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.

L'objectif de ces travaux de thèse était d'améliorer les performances d'étanchéité de connecteurs automobiles fabriqués en silicone. La première approche visait à comprendre les relations entre les structures chimiques présentes dans les formulations LSR et les propriétés mécaniques afin de proposer des additifs favorisant la résistance à la déchirure. Lors d'une étude préalable, nous avons étudié l'effet synergétique du platine et de la silice sur la dégradation thermique de formulations silicone. Ce travail a permis de décrire le mécanisme et de proposer de nouvelles formulations plus performantes en terme de taux de résidu après pyrolyse. Cette première étude alliée à d'autres techniques a permis d'analyser les structures chimiques présentes dans huit formulations commerciales. Nous avons également caractérisé la réactivité ainsi que la structure du réseau polymère obtenu après réticulation. Les relations liant les structures chimiques à la structure des réseaux ont été établies. Enfin, les propriétés mécaniques telles que la déformation rémanente à la compression, les propriétés ultimes (force et élongation à la rupture) et la résistance à la déchirure des matériaux ont été corrélées avec les différentes structures des réseaux.La seconde partie était dédiée à la synthèse d'un additif fonctionnel thermiquement activable permettant de réparer a posteriori une déchirure. Afin de sélectionner le meilleur système correspondant au cahier des charges, une revue complète de la bibliographie a été réalisée sur la réversibilité des fonctions urées et uréthanes, en portant une attention particulière sur la chimie des isocyanate bloqués. Deux molécules bloquantes ont été sélectionnées après étude de la réactivation thermique de la fonction isocyanate. Un monomère portant cette fonction isocyanate bloqué a été engagé dans une réaction de copolymérisation afin d'obtenir plusieurs générations d'additifs testés selon les normes appliquées aux connecteurs. / This PhD work aimed at improving the water and air-proofing properties of automotive connectors made of silicones. The first approach consisted of understanding the relationships between the chemical structures added in the LSR formulations and their ultimate mechanical performances so as to propose additives which would improve tear resistance of the materials. In a preliminary study, we investigated the synergistic role of platinum catalyst and silica on the thermal degradation of silicone formulations. These investigations allowed us to describe the degradation mechanism and to suggest new formulations in order to improve the residue content at high temperature. This first study, combined with other techniques, allowed us to analyze the chemical structures present in eight commercial formulations. We also characterized the reactivities as well as the network topologies obtained after curing the formulations. Correlations between the chemical structures and the network topology were then established. Finally, some mechanical properties, i.e. the compression set, the ultimate properties (tensile strength and elongation at break) and the tear resistance of final materials were matched with network topologies. The second part was dedicated to the synthesis of a functional additive which could be thermally reactivated to heal a tear. In order to select the best system according to the strict specifications of this work, a complete literature review on the reversibility of urea and urethane bonds was done, with special emphasis on blocked isocyanate chemistry. After a study on the isocyanate group thermal reactivation, two blocking molecules were chosen. A monomer bearing this blocked isocyanate function was then copolymerized to obtain different generations of additives which were finally tested according to standard norms applied to connectors.