigate the catalysts and initiators effects on the ATRP reactivity. In this thesis, I applied multiple computational approaches, including DFT calculations, Marcus theory calculations, energy decomposition analysis, and multivariate linear regression, to investigate the mechanism and the catalysts and initiators effects in ATRP. Our computational studies revealed that copper-catalyzed ATRP occurs via an inner-sphere electron transfer transition state in activation/deactivation process. Detailed analysis of the ATRP transition states suggested key factors controlling the reactivities of catalysts and initiators, which were further utilized to establish predictive models for the catalyst and initiator effects via a multivariate regression approach. On the other hand, the photoredox-mediated ATRP prefers an outer-sphere electron transfer mechanism in activation/deactivation process. Furthermore, although the mechanism for ROMP is well-established, it is unclear regarding the origin of the selectivity for cis-macrocyclic olefin monomers over trans-monomers in SEED-ROMP. By integrating DFT calculations and molecular dynamics simulation, it is suggested that enhanced polymerization reactivity was due to the cis-macrocyclic olefin being less flexible and having a larger population of metathesis-reactive conformers. In addition, I applied DFT calculations and quasi-classical Born-Oppenheimer molecular dynamics simulations to investigate the mechanisms, dynamics effect, and the origin of reactivities and chemoselectivities in the hydroboration of alkynes and arynes with NHC-borane. Our calculations revealed that the hydroboration of alkynes occurs through a trans-selective hydride transfer process followed by a bifurcation pathway leading to both trans-alkenylborane and trans-borirane products. The hydroboration of arynes is a dynamically-stepwise hydride transfer process, in which the hydride prefers to attack more positively charged and more linear sp-hybridized carbons in substituted arynes, leading to the high levels of regioselectivity. Chemistry ; ROMP ; Atom-transfer radical-polymerization ; Polymerization ; Carbene ; Reactivity (psychology) ; Photochemistry ; Catalysis ; Combinatorial chemistry ; Metathesis ; Organic chemistry ; Polymer Synthetic Organic Chemistry Methods ; Organometallic Complex Synthesis and Catalysis ; Advanced Polymer Synthesis and Characterization