Liaison carbone-carbone

Résumé

Une liaison carbone-carbone est une liaison covalente entre deux atomes de carbone. La forme la plus courante est la liaison simple qui engage deux électrons, un de chaque atome. La liaison simple est une liaison σ formée entre les orbitales hybridées des atomes de carbone. Elle est forcément formée entre deux carbones d'hybridation sp3 mais peut aussi apparaître entre deux atomes de carbone avec une autre hybridation comme les carbones sp ou sp2. La liaison double qui apparaît entre carbone sp2, est caractéristique des alcènes et engage quatre électrons. Elle est formée d'une liaison σ et d'une liaison π issue des orbitales atomiques px ou py non hybridées de chaque atome de carbone. La liaison triple qui apparaît entre carbone sp, est caractéristique des alcynes et engage six électrons. Elle est formée d'une liaison σ et de deux liaisons π issues des orbitales atomiques px et py non hybridées de chaque atome de carbone. Le carbone a une caractéristique unique parmi tous les éléments

Source officielle

https://fr.wikipedia.org/wiki/Liaison_carbone-carbone

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Séances de cours associées

Chargement

C-C and C-N bonds are some of the most common structures in molecules ranging from drugs to catalysts and to food additives. Many coupling reactions were developed to form these types of bonds with excellent selectivity and good performance. Still, the synthesis of some of those species either depends on traditional methods or requires expensive and rare reagents and catalysts. The use of precious metals in chemistry, and particularly in the industry, can make molecular synthesis expensive and energy-intensive since the extraction of those materials is costly. Moreover, in homogeneous catalysis, the recovery of metal-based catalysts is almost impossible, making these processes unsustainable for the future. Thus, the discovery of chemical reactions that donât involve precious materials is indispensable. This thesis concentrates on the development of two chemical transformations, one performing an sp2-sp3 radical reductive coupling, one performing a photocatalyzed alkyl amination, involving inexpensive reagents and catalysts. The first chapter reviews the existing methodologies which use alkyl radicals to form carbon-carbon and carbon-nitrogen bonds. Alkyl radicals are formed from alkyl halides and pseudo-halides (Acids, esters, tosylates, etc.) via oxidation or reduction performed by metal or photoredox catalyst. The wide variety of available methodologies was illustrated, highlighting the advantages and weaknesses of each reaction. The second chapter of this dissertation is devoted to the development of a new iron-based catalytic method to form stereoselectively Z-alkene boron species from the corresponding boron-alkyne. This novel stereoselective method opens a new way to obtain these types of molecules with cheap reagents, in high yields, with excellent stereoselectivity (Z:E ratio >10:1), and with good tolerance of functional group. The reaction is radical based, avoiding the problem of beta-hydride elimination, which usually occurs in ionic reactions involving metal centers. In order to prove the versatility of this method, the vinyl boron compound formed from this reaction was used for further functionalizations. The formal total synthesis of a 5-HT2c receptor agonist was successfully performed, improving the overall yield of the process, and avoiding the use of expensive and polluting chemicals. The third chapter of this thesis is concentrated on the development of a tandem photoredox/metal base catalyzed functionalization of anilines and imines with alkyl carboxylic esters. In this reaction, no late transition metal is used, and the traditional metal-based photocatalyst is substituted with an inexpensive and more efficient organic dye. The alkylation of amines is a challenging topic in organic chemistry and classical nucleophilic substitution lacks selectivity and scope. Harvesting visible light with a photoredox catalyst opens the possibility to a radical pathway, making this coupling easy and with high tolerance for functional groups. In order to understand the catalytic cycle of the reaction, additional mechanistic studies were conducted, leading to some insight on how this reaction works.

EPFL2020

Chargement

Rh-catalyzed C-H Bond Heteroarylation with Hypervalent Iodine Reagents and Pd-Catalyzed Tethered Functionalizations of Carbon-Carbon Multiple Bonds.

Ashis Kumar Das

Transition metal-catalyzed C-H activation is a powerful tool for the synthesis of organic building blocks, materials, and natural products. Over the last decade, directing-group-assisted metal-catalyzed C-H functionalization has attracted increasing interest as a means to target specific C-H bonds. Moreover, a large variety of electrophiles has been employed in this kind of processes. In this thesis, we describe a novel C-6 selective C-H heteroarylation of pyridine-2-ones using indoleBX reagents as coupling partners under Rh(I)-catalysis. The reaction worked efficiently also with other analogous heterocyclic substrates and with a quinoline-N-oxide. We were able to obtain 6-(indol-3-yl)pyridinone and 8-(indol-3-yl)quinolone after cleavage of the directing group or rearrangement of the N-oxide moiety. 1,2-Amino alcohols are important structural motifs in organic chemistry that can be observed in natural products, pharmaceutically and biologically active molecules. The palladium-catalyzed difunctionalization of C-C multiple bonds has been broadly investigated as an efficient strategy to access such substructures. This transformation has been reported using either Pd(0)/Pd(II) or Pd(II)/Pd(IV) catalytic cycles, in which the nucleophilic functionalization of a putative Pd(II)-alkyl intermediate is achieved prior to beta-hydride elimination. As a further topic of this thesis, we developed new methods for the synthesis of amino alcohols whereby the Pd-catalyzed difunctionalization of alkynes and alkenes was combined with the tethering approach previously established in our laboratories. We first investigated the Pd-catalyzed enantioselective hydroalkoxylation of propargylic amines. The latter were tethered in situ with a trifluoroacetaldehyde hemiacetal. Best results were obtained with commercially available DACH-Phenyl Trost ligand. Aryl iodides were required as an additive in catalytic amount for successful outcome. A large variety of chiral oxazolidines were obtained in good to excellent yields and ee, followed by a diastereoselective reduction to give protected enantioenriched amino alcohols. A possible mechanism for this transformation relies on a Pd(0)/Pd(II) catalytic cycle involving oxidative addition of aryl iodide with a subsequent oxypalladation/protodemetallation (or reductive elimination) reaction. This is necessary to generate the active complex, not as the start of a cross-coupling event. The use of a commercially available DACH-Ph Trost ligand to produce high enantioselectivity in an unprecedented DYKAT process was critical to success.The Pd-catalyzed amino acetoxylation of cinnamyl alcohols was next taken into consideration towards the synthesis of valuable amino diol derivatives. In this case, preformed substrates were used, where the alcohol moiety was tethered to N-nucleophilic groups. Conditions involving Pd(II)/Pd(IV) catalysis and employing PIDA as a strong oxidant were employed. The use of this hypervalent iodine reagent was key to the success of the transformation. Amino acetoxylated products were obtained in moderate to good yields and dr. The scope of this reaction remains mostly limited to cinnamyl derived substrates. The reaction conditions did not tolerate substituents on the alpha, beta, and gamma positions of the allylic chains. Alkyl chain containing unsaturated alcohols delivered Aza-Wacker-type products.

EPFL2022

Chargement

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

EPFL2013

Chargement

EPFL2013

Rh-catalyzed C-H Bond Heteroarylation with Hypervalent Iodine Reagents and Pd-Catalyzed Tethered Functionalizations of Carbon-Carbon Multiple Bonds.

Ashis Kumar Das

EPFL2022

EPFL2020