Browsing by Subject "reactivity"

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    Electrostatically Enhanced Thioureas: Synthesis, Reactivity and Selectivity
    (2018-07) Fan, Yang
    Hydrogen bonding exhibits its importance in enzyme-catalyzed chemical transformations, naturally occurring three-dimensional architectures and molecular recognition. In recent years, synthetic chemists have successfully exploited hydrogen bonds and developed many enantioselective organocatalysts. As a result, small molecule hydrogen bond donors along with organometallic species and enzymes are now recognized as playing a major role in asymmetric synthesis. Thiourea derivatives are among the most common and widely-developed hydrogen bond catalysts. Impressive results in terms of both yields and enantioselectivities in asymmetric syntheses have been obtained. A key feature in their success is the ability of these compounds to simultaneously donate two hydrogen bonds to a substrate, despite their relatively weak acidity. This provides highly stereoconfined environments when chiral moieties are incorporated into the thiourea and has made them the subject of extensive research efforts. The work described in this thesis focuses on the development of a class of positively charged acidity-enhanced thiourea catalysts which make use of an alkylated pyridinium substituent and an appropriate non-coordinating counteranion to enhance their N‒H acidities and improve their catalytic activities by orders of magnitude in a variety of transformations. A series of these catalysts have been synthesized and their reactivities in both asymmetric and non-asymmetric transformations were explored. Simple and highly efficient synthetic schemes and excellent catalytic results have been discovered for these novel species.
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    Structural- and Spectroscopic-Reactivity Relationships of Nonheme Oxoiron(IV) Complexes
    (2019-05) Rasheed, Waqas
    Non-heme oxoiron(IV) motifs have been identified as key intermediates that activate strong C—H bonds. Unlike the enzymatic intermediates however, most oxoiron(IV) complexes in synthetic chemistry have a triplet ground spin state and thus differ in their functional and electronic properties from the S = 2 units characterized in the enzymes. One striking exception is the complex [FeIV(O)(TQA)(L)]2+, where TQA = tris(2-quinolylmethyl)amine, which has Mössbauer parameters that closely resemble those of TauD-J, an enzymatic intermediate that has been relatively well-characterized. This oxoiron(IV) complex contains quinoline donors, and its thermal instability precludes its structural characterization (half-life = 15 minutes at 233 K). In this dissertation, several oxoiron(IV) complexes supported by pentadentate and tetradentate ligands are characterized, and examined for their reactivity and spectroscopic features. Crystallographic characterization of a few of these molecules is also reported. The structurally characterized oxoiron(IV) complexes along with some previously reported oxoiron(IV) complexes are used to set up structure-reactivity and spectroscopic-reactivity relationships, and show linear correlations with increasing isomer shifts, λmax values as well as metal-ligand distances. In addition, this thesis also uses 1H-NMR spectroscopy as an effective tool to identify solution-state structure as well the spin state of oxoiron(IV) complexes. We also characterize the first example of a spin crossover oxoiron(IV) complex, examples of which are only seen in iron(II) and iron(III) complexes.