Cycloadditions of diazabicyclooctenes, tethered cyclobutadienes, and styrene: Computational investigations on the concerted/diradical frontier

This dissertation describes the investigation of cycloaddition and cycloreversion mechanisms using a variety of computational methods. The reactions of interest can potentially occur via a concerted pericyclic transition state or via a singlet diradical intermediate.; In Chapter 1, the loss of N2 from diazabicyclo[2.2.2]oct-2-enes, azomethane, and diazabicyclo[2.2.1]hept-2-ene was studied with B3LYP, CASSCF, and CASPT2, resulting in a unified description of azoalkane decomposition. Fused-ring substituents were found to vary the mechanism for thermolysis from three-bond cleavage (pericyclic) to two- or one-bond cleavage (diradical). A cyclopropane bridge imparts a strong stereoelectronic effect, leading to a large preference for either three-bond or one-bond cleavage. In the absence of this stereoelectronic effect, two-bond cleavage and one-bond cleavage are highly competitive.; Intramolecular cycloadditions of cyclobutadiene with tethered alkenes are the subject of Chapter 2. B3LYP/6-31G(d) calculations indicate that various tethers exert electronic effects and geometric strain, thereby influencing the rate of the intramolecular cycloadditions. Subtle changes in the rate have a profound influence on the experimental yield because the intramolecular cycloaddition must compete with the diffusion-controlled dimerization of cyclobutadiene.; Chapter 3 is a B3LYP/6-31G(d) study of intramolecular cycloadditions of cyclobutadiene with tethered dienes. Cyclobutadiene reacts with a diene in a (4+2) fashion with a long tether and in a (2+2) fashion with a short tether. The potential energy surface of the intramolecular cycloaddition is characterized by a bifurcation subsequent to the initial cycloaddition transition state. The alignment of the cycloaddition TS and bifurcation explains the observed (2+2):(4+2) selectivity of the cycloaddition.; The mechanism of styrene self-initiation is the focus of Chapter 4. Previous experimental results failed to provide conclusive evidence for either the Mayo or Flory mechanisms. B3LYP and BPW91 calculations indicate that the Diels-Alder adduct is the key intermediate for formation of monoradical initiating species. Although stepwise formation of diphenylcyclobutane is energetically preferred, dynamic factors can account for formation of the Diels-Alder adduct at the expense of the 1,4-diradical.