Bioorthogonal reactions, due to its excellent selectivity and time-efficiency, have emerged as a popular tool for protein and cell probing. Among all the bioorthogonal reactions, the inverse electron-demand Diels-Alder reaction (IEDDA) reaction has its advantage of bearing the fastest kinetics. Although the IEDDA reaction drew considerable attention in chemical biology in the last decade, challenges lie in finding the suitable dienophiles. Strained dienophiles, for example, trans-cyclooctene derivatives, can undergo ultrafast IEDDA reactions and therefore have been extensively developed. Unstrained alkenes and alkynes, however, have not been well investigated as IEDDA handles. In general, unstrained dienophiles are more straightforward to synthesise compared with strained dienophiles, therefore they are more accessible to researchers. In addition, the absence of a highly reactive bond makes unstrained dienophiles inert to biological nucleophiles, which allows effectively cellular labelling. In this dissertation, I described three different unstrained dienophiles for different biological purposes. Allyl handle is thiol-stable and non-toxic, which was utilised to label apoptotic cells in a pre-targeting manner. Enol ethers can react with tetrazines to decage protected amino acids and prodrugs. Potassium arylethynyltrifluoroborate, as a novel dienophile, was shown to react fast with pyridyl tetrazines controllably and this new IEDDA was applied to label proteins site-selectively and to fluorescently label two proteins orthogonally. In addition to IEDDA reactions, other bioorthogonal reactions were also developed using these versatile unstrained handles. Allyl-bearing amino acids and proteins can undergo an acetophenone-mediated hetero-[2+2] photocycloaddition with maleimide derivatives, expanding the toolbox of photo-triggered chemistry for protein modification. The potassium arylethynyltrifluoroborate handle was also found reactive in copper(I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC) and showcased the huge potential for protein labelling and multicolour cellular labelling.

<p dir="ltr">Ring strain is the driving force for numerous ring-opening reactions of three- and four-membered heterocycles. By comparison, five-membered heterocycles lack this thermodynamic driving force. As a result, only a few methods exist for the ring-opening of five-membered heterocycles using transition metal catalysts. For unstrained and unactivated 2,5-dihydrofurans this is achieved via a β-O elimination process, wherein, gaining selectivity over a competing β-H elimination is challenging. We report a novel strategy for the asymmetric ring-opening of 2,5-dihydrofurans with dichloroalkenes utilizing an earth-abundant cobalt catalyst. We propose that the dichloroalkenes form reactive vinylidene intermediates with the chiral catalyst, followed by a [2+2] cycloaddition with the heterocyclic alkene. This cobaltacyclobutane exclusively undergoes an outer-sphere β-O elimination assisted by zinc halide. Alternative inner-sphere β-O and β-H elimination pathways are inaccessible from this four-membered metallacycle. This is followed by a transmetallation step to form a zinc metallacycle, which subsequently gives rise to homoallylic alcohols, upon quenching, with high diastero- and enantioselectivity. Additionally, the organozinc intermediate can be trapped in situ by various electrophiles for further derivatizations. DFT model predicts the origin of the high diastereo- as well as enantioselectivity observed in the reaction.</p><p dir="ltr">Furthermore, the cobaltacyclobutane intermediate serves as a dynamic platform, facilitating access to a diverse array of products depending on the alkene partners employed. Utilizing chiral allylic alcohols as alkene partners leads to the translation of stereochemical information enabling the stereospecific synthesis of both <i>E</i>- and <i>Z</i>-isomers of alkenes. Alkenes are important motifs found in various natural products and bioactive compounds. A catalytic approach for the precise control of the alkene geometry is highly valuable since the stereochemistry of alkenes plays a pivotal role in determining the properties of molecules. Our strategy provides access to organozinc dienes which could be functionalized further to form highly substituted 1,4-skipped dienes. Additionally, meso-diols can undergo a desymmetrizing β-O elimination from the cobaltacyclobutane intermediate yielding chiral cyclopentenols with contiguous stereocenters</p>

