Mareva Fevre

Imidazol(in)ium Hydrogen Carbonates as a Genuine Source of N-Heterocyclic Carbenes (NHCs): Applications to the Facile Preparation of NHC Metal Complexes and to NHC-Organocatalyzed Molecular and Macromolecular Syntheses

Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations.

Imidazolium Hydrogen Carbonates versus Imidazolium Carboxylates as Organic Precatalysts for N-Heterocyclic Carbene Catalyzed Reactions

Imidazolium-2-carboxylates (NHC-CO(2) adducts, 3) and (benz)imidazolium hydrogen carbonates ([NHC(H)][HCO(3)], 4) were independently employed as organic precatalysts for various molecular N-heterocyclic carbene (NHC) catalyzed reactions. NHC-CO(2) adducts were obtained by carboxylation in THF of related free NHCs (2), while the synthesis of [NHC(H)][HCO(3)] precursors was directly achieved by anion metathesis of imidazolium halides (1) using potassium hydrogen carbonate (KHCO(3)) in methanolic solution, without the need for the prior preparation of free carbenes. Thermogravimetric analysis (TGA) and TGA coupled with mass spectrometry (TGA-MS) of most [NHC(H)][HCO(3)] precursors 4 showed a degradation profile in stages, with either a concomitant or a stepwise release of H(2)O and CO(2), between 108 and 280 °C, depending on the nature of the azolium and substituents. In solution, NHC generation from both [NHC(H)][HCO(3)] salts and NHC-CO(2) adducts could be achieved at room temperature, most likely by a simple solvation effect. Both types of precursors proved efficient for organocatalyzed molecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficiencies of NHC-CO(2) adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)][HCO(3)] counterparts 4.

Catalysis as an Enabling Science for Sustainable Polymers

The replacement of current petroleum-based plastics with sustainable alternatives is a crucial but formidable challenge for the modern society. Catalysis presents an enabling tool to facilitate the development of sustainable polymers. This review provides a system-level analysis of sustainable polymers and outlines key criteria with respect to the feedstocks the polymers are derived from, the manner in which the polymers are generated, and the end-of-use options. Specifically, we define sustainable polymers as a class of materials that are derived from renewable feedstocks and exhibit closed-loop life cycles. Among potential candidates, aliphatic polyesters and polycarbonates are promising materials due to their renewable resources and excellent biodegradability. The development of renewable monomers, the versatile synthetic routes to convert these monomers to polyesters and polycarbonate, and the different end-of-use options for these polymers are critically reviewed, with a focus on recent advances in catalytic transformations that lower the technological barriers for developing more sustainable replacements for petroleum-based plastics.

Facile carbohydrate-mimetic modifications of poly(ethylene imine) carriers for gene delivery applications

Commercially-available linear and branched PEIs (LPEI and BPEI) were chemically-modified with carbohydrates and carbohydrate-mimetics to improve biocompatibility. Hydroxyl moieties were installed in a close proximity via reaction of PEIs amines with paraformaldehyde (pF) or glycidol. Mixing PEI with pF led to the formation of hemiaminal moieties as well as N-methylation of the backbone through an Eschweiler–Clarke-type rearrangement. The amount of attached hydroxyl groups depended on the initial amount of pF and the results were in agreement with NMR studies on model reactions with primary and secondary amines. The primary amines of BPEI triggered the ring-opening of glycidol and sugar-containing epoxides, in methanol and at room temperature. PEI chains modified with pF displayed the same cytotoxicity as the