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Virginia Tech

1. Hancock, Amber N. A Radical Approach to Syntheses and Mechanisms.

Degree: PhD, Chemistry, 2011, Virginia Tech

The critically important nature of radical and radical ion mechanisms in biology and chemistry continues to be recognized as our understanding of these unique transient species grows. The work presented herein demonstrates the versatility of kinetic studies for understanding the elementary chemical reactions of radicals and radical ions. Chapter 2 discusses the use of direct ultrafast kinetics techniques for investigation of crucially important enzymatic systems; while Chapter 3 demonstrates the value of indirect competition kinetics techniques for development of synthetic methodologies for commercially valuable classes of compounds. The mechanism of decay for aminyl radical cations has received considerable attention because of their suspected role as intermediates in the oxidation of tertiary amines by monoamine oxygenases and the cytochrome P450 family of enzymes. Radical cations are believed to undergo deprotonation as a key step in catalysis. KIE studies performed by previous researchers indicate N,N-dimethylaniline radical cations deprotonate in the presence of the bases acetate and pyridine. By studying the electrochemical kinetics of the reaction of para substituted N,N-dimethylaniline radical cations with acetate anion, we have produced compelling evidence to the contrary. Rather than deprotonation, acetate reacts with N,N-dimethylaniline radical cation by electron transfer, generating the neutral amine and acetoxyl radical. Transport properties of reactants and solvent polarity changes were investigated and confirmed not to influence the electrochemical behavior forming the basis for our mechanistic hypothesis. To reconcile our conclusion with earlier results, KIEs were reinvestigated electrochemically and by nanosecond laser flash photolysis. Rather than a primary isotope effect (associated with C-H bond cleavage), we believe the observed KIEs are secondary, and can be rationalized on the basis of a quantum effect due to hyperconjugative stabilization in aromatic radical cations during an electron transfer reaction. Product studies performed by constant potential coulometry indicate N,N-dimethylaniline radical cations are catalytic in carboxylate oxidations. Collectively, our results suggest that aminyl radical cation deprotonations may not be as facile as was previously thought, and that in some cases, may not occur at all. Interest in design and synthesis of selenium containing heterocycles stems from their ability to function as antioxidants, anti-virals, anti-inflammatories, and immunomodulators. To establish synthetic feasibility of intramolecular homolytic substitution at selenium for preparation of selenocycles, we set out to determine what factors influence cyclization kinetics. A series of photochemically labile Barton and Kim esters have been syntheisized and employed as radical precursors. The effect of leaving radical stability on kinetics has been investigated through determination of rate constants and activation parameters for intramolecular homolytic substitution of the corresponding… Advisors/Committee Members: Tanko, James M. (committeechair), Marand, HervĂ© L. (committee member), Marand, HervĂ© L. (committee member), Long, Timothy E. (committee member), Madsen, Louis A. (committee member), Troya, Diego (committee member).

Subjects/Keywords: Carboxylate; Deprotonation; Electron Transfer; Radical Cyclization; Homolytic Substitution; Selenium; Radical Trap; Rate Constant; Deuterium Isotope Effect; Arrhenius Parameter; N; Mechanism; Kinetics; Cyclic Voltammetry; Constant Potential Coulometry; Competition Kinetics.; Amine Radical Cation; N-dimethylaniline; Digital Simulations; Laser Flash Photolysis

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APA (6th Edition):

Hancock, A. N. (2011). A Radical Approach to Syntheses and Mechanisms. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/77139

Chicago Manual of Style (16th Edition):

Hancock, Amber N. “A Radical Approach to Syntheses and Mechanisms.” 2011. Doctoral Dissertation, Virginia Tech. Accessed April 19, 2021. http://hdl.handle.net/10919/77139.

MLA Handbook (7th Edition):

Hancock, Amber N. “A Radical Approach to Syntheses and Mechanisms.” 2011. Web. 19 Apr 2021.

Vancouver:

Hancock AN. A Radical Approach to Syntheses and Mechanisms. [Internet] [Doctoral dissertation]. Virginia Tech; 2011. [cited 2021 Apr 19]. Available from: http://hdl.handle.net/10919/77139.

Council of Science Editors:

Hancock AN. A Radical Approach to Syntheses and Mechanisms. [Doctoral Dissertation]. Virginia Tech; 2011. Available from: http://hdl.handle.net/10919/77139

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