<p> Ring opening of heterocyclic compounds through C-X bond cleavage is a useful strategy that provides rapid access to highly functionalized acyclic building blocks. In recent years, much work has focused on using transition metal catalysts to activate the C-X bonds of heterocycles and initiate ring opening. Metal-catalyzed ring opening of unstrained heterocycles is less prevalent than catalytic activation of strained heterocycles, which is advantageously driven by relief of ring strain. Methods for catalytic ring activation of unstrained heterocycles exist but are limited. Herein, we report the use of chiral cobalt complexes as catalysts for enantioselective ring opening of dihydrofuran and nitrogen-protected pyrrolines by utilizing dichloroalkenes as vinylidene precursors, Zn as a reductant, and ZnCl2 as an additive. Based on preliminary mechanistic studies, we believe this method proceeds through [2 + 2] cycloaddition between the ligated cobalt vinylidene species and the heterocycle, followed by β-heteroatom elimination, cobalt to zinc transmellation, and protonation to give rise to synthetically useful chiral allylic alcohol and amine products. </p>

De par ses extraordinaires propriétés physiques, on s'attend à ce que le graphène devienne un matériau de nouvelle génération, susceptible de compléter les semi-conducteurs traditionnels dans les technologies de dispositifs électroniques. Depuis sa découverte expérimentale en 2004, de nombreux travaux ont cherché à en évaluer les potentialités. Toutefois, en vue d'applications en électronique, le graphène souffre d'un inconvénient majeur : l'absence de bande interdite dans sa structure de bandes. Ainsi, il est très difficile de moduler et couper le courant dans un transistor de graphène, ce qui restreint considérablement son champ d'applications. Du point de vue des propriétés thermoélectriques, l'absence de bande interdite empêche la séparation des contributions opposées des électrons et des trous au coefficient Seebeck, qui reste donc faible dans le graphène parfait. Aussi, l'ouverture d'une bande interdite (gap) dans le graphène est une nécessité pour contourner les inconvénients de ce matériau et bénéficier pleinement de ses excellentes propriétés de conduction. Il a été montré que de nombreuses approches de nanostructuration peuvent être utilisées dans ce but : découpage de nanorubans de graphène, bicouche de graphène avec application d'un champ électrique transverse, percement d'un réseau périodique de nano-trous (nanomesh), structures mixtes de graphène et de nitrure de bore, dopage du graphène à l'azote. Cependant, toutes ces approches ont leurs propres difficultés de fabrication et/ou restent encore à confirmer expérimentalement. Dans ce travail, je me suis focalisée sur une autre approche : l'ingénierie de contrainte, qui offre un large éventail de possibilités pour moduler les propriétés électroniques des nanostructures de graphène. Pour ce travail théorique, tous les calculs ont été faits en utilisant essentiellement deux méthodes : un modèle atomistique de Hamiltonien de liaisons fortes pour décrire les propriétés électroniques du matériau et l'approche des fonctions de Green hors-équilibre pour le calcul du transport quantique. Après une introduction du contexte général de ce travail et des techniques de calcul développées dans ce but, j'ai d'abord analysé les effets de contrainte. En fait, une contrainte d'amplitude supérieure à 23% est nécessaire pour ouvrir un gap dans la structure de bande du graphène. Mais je montre qu'avec une contrainte de quelques pourcents, le décalage du point de dirac induit par la contrainte peut suffire à ouvrir un gap de conduction très significatif (500 meV ou plus) dans des hétérostructures de graphène constituées de jonctions graphène contraint/graphène non contraint, alors que chacun des matériaux reste semi-métallique. Après l'analyse détaillée de cette propriété en fonction de l'amplitude de la contrainte, de sa direction et de la direction du transport, j'exploite cet effet dans des jonctions appropriées pour améliore le comportement et les performances de différents types de dispositifs. En particulier, je montre qu'avec une contrainte de seulement 5% il est possible de couper efficacement le courant dans les transistors, de sorte que le rapport ON/OFF peut atteindre 100000, ce qui constitue une très forte amélioration par rapport aux transistors de graphène pristine où ce rapport ne peut