A valence bond view of isocyanides' electronic structure

International audienceHigh level Valence Bond calculations support a predominantly carbenic electronic structure for isocyanides, with a secondary zwitterionic character, despite their linear geometry. This geometry results from the significant energetic stabilization due to nitrogen π lone pair donation. Results are not changed by substitution or solvation effects

Approche théorique de la réactivité des isonitriles en chimie organique

Les isonitriles sont des espèces connues depuis longtemps, mais étudiées depuis peu. Une approche théorique a permis de s'intéresser en détails aux réactions de Nef et de Ugi. Nous nous sommes tout d'abord focalisés sur la première. Après en avoir élucidé le mécanisme, nous avons étudié l'effet du solvant et nous avons proposé de nouvelles conditions expérimentales. Nous avons ensuite étudié l'influence des groupements de l'acyl, de l'isonitrile et du groupe partant. L'ensemble des variations considérées a pu être rationalisé en reliant l'énergie d'activation au pKa du groupe partant. En parallèle, nous avons étudié la réaction de Ugi. Le mécanisme proposé par Ugi pour cette réaction complexe n'avait toujours pas été vérifié 50 ans après sa découverte. Une étude quasi-exhaustive des différents mécanismes possibles a été menée, en utilisant une approche originale mêlant théorie et expériences. Le mécanisme de cette réaction a ainsi été démontré, tant dans le méthanol que dans le toluène. Les étapes cinétiquement déterminantes et les forces motrices ont été mises en lumière et diffèrent de celles proposées par Ugi. Une variation de la réaction de Ugi est le couplage Ugi-Smiles pour lequel de nombreux résultats expérimentaux n'ont toujours pas trouvé d'explications. Nous nous sommes donc intéressés au réarrangement de Smiles. Nous avons montré l'importance d'une liaison hydrogène intramoléculaire sur la faisabilité de la réaction, et nous avons étendu cette observation aux réactions intermoléculaires. Nous avons également étudié l'influence des substituants des quatre réactifs sur les barrières afin de construire un modèle prédictif.Isocyanides are known for a long time, but have been studied only recently. A theoretical approach allowed us to investigate in details the Nef and the Ugi reactions. We first focused on the former. After elucidating its mechanism, we studied solvent effects and proposed new experimental conditions. We then studied the acyl moiety and isocyanide influences, such as the leaving group one. All the variations were rationalized by linking the activation energy to the leaving group pKa. Simultaneously, we studied the Ugi reaction. The mechanism proposed by Ugi for this complex reaction was still unverified 50 years after its discovery. A thorough and quasi-complete study of all the possible mechanisms were lead, using a mixed theoretical and experimental approach. The mechanism of this reaction was demonstrated, in both methanol and toluene. Rate determining steps and driving forces were highlighted and differ from those proposed by Ugi. A variation of the Ugi reaction is the Ugi-Smiles coupling, for which numerous experimental results are still unexplained. We thus studied the Smiles rearrangement. We showed the key-role of an intramolecular hydrogen bond on the reaction feasibility, and extended this observation to intermolecular reactions. We also studied the four substituent influence on the barrier, aiming to build a predictive model.LYON-ENS Sciences (693872304) / SudocSudocFranceF

Structure of hexakis-methylthiobenzene: A theoretical study

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Performance of recent density functionals to discriminate between olefin and nitrogen binding to palladium

International audienceIn the last decades, density functional theory has become unavoidable in theoretical studies of organometallic chemistry. Most of the recent functionals contain many parameters that are adjusted using carefully chosen reaction sets. However, these sets only contain a few entries involving late transition metal reaction, so that choosing a functional for such a study is difficult. In this work, the theoretical description of the oxidative addition of Pd(PH3)2 to 2-iodo-allyl-aniline was chosen as a representative reaction of palladium. The competitive binding of the palladium to the alkene or the nitrogen atom was used to assess the accuracy of ab initio methods (MP2, MP3, MP2.5, SCS-MP2, SCS-MP3) and 56 functionals ranging from local density approximation to the costly double-hybrid approaches (such as B2PLYP), against a CCSD(T)/CBS reference value. Model systems [(PH3)2ClPd(NH3)]+ and [(PH3)2ClPd(H2C=CH2)]+ were first considered: all functionals correctly predict that the azane complex is the most stable. However, some functionals overestimate its stability compared to the alkene complex. This is amplified in the 2-iodo-allyl-aniline study: SCS-MP3, B2PLYP as well as BP86, most of the meta-GGA (generalized gradient approximation), hybrid GGAs and hybrid meta-GGAs are predicting that oxidative addition proceeds directly. On the contrary, many functionals, among which B3LYP, M06-2X and most range-separated methods, wrongly predict that palladium first binds to the nitrogen atom before proceeding to the olefin insertion. Resorting to these functionals to study inorganic reactions with palladium might thus result in predicting wrong mechanisms

ETUDE THEORIQUE DU ROLE DES LIGANDS DONNEURS PI DANS LES COMPOSES DES METAUX DE TRANSITION ET DU GROUPE PRINCIPAL

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Density Functional Study of Indole Formation by an Intramolecular Heck Reaction

International audienceThe mechanism of an intramolecular palladium-catalyzed Heck–Mizoroki reaction to form indole derivatives was studied using density functional theory (DFT). Starting from the N-allyl-2-iodo-N-methylaniline, four routes were detailed: two lead to five-membered rings and two to six-membered rings. In agreement with previous studies, oxidative addition was found to be the rate-determining step with a rather small activation free enthalpy of 10.2 kcal/mol. The five-membered ring is both kinetically and thermodynamically favored: the activation barriers for the ring closure differ by more than 4 kcal/mol, and the obtained five-membered product is more stable than the six-membered product by 1 kcal/mol . Because of its aromatic character, indole is more stable than the other three products by more than 20 kcal/mol: in agreement with experiments, it is the main product of this reaction. This study also revealed that the nitrogen atom does not play any role in the indole formation, while it discriminates between the two six-membered ring