| Summary: | Thesis advisor: James P. Morken === Sesquiterpene quinones are a prolific class of marine natural products that are particularly interesting due to their antibacterial, antiviral, and anti-inhibitory properties. Hundreds of these biologically active molecules are based on decalin frameworks, both cis- as well as trans-fused, however, significantly less synthetic work has focused on targeting the cis-fused series of compounds. In this chapter, progress towards an asymmetric, general route to various sesquiterpene quinones in the cleordane family of natural products will be described. The key steps of the synthesis include a highly convergent and diastereoselective reductive alkylation to forge both the requisite cis-ring fusion well as the all carbon quaternary center, as well as a scandium-catalyzed ring expansion of a 6,5-ring system to deliver the decalin core of the molecule. Additionally, the chapter includes the development and substrate scope of both methodologies utilized in the key complexity building reactions. Iron complexes ligated by bis(imino)pyridine ligands are remarkably active catalysts for a vast range of organic transformations including polymerization, hydrogenation, hydrosilylation, and hydroboration. Whereas much work has been done to probe the importance of the imine-substituents on catalysis, significantly less information is known about the nature of the central pyridine donor. To study the effects of a more donating ligand in which the pyridine is replaced with an N-heterocyclic carbene, a series of novel ligands and their corresponding iron complexes were synthesized and characterized. Whereas imidazole-derived complexes exhibited exclusively bidentate binding modes, 4,5,6-trihydropyrimidylidene-based ligands adopted a tridentate pincer conformation analogous to complexes of bis(imino)pyridines. Bonding in the five-coordinate bis(imino)-N-heterocyclic carbene complex displayed considerably contracted iron-ligand bond distances compared to the analogous bis(imino)pyridine iron complex. The study of physical and electronic structure and bonding in organometallic compounds is a critical for understanding and predicting complex behavior and reactivity. Having synthesized a completely new type of N-heterocyclic carbene (NHC) ligand and the corresponding iron complex, a rigorous study of metal-NHC bonding, magnetism, and redox activity in bis(imino)-NHC (or carbenodiimine, CDI) complexes of iron was carried out. A series of oxidation and reduction reactions on CDI complexes of iron were performed, enabling access to complexes spanning from formally iron(0) to iron (III) oxidation states. A battery of spectroscopic and computational methods, including X-ray crystallography, Mössbauer spectroscopy, SQUID magnetometry, and EPR spectroscopy established the CDI ligand as a redox active chelator. Additionally, a unique iron-carbene interaction was discovered, in which the metal center antiferromagnetically couples with the carbon of the NHC. Intent on developing CDI complexes of iron into practical catalysts for both synthetic organic transformations and polymerization, a series of stoichiometric as well as catalytic reactions were carried out to evaluate the reactivity profile of the novel complexes. Halide atom abstraction generated a new cationic species, which demonstrated different coordination chemistry compared to the bis(imino)pyridine analogue. Furthermore, the addition of a hydride or alkyl lithium reagent to the parent (CDI)FeCl2 species resulted in interesting and unexpected reactivity involving the carbene ligand. Preliminary catalytic hydrogenation experiments established (CDI)FeCl2 as a competent catalyst for the reduction of simple alkenes in the presence of Na(Hg) as a reductant under 80 psi of hydrogen. Additionally, the dichloride species could be readily converted into bis(aryloxide) complexes that were active for the polymerization of lactide to produce poly(lactic acid). The polymerization is very controlled (PDI values are <1.3), and polymers with molecular weights of around 35 kDa can be obtained after 3 hours at room temperature. === Thesis (PhD) — Boston College, 2014. === Submitted to: Boston College. Graduate School of Arts and Sciences. === Discipline: Chemistry.
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