Jean-Valère Naubron

Catalyzed Dehydrogenative Coupling of Primary Alcohols with Water, Methanol, or Amines†

A working partnership: Metal-ligand cooperativity is responsible for the high activity of the rhodium amido complex 1 in the dehydrogenative coupling of primary alcohols with water, methanol, or amines, including ammonia (see scheme), to give carboxylic acids, methyl carboxylates, or amides, respectively. The catalysis proceeds under mild reaction conditions in the presence of a recyclable hydrogen acceptor A. The multistep mechanism was elucidated by computational methods.

Ethanol as Hydrogen Donor: Highly Efficient Transfer Hydrogenations with Rhodium(I) Amides

Homogeneously catalyzed transfer hydrogenation has become a powerful tool in synthetic chemistry, and a wide range of unsaturated substrates can be employed in this reaction. Impressive activities (turnover frequencies TOF> 1 # 10 h ) and selectivites have been reached. Ruthenium(II) arene complexes and rhodium(III) cyclopentadienyl complexes in combination with 2-propanol or formic acid/ triethylamine mixtures as hydrogen donors are among the most popular catalytic systems. Ethanol is a renewable resource and has spurred considerable interest as an alternative to fossil fuels and as a potential feedstock for the chemical industry. Although reduced organometallic complexes are often prepared by reacting a complex with the metal in a higher oxidation state with ethanol (for example, Rh!Rh or Ru!Ru), ethanol has not been investigated systematically as a hydrogen source in transfer hydrogenation. This may be due to the fact that ethanol frequently poisons the catalyst by forming stable and inactive carbonyl complexes and that under basic conditions, aldol condensation products are easily formed with acetaldehyde. We reported that the d Rh diolefin amide [Rh(trop2N)(PPh3)] (2a) is an active catalyst for ketone and imine hydrogenation with H2 (trop2N= bis(5-H-dibenzo[a,d]cyclohepten-5-yl)amide). We report herein that such Rh amide complexes are very efficient catalysts for the reaction in Equation (1).

Tandem radical addition–aldol condensations: evidence for the formation of zinc enolates in diethylzinc mediated radical additions to N-enoyloxazolidinones

Diethylzinc mediated addition of alkyl radicals to chiral N-enoyloxazolidinones is immediately followed by homolytic substitution at zinc leading to a zinc enolate; the trapping of the latter in a subsequent aldol condensation serves as a useful mechanistic probe; overall this reaction sequence constitutes a novel example of a one pot, three-component, radical-polar crossover reaction.

One-Step Catalytic Asymmetric Synthesis of Configurationally Stable Tröger Bases

Bridging the gap: Configurationally stable ethano-bridged Troger bases have been prepared in a single step by the direct rhodium(II)-catalyzed reaction of methano-bridged Troger bases and diazo esters (see scheme). The process is general, enantiospecific, diastereoselective (with introduction of a new quaternary carbon center), and regioselective.

Electron cryo-microscopy of TPPS4⋅2HCl tubes reveals a helical organisation explaining the origin of their chirality.

A widely studied achiral porphyrin, which is highly soluble in aqueous solutions (TPPS4), is shown to self‐assemble into helical nanotubes. These were imaged by electron cryo‐microscopy and a state‐of‐the‐art image analysis allows building a map at ∼5 Å resolution, one of the highest obtained so far for molecular materials. The authors were able to trace the apparent symmetry breaking to existing nuclei in the “as received samples”, while carefully purified samples show that both handnesses occur in equal amounts.magnified image

Double Transfer of Chirality in Organocopper-Mediated bis(Alkylating) Cycloisomerization of Enediynes†

An original synthesis of chiral benzofulvenes triggered by organocopper reagents is reported. These enantiopure products are available through a highly chemo-, regio-, diastereo-, and enantioselective bis(alkylating) cycloisomerization process. A double chirality transfer (central-to-axial-to-central) is observed.

A switchable self-assembling and disassembling chiral system based on a porphyrin-substituted phenylalanine–phenylalanine motif

Artificial light-harvesting systems have until now not been able to self-assemble into structures with a large photon capture cross-section that upon a stimulus reversibly can switch into an inactive state. Here we describe a simple and robust FLFL-dipeptide construct to which a meso-tetraphenylporphyrin has been appended and which self-assembles to fibrils, platelets or nanospheres depending on the solvent composition. The fibrils, functioning as quenched antennas, give intense excitonic couplets in the electronic circular dichroism spectra which are mirror imaged if the unnatural FDFD-analogue is used. By slightly increasing the solvent polarity, these light-harvesting fibres disassemble to spherical structures with silent electronic circular dichroism spectra but which fluoresce. Upon further dilution with the nonpolar solvent, the intense Cotton effects are recovered, thus proving a reversible switching. A single crystal X-ray structure shows a head-to-head arrangement of porphyrins that explains both their excitonic coupling and quenched fluorescence.

Synthesis, structural analysis, and chiral investigations of some atropisomers with EE-tetrahalogeno-1,3-butadiene core.

The atropenantiomers of stable 1,2,3,4-tetrahalo-1,3-butadiene derivatives (where halogeno stands for bromine or iodine) were separated with use of chiral HPLC. The barriers for the enantiomerization process were determined on-line by dynamic HPLC (DHPLC) or off-line by classical kinetic measurements. In the case of the tetrachloro compound, the barrier was too low for DHPLC and its value was obtained by dynamic NMR experiments. The obtained barriers for chloro, bromo, and iodo derivatives correlate with the van der Waals radii of the halogens. The absolute configuration of the isolated enantiomers of the tetraiodo and tetrabromo compounds was assigned by comparison of the experimental and conformations averaged calculated VCD spectra. The identification of a signature band of the absolute configuration of the butadiene core, the sign and location of which are independent from the different conformations and substituents, allowing the safe assignment of the absolute configuration of the enantiomers of chiral 1,3-butadienes, is also reported.

Origin of the Enantioselectivity in Organocatalytic Michael Additions of β-Ketoamides to α,β-Unsaturated Carbonyls: A Combined Experimental, Spectroscopic and Theoretical Study

The organocatalytic enantioselective conjugate addition of secondary β-ketoamides to α,β-unsaturated carbonyl compounds is reported. Use of bifunctional Takemotos thiourea catalyst allows enantiocontrol of the reaction leading either to simple Michael adducts or spirocyclic aminals in up to 99 % ee. The origin of the enantioselectivity has been rationalised based on combined DFT calculations and kinetic analysis. This study provides a deeper understanding of the reaction mechanism, which involves a predominant role of the secondary amide proton, and clarifies the complex interactions occurring between substrates and the catalyst.

Chiral Nanographene Propeller Embedding Six Enantiomerically Stable [5]Helicene Units

A one-step synthesis of a nanographene propeller with a D3-symmetry was obtained starting from 7,8-dibromo[5]helicene by Yamamoto nickel(0) couplings. It afforded a chiral polyaromatic hydrocarbon (PAH) embedding six enantiomerically stable [5]helicene units. This dense accumulation of helical strain resulted in a distorted geometry, but stable stereochemistry. The conformational, structural, chiroptical, and photophysical properties of the molecule are reported.