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Isocyanates are invaluable bulk chemicals that play a central role in the synthesis of various polymers and provide a key platform for the synthesis of nitrogen-containing molecules such as carbamates and ureas. Unfortunately, isocyanates suffer from high toxicity, low functional group tolerance, and a propensity to undergo deleterious side-reactions. Consequently, blocked (masked) isocyanate derivatives have been the subject of increased interest resulting from their reduced toxicity and exceptional control over isocyanate reactivity. This strategy has largely been relegated to the polymerization literature, although its use in the synthesis of complex urea and carbamate derivatives is well established in synthetic organic chemistry. However, prominent gaps in the blocked isocyanate literature were clear at the outset of this research project. First and foremost, the development of heteroatom-substituted isocyanates, such as N- and O-substituted derivatives, remained relatively scarce despite their potential for the synthesis of important nitrogen-containing derivatives. Furthermore, the additions of carbon-centred nucleophiles on blocked N-, O-, and even C-substituted blocked isocyanates were exceedingly rare. Finally, the use of a blocking group strategy in catalytic transformations of isocyanates remained largely absent from the literature. This was particularly striking given the widespread development of catalytic transformations of isocyanates. As such research efforts began focusing on furthering the development of blocked N-isocyanates as a vital platform for heterocyclic synthesis (chapter 2). Initially, the cascade reactivity of blocked N-iso(thio)cyanates was expanded to incorporate electrophiles such as alkynes (section 2.2). This readily provided access to imidazolone and thiazolidine products. Subsequently, the development of a cascade reaction providing access to 1,2,4-triazin-3(2H)-ones was explored (section 2.3). This provided the first examples of an N-isocyanate cascade which hinged on the use of acid catalysis. Moreover, insight into hydrazone isomerization was gained. Finally, these efforts culminated in the development of cascade reactions providing access to a rare class of 1,2,4-triazinones as well as 5-aminopyridazinones (section 2.4). This provided the first example of a cascade reaction involving a C-C bond formation onto a blocked N-isocyanate derivatives. Furthermore, this development was pivotal in re-focusing attention on the development of general strategies to achieve addition of carbon nucleophiles onto blocked isocyanate derivatives. Towards this end, the development of two strategies to achieve carbon-centred nucleophilic additions on both blocked N- And O-isocyanates were developed (chapter 3). Inspiration from the isocyanate literature led to the development of carboxylic acids as formal carbon nucleophiles (section 3.2). This strategy was found to be quite general for the synthesis of hydroxamates from blocked O-isocyanates. Furthermore, encouraging results were generated on the ability of Grignard reagents to form similar products (section 3.3). Particularly important is the paradigm shift this allows from C-N bond formation to C-C bond formation for the synthesis of hydroxamate derivatives. Furthermore, lead results suggest the potential of this reactivity to translate to blocked N-substituted derivatives, a transformation which had failed with carboxylic acids. Finally, the development of a catalytic amide synthesis from blocked isocyanate precursors was targeted (chapter 4). The use of a blocking group strategy was able to address the current major limitation of isocyanates as amide precursors, that is functional group tolerance (section 4.2). Indeed, a commercially available rhodium catalyst was found to allow efficient amidation of various ambiphilic blocked isocyanate derivatives using arylboroxines as nucleophiles. Mechanistic studies including the use of variable time normalization analysis supported the presence of two alternative kinetic regimes contingent on the reaction conditions employed. Furthermore, these data suggested the success of this transformation, in the case of ambiphilic derivatives, hinged on a rate determining isocyanate release (chapter 4). Finally, initial results strongly support the potential for Boc-carbamates to provide a general platform for amidation in the presence of strong nucleophiles such as primary amines. The potential of a blocking group strategy in catalytic reaction development was further displayed with the development of a palladium catalyzed amidation of blocked derivatives with arylboroxine nucleophiles (section 4.3). Indeed, the use of blocked isocyanates was found to be absolutely key in achieving efficient reactivity with the palladium catalyst. This result, coupled with the sparse reports on blocked isocyanates in catalysis, strongly suggest that the use of such a strategy could allow the development of reactivity otherwise unattainable when using free isocyanates.

