Les diradicaux sont des espèces réputées extrêmement instables. A titre d'exemple, le<br />cyclobutanediyle qui est l'archétype de ce genre d'espèce en série carbonée n'a en effet pu<br />être isolé que dans des matrices à basse température.<br />La première partie de cet exposé sera consacrée aux diradicaux formés des éléments<br />lourds des groupes principaux.<br />Nous montrerons dans une introduction bibliographique comment l'utilisation de ces<br />hétéroéléments a permis l'isolation du premier diradical localisé singulet stable basé sur un<br />squelette formé uniquement des hétéroéléments bore et phosphore.<br />Dans un deuxième chapitre, nous nous intéresserons aux isomères de ce diradical, en<br />particulier ses isomères de valence. Après un bref retour sur le concept, nous étudierons les<br />facteurs qui permettent de stabiliser les différents isomères déjà connus. Nous montrerons<br />ensuite comment il a été possible d'isoler de nouveaux isomères.<br />Dans un troisième chapitre, nous nous arrêterons sur deux isomères de valence<br />particuliers et étudierons l'isomérie d'élongation (bond-stretch isomerism). Dans notre cas,<br />deux isomères qui ne diffèrent que par la longueur d'une liaison sont en équilibre. Grâce à<br />des études de RMN à température variable, nous mettrons en évidence l'influence des<br />facteurs électroniques dans cet équilibre.<br />Ce travail nous a amenés à nous interroger sur la possibilité de préparer un diradical<br />purement organique formé uniquement d'éléments de la seconde ligne : carbone et azote.<br />Cette réflexion nous a conduits à mettre au point une nouvelle méthode de synthèse de sels<br />d'iminium et d'imidazolinium que nous présenterons dans une deuxième partie.

This thesis describes the catalytic asymmetric epoxidation of olefins mediated by chiral iminum salts. The first chapter introduces some of the most novel and effective catalytic asymmetric methods for preparing chiral oxiranes. The second chapter is divided into three sections. The first section of chapter two is dedicated to our efforts to develop new aqueous oxidative conditions using both hydrogen peroxide and sodium hypochlorite as efficient, green oxidants that remove the temperature boundaries observed with the use of Oxone® as the stoichiometric oxidant. A wider range of available temperatures was examined allowing optimization of both oxidative systems. Ethereal hydrogen peroxide was observed to mediate asymmetric epoxidation within an acetonitrile monophasic co-solvent system giving enantioselectivities of up to 56%. When sodium hypochlorite was used in a biphasic solvent system in conjunction with dichloromethane; it was observed to mediate oxidation of the substrate alkenes in up to 71% ee. The second and third sections of chapter two are dedicated to our efforts to synthesize chiral iminium salts as catalysts for asymmetric epoxidation based on a biphenyl azepinium salt catalyst structure. From previous work within the Page group, the asymmetric synthesis and subsequent defined stereochemistry of a chiral carbon atom α to the iminium nitrogen atom was shown to have significant effect on the enantiocontrol of epoxidation using the iminium salt catalyst. Work was completed on biphenyl azepinium salt catalysts, inserting an alkyl or aryl Grignard reagent into the iminium bond using a pre-defined dioxane unit as a chiral auxiliary. Oxidation of the subsequent azepine gave a single diastereoisomerically pure azepinium salt. The methyl analogue of this sub-family of azepinium catalysts has been shown to give up to 81% ee for epoxidation of 1-phenylcyclohexene, furthermore, the binaphthalene azepinium salt with an additional methyl group was also synthesized and was shown to give up to 93% for epoxidation of 1-phenylcyclohexene. Continuation of the substitution α to the nitrogen atom gave rise to an interesting tetracyclic (biphenyl) azepinum salt catalyst. Construction of an asymmetric oxazolidine ring unit encapsulating the azepinium nitrogen and one of the methylene carbon atoms was achieved. In doing so two chiral centres α to the nitrogen atom were generated. The azepinium chiral carbon atom was populated by an addition methyl group with variation in the substitution on the oxazolidine chiral carbon atom. The benzyl analogue of this sub-family of tetracyclic azepinium catalysts has shown to give up to 79% ee for epoxidation 1-phenylcyclohexene. The third chapter is the experimental section and is dedicated to the methods of synthesis and characterization of the compounds mentioned in the previous chapter. X-ray reports regarding the crystallographic analysis of the structures presented in chapter two are provided in appendix A. Appendix B contains the analytical spectra for the determination of enantiomeric excess of the epoxides.