MECHANISTIC AND PHOTOLYTIC STUDIES OF CYCLOPROPYL, BENZIMIDAZOLE, AND BENZOTRIAZOLE NITRENIUM IONS

Abstract

Nitrenium ions are short-lived, reactive intermediates that play an integral role in both biology and chemistry. These ions have been implicated as carcinogens in DNA-damaging reactions but have also been utilized as new electrophilic reagents in chemical synthesis. The work presented in this dissertation investigated three nitrenium ions using photolytic studies to understand their chemical and kinetic behaviors. Chapter 1 starts with an introduction to organic photochemistry and its use not only in synthetic research but also in the generation of short-lived intermediates. The chapter also presents laser-flash photolysis, a technique vital to studying reactive nitrenium ion intermediates generated in the heterolysis of N-aminopyridinium salts. Chapter 2 provides a background of nitrenium ions detailing the conception of the divalent cationic nitrogen species and early studies that led to the eventual proof of the short-lived, reactive intermediates through competitive trapping experiments and isolation of adducts. Chapter 3 explores two cyclopropyl nitrenium ions, one where the substituent is an aromatic ring capable of delocalizing the positive charge through pi-conjugation and another where the substituent is an alkyl group incapable of delocalizing positive charge. In the case of the aromatic cyclopropyl nitrenium ion, stable products result from a mix of ring expansion, nucleophilic addition and ethylene elimination. In contrast, the only observed product for the alkyl cyclopropyl nitrenium ion results from ethylene elimination. Chapter 4 examines the benzimidazolenium ion which DFT calculations predict to be a very reactive nitrenium ion intermediate. Experimental studies have shown that this nitrenium ion primarily decays through H-atom abstraction from the solvent and in-cage recombination with the pyridine leaving group. Chapter 5 probes the benzotriazolenium ion to see if it has similar reactivity to the previously studied benzimidazolenium ion. Initial experimental results have shown that the benzotriazolenium ion decays through H-atom abstraction from the solvent and in-cage recombination with the pyridine leaving group. In addition, another decay pathway appears to be nucleophilic trapping with alcohols (ROH) to form alkoxy adducts to the benzotriazole ring.