pas excéder 10. Puis, nous montrons qu'en combinant ingénieries de contrainte et de dopage dans de telles jonctions, le coefficient Seebeck peut atteindre des valeurs aussi fortes que 1.4 mV/K, ce qui est 17 fois plus élevé que dans le graphène sans gap. Cela peut contribuer à faire du graphène un excellent matériau thermoélectrique. Enfin, j'ai étudié l'effet de conductance différentielle négative (CDE) dans des diodes de graphène, constituées soit d'une simple-barrière contrainte contrôlée par une grille, soit d'une jonction PN. Je montre qu'une ingénierie de contrainte appropriée peut induire de forts effets de CDE, avec un rapport pic/vallée de quelques centaines à température ambiante. / Due to its outstanding physical properties, graphene is expected to become a new generation material, able to replace or complement traditional semiconductors in device technology. Hence, many studies have been led to explore the potential of this material immediately after the successful fabrication of a single layer of graphene in 2004. However, applications of graphene in electronic devices are still questionable due to the gapless character of this material. In particular, regarding electronic applications, the absence of energy bandgap in the band structure makes it difficult to switch off the current in graphene devices like transistors. Regarding thermoelectric properties, the gapless character is also a strong drawback since it prevents the separation of the opposite contributions of electrons and holes to the Seebeck coefficient. Thus, a sizable band gap in graphene is a requirement to overcome the disadvantages of graphene and to fully benefit from its excellent conduction properties. It has been shown that many Nano structuring techniques can be used to open such a bandgap in graphene, e.g., graphene nanoribbons, graphene bilayer with a perpendicular electric field, graphene nanotech lattices, channels based on vertical stack of graphene layers, mixed graphene/hexagonal boron nitride structures, nitrogen doped graphene, and so on. However, each of these methods has its own fabrication issues and/or need to be further confirmed by experiments. In this work, we focus on strain engineering, which offers a wide range of opportunities for modulating the electronic properties of graphene nanostructures. For this theoretical work, all calculations were performed using essentially two main methods, i.e., an atomistic tight binding Hamiltonian model to describe the electronic structure and the non-equilibrium Green's function approach of quantum transport. The main aim is to analyze in details the strain effects in graphene and to provide strategies of strain engineering to improve the performance of both electronic (transistors and diodes) and thermoelectric devices. After introducing the general context if this work and the numerical techniques developed for this purpose, we first analyze the only effect of strain. Actually, if uniformly applied, a strain of large amplitude (> 23%) is required to open a bandgap in the band structure of graphene. However, we show that with a strain of only a few percent, the strain-induced shift of the Dirac point in k-space may be enough to open a sizable conduction gap (500 meV or more) in graphene heterojunctions made of unstrained/strained junctions, though the strained material remains gapless. After analyzing in details this property according the amplitude and direction of strain and the direction of transport, we exploit this effect using appropriate strain junctions to improve the behavior and performance of several types of devices. In particular, we show that with a strain of only 5%, it is possible to switch-off transistors efficiently, so that the ON/OFF current ratio can reach 100000, which is a strong improvement with respect to pristine graphene transistors where this ratio cannot exceed 10. Then we show that by combining strain and doping engineering in such strain junctions the Seebeck coefficient can reach values higher than 1.4 mV/K, which is 17 times higher than in gapless pristine graphene. It can contribute to make graphene an excellent thermoelectric material. Finally, we study the effect of negative differential conductance (NDC) in graphene diodes made of either as single gate-induced strained barrier or a p-n junction. We show that appropriate strain engineering in these devices can lead to very strong NDC effects with peak-to-valley ratios of a few hundred at room temperature.