Céline Besnard

Fused Indolines by Palladium‐Catalyzed Asymmetric CC Coupling Involving an Unactivated Methylene Group

Selectivity at high temperatures: Bulky, thermally stable, chiral palladium complexes generated from N-heterocyclic carbenes (NHCs; see picture) were successfully applied to the synthesis of highly enantioenriched trans-fused indolines. Remarkably, this reaction occurs at elevated temperatures with excellent asymmetric recognition of an enantiotopic C-H bond in an unactivated methylene unit.

Enantioselective One‐Pot Organocatalytic Michael Addition/Gold‐Catalyzed Tandem Acetalization/Cyclization

A one-pot process consisting of a Michael addition to a nitroenyne and a subsequent acetalization/cyclization is reported (see scheme; TMS=trimethylsilyl), which results in the formation of nitro-substituted tetrahydrofuranyl ethers with high diastereo- and enantioselectivities. Organocatalysis and gold catalysis are compatible and complementary in a one-pot process.

Near-Infrared→Visible Light Upconversion in a Molecular Trinuclear d-f-d Complex

Giving the green light: The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission. Copyright

Iridium-Catalyzed Asymmetric Isomerization of Primary Allylic Alcohols†

Nothing to sm(Ir)k at: Under appropriate reaction conditions, iridium hydride catalysts promote the isomerization of primary allylic alcohols. The best catalysts, like (R)-1 (P green, O red, N blue, Ir yellow), deliver the desired chiral aldehydes with excellent enantioselectivity and good yields. Mechanistic hypotheses have been developed on the basis of preliminary investigations.

One-Step Synthesis of Nitrogen-Containing Medium-Sized Rings via α-Imino Diazo Intermediates

Eight- and 9-membered dioxazocines and dioxazonines are readily synthesized starting from N-sulfonyl-1,2,3-triazoles in a single-step procedure. A perfect regioselectivity and generally good yields (up to 92%) are obtained under dirhodium catalysis using 1,3-dioxolane and 1,3-dioxane as solvents and reagents.

Modular Synthesis, Orthogonal Post-Functionalization, Absorption, and Chiroptical Properties of Cationic (6)Helicenes**

Pick and choose: Novel cationic diaza-, azaoxo-, and dioxo[6]helicenes are readily prepared and functionalized selectively by orthogonal aromatic electrophilic and vicarious nucleophilic substitutions (see scheme). Reductions, cross-coupling, or condensation reactions introduce additional diversity and allow tuning of the absorption properties up to the near-infrared region. The diaza salts can be resolved into single enantiomers.

Rational design of a ternary supramolecular system: self-assembly of pentanuclear lanthanide helicates.

The self-assembly of the first pentanuclear helicate was predicted on the structural basis obtained for linear and tetranuclear parent supramolecular compounds. Accordingly, the designed ternary supramolecular system requires appropriate polytopic organic receptors, which were successfully synthesized. Indeed, the formation of pentanuclear complexes was experimentally evidenced with NMR and ESMS spectra that perfectly reflect the expected pattern. The structural features in the europium pentanuclear complex are highlighted with semiempirical molecular modeling. The present work validates the combinatorial approach leading to the thermodynamically driven formation of tower-like pentanuclear edifices.

Synthesis of 3,3-Disubstituted Oxindoles by Palladium-Catalyzed Asymmetric Intramolecular α-Arylation of Amides: Reaction Development and Mechanistic Studies

Palladium complexes incorporating chiral N-heterocyclic carbene (NHC) ligands catalyze the asymmetric intramolecular α-arylation of amides producing 3,3-disubstituted oxindoles. Comprehensive DFT studies have been performed to gain insight into the mechanism of this transformation. Oxidative addition is shown to be rate-determining and reductive elimination to be enantioselectivity-determining. The synthesis of seven new NHC ligands is detailed and their performance is compared. One of them, L8, containing a tBu and a 1-naphthyl group at the stereogenic centre, proved superior and was very efficient in the asymmetric synthesis of fifteen new spiro-oxindoles and three azaspiro-oxindoles often in high yields (up to 99 %) and enantioselectivities (up to 97 % ee; ee=enantiomeric excess). Three palladacycle intermediates resulting from the oxidative addition of [Pd(NHC)] into the aryl halide bond were isolated and structurally characterized (X-ray). Using these intermediates as catalysts showed alkene additives to play an important role in increasing turnover number and frequency.

Improved Catalysts for the Iridium-Catalyzed Asymmetric Isomerization of Primary Allylic Alcohols Based on Charton Analysis

An improved generation of chiral cationic iridium catalysts for the asymmetric isomerization of primary allylic alcohols is disclosed. The design of these air-stable complexes relied on the preliminary mechanistic information available, and on Charton analyses using two preceding generations of iridium catalysts developed for this highly challenging transformation. Sterically unbiased chiral aldehydes that were not accessible previously have been obtained with high levels of enantioselectivity, thus validating the initial hypothesis regarding the selected ligand-design elements. A rationale for the high enantioselectivities achieved in most cases is also presented.

Scope and mechanism of asymmetric C(sp3)–H/C(Ar)–X coupling reactions: computational and experimental study

Advances in the efficient palladium–NHC catalysed synthesis of highly enantioenriched 2,3-trans-fused and 2-alkyl indolines via asymmetric C(sp3)–H activation of an unactivated methylene/methyl group are reported. Very high asymmetric inductions (up to 99% ee) were achieved at reaction temperatures ranging from 120 to 160 °C. Factors influencing the efficiency of the reaction (halide, pseudohalide, N-protecting group) were investigated. The reaction pathway and enantioselection were probed by detailed density functional theory (DFT) calculations (M06-L functional). The combined theoretical and experimental study shows that the Pd–NHC catalysed C(sp3)–H arylation proceeds via a concerted metalation–deprotonation (CMD) mechanism. The CMD step is shown by DFT calculations and kinetic isotope effect measurements to be selectivity-determining. A good agreement between experimental enantioselectivities and calculated differences amongst diastereomeric activation barriers is observed.