Eric Fouquet

Heterogeneous multifunctional catalysts for tandem processes: an approach toward sustainability.

The development of sustainable organometallic chemistry is one of the many challenges that chemists must address to solve environmental issues. Until now, the classical approach was to develop highly selective homogeneous catalysts for a single transformation. However, a new area of research focuses on the execution of two or more reactions in the same vessel using heterogeneous multitask catalysts. This approach results in a reduced number of operations leading to time and cost benefits. Moreover, owing to the scarcity of natural resources and the recent dramatic cost increase of these resources and metals, the development of recyclable heterogeneous catalysts is urgent. This Minireview focuses on the latest developments in tandem reactions promoted by heterogeneous multitask catalysts.

Room-Temperature, Ligand- and Base-Free Heck Reactions of Aryl Diazonium Salts at Low Palladium Loading: Sustainable Preparation of Substituted Stilbene Derivatives

The Pd(OAc)(2)-catalyzed Heck reaction of aryl diazonium salts with 2-arylacrylates led to cis-stilbenes with good to excellent stereoselectivity. The environmentally friendly protocol developed in this work features low palladium loading in technical-grade methanol at room temperature under base-, additive-, and ligand-free conditions. The same protocol applied to simple Heck coupling of aryl diazonium salts with methyl acrylate allows astonishingly low palladium loading, down to 0.005 mol%. The stereoselectivity experimentally observed for the synthesis of cis-stilbenes has been rationalized by DFT calculations. Moreover, the role of methanol in promoting the reaction has been clarified by a computational study.

Synthesis of oxindoles by tandem Heck-reduction-cyclization (HRC) from a single bifunctional, in situ generated Pd/C catalyst.

A tandem sequence involving palladium-catalyzed sequential Heck-reduction-cyclization transformations in mild conditions has been developed for the synthesis of oxindoles. The protocol involves inexpensive reagents and does not require any additives such as base or ligands.

NMR and molecular modeling of wine tannins binding to saliva proteins: revisiting astringency from molecular and colloidal prospects

In organoleptic science, the association of tannins to saliva proteins leads to the poorly understood phenomenon of astringency. To decipher this interaction at molecular and colloidal levels, the binding of 4 procyanidin dimers (B1–4) and 1 trimer (C2) to a human saliva proline‐rich peptide, IB714, was studied. Interactions have been characterized by measuring dissociation constants, sizes of complexes, number, and nature of binding sites using NMR (chemical shift variations, diffusion‐ordered spectroscopy, and saturation transfer diffusion). The binding sites were identified using molecular mechanics, and the hydrophilic/hydrophobic nature of the interactions was resolved by calculating the molecular lipophilicity potential within the complexes. The following comprehensive scheme can be proposed: 1) below the tannin critical micelle concentration (CMC), interaction is specific, and the procyanidin anchorage always occurs on the same three IB714 sites. The tannin 3‐dimensional structure plays a key role in the binding force and in the tannins ability to act as a bidentate ligand: tannins adopting an extended conformation exhibit higher affinity toward protein and initiate the formation of a network. 2) Above the CMC, after the first specific hydrophilic interaction has taken place, a random hydrophobic stacking occurs between tannins and proteins. The whole process is discussed in the general frame of wine tannins eliciting astringency.—Cala, O., Pinaud, N., Simon, C., Fouquet, E., Laguerre, M., Dufourc, E. J., Pianet, I. NMR and molecular modeling of wine tannins binding to saliva proteins: revisiting astringency from molecular and colloidal prospects. FASEB J. 24, 4281–4290 (2010). www.fasebj.org

Unprecedented substoichiometric use of hazardous aryl diazonium salts in the Heck-Matsuda reaction via a double catalytic cycle.

The first Heck-Matsuda reaction using a catalytic amount of diazonium salts has been discovered. The reaction proceeds through an unprecedented double catalytic cycle in which the electrophile is in situ generated through the reaction advancement. This concept features mild and safe conditions as hazardous aryl diazonium salts are not isolated anymore. Importantly, this sustainable procedure generates only environmentally friendly byproduct such as t-BuOH, H(2)O, and N(2).

A Useful, Reliable and Safer Protocol for Hydrogenation and the Hydrogenolysis of O‐Benzyl Groups: The In Situ Preparation of an Active Pd0/C Catalyst with Well‐Defined Properties

A simple, highly reproducible protocol for the hydrogenation of alkenes and alkynes and for the hydrogenolysis of O-benzyl ethers has been developed. The method features the in situ preparation of an active Pd(0)/C catalyst from Pd(OAc)(2) and charcoal, in methanol. The mild reaction conditions (25°C) and low catalyst loading required (0.025 mol%), as well as the absence of contamination of the product by palladium residues (<4 ppb), make this a sustainable, useful process for organic chemists. Alternatively, the protocol can be carried out under microwave activation, to shorten the reaction times, with cyclohexene as the hydrogen source.

Modeling Procyanidin Self-Association Processes and Understanding Their Micellar Organization : A Study by Diffusion NMR and Molecular Mechanics

The colloidal behavior of eight synthetic procyanidins (three monomers, four dimers, and a trimer) has been investigated in water or in a winelike medium using DOSY NMR spectroscopy and molecular dynamics simulations. Different behavior was observed for monomers and oligomers. Monomers self-associate with a high affinity constant (37-53 M(-1)) to form micelles at low cmc (critical micelle concentration) values (1-5 g.L(-1)). These micelles undergo a time-dependent coalescence process to form hazes and precipitates. As for dimers and the trimer, self-association also occurs but with a lower affinity (approximately 6 M(-1)) and at higher cmc values (10-20 g.L(-1)) to form small micelles (<5 nm) that remain stable throughout the experiment. The presence of 10% ethanol does not significantly affect the self-association constant for monomers and oligomers but increases their cmc values by approximately 50% and decreases the micelle size by a factor 2. However, the presence of 20 mM NaCl appears to negate the effect of ethanol. This study helps to clarify the role of procyanidin monomers versus oligomers in wine turbidity and demonstrates that procyanidin oligomers are fully available to interact with saliva proteins.

Sustainable Heck–Matsuda Reaction with Catalytic Amounts of Diazonium Salts: An Experimental and Theoretical Study

The palladium-catalyzed Heck-Matsuda reaction with a catalytic amount of an in-situ-generated diazonium salt proceeded under mild and sustainable conditions. The reaction proceeded at room temperature, under base-free conditions, and only generated tBuOH, H(2)O, and N(2) as by-products. Ortho-substituted diazonium salts were more-efficiently coupled to methyl acrylate than their corresponding para isomers, which required the addition of anisole as an additive. In support of these experimental data, we carried out theoretical studies to gain a deeper understanding of these reaction outcomes.

Silicon‐Based Chemistry: An Original and Efficient One‐Step Approach to [18F]‐Nucleosides and [18F]‐Oligonucleotides for PET Imaging

Take it eaSi! Nucleosides, dinucleotides, and one oligonucleotide, all modified by click chemistry, have for the first time been directly and very efficiently labeled with (18)F by using a silicon-based, one-step approach that opens the way for the development of a new class of positron emission tomography (PET) tracers (see graphic).

Stabilisation of Carbon‐Supported Palladium Nanoparticles through the Formation of an Alloy with Gold: Application to the Sonogashira Reaction

Oh my Gold! Gold atoms stabilise catalytically active palladium nanoparticles when engaged in an alloy heterogenised on carbon. The increased durability makes the Pd-Au/C catalyst more recyclable than the gold-free Pd/C catalyst for the Sonogashira reaction.