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Microcalorimetric measurements and contact angle measurements were conducted to study the surface chemistry of powdered minerals. The contact angle measurements were conducted on both flat and powdered talc samples, and the results were used to determine the surface free energy components using Van Oss-Chaudhury-Good (OCG) equation. It was found that the surface hydrophobicity of talc increases with decreasing particle size. At the same time, both the Lifshitz-van der Waals (gSLW) and the Lewis acid-base (gSAB) components (and, hence, the total surface free energy (gS)) decrease with decreasing particle size. The increase in the surface hydrophobicity and the decrease in surface free energy (gS) can be attributed to preferential breakage of the mineral along the basal plane, resulting in the exposure of more basal plane surfaces to the aqueous phase. Heats of immersion measurements were conducted using a flow microcalorimeter on a number of powdered talc samples. The results were then used to calculate the contact angles using a rigorous thermodynamic relation. The measured heat of immersion values in water and calculated contact angles showed that the surface hydrophobicity of talc samples increase with decreasing particle size, which agrees with the direct contact angle measurements. A relationship between advancing water contact angle qa, and the heat of immersion (-DHi) and surface free energies was established. It was found that the value of -DHi decrease as qa increases. The microcalorimetric and direct contact angle measurements showed that acid-base interactions play a crucial role in the interaction between talc and liquid. Using the Van Oss-Chaudhury-Good's surface free energy components model, various talc powders were characterized in terms of their acidic and basic properties. It was found that the magnitude of the Lewis electron donor, gS-, and the Lewis electron acceptor, gS+, components of surface free energy is directly related to the particle size. The gS- of talc surface increased with decreasing particle size, while the gS+ slightly decreased. It was also found that the Lewis electron-donor component on talc surface is much higher than the Lewis electron-acceptor component, suggesting that the basal surface of talc is basic. The heats of adsorption of butanol on various talc samples from n-heptane solution were also determined using a flow microcalorimeter. The heats of adsorption values were used to estimate % hydrophilicity and hydrophobicity and the areal ratios of the various talc samples. In addition, contact angle and heat of butanol adsorption measurements were conducted on a run-of-mine talc sample that has been ground to two different particle size fractions, i.e., d50=12.5 mm and d50=3.0 mm, respectively. The results were used to estimate the surface free energy components at the basal and edge surfaces of talc. It was found that the total surface free energy (gS) at the basal plane surface of talc is much lower than the total surface free energy at the edge surface. The results suggest also that the basal surface of talc is monopolar basic, while the edge surface is monopolar acidic. The results explain why the basicity of talc surface increases with decreasing particle size as shown in the contact angle and microcalorimetric measurements. Furthermore, the effects of the surface free energies of solids during separation from each other by flotation and selective flocculation were studied. In the present work, a kaolin clay sample from east Georgia was used for the beneficiation tests. First, the crude kaolin was subjected to flotation and selective flocculation experiments to remove discoloring impurities (i.e., anatase (TiO2) and iron oxides) and produce high-brightness clay with GE brightness higher than 90%. The results showed that a clay product with +90% brightness could be obtained with recoveries (or yields) higher than 80% using selective flocculation technique. It was also found that a proper control of surface hydrophobicity of anatase is crucially important for a successful flotation and selective flocculation process. Heats of immersion, heats of adsorption and contact angle measurements were conducted on pure anatase surface to determine the changes in the surface free energies as a function of the surfactant dosage (e.g. hydroxamate) used for the surface treatment. The results showed that the magnitude of the contact angle and, hence, the surface free energy and its components on anatase surface varies significantly with the amount of surfactant used for the surface treatment. / Ph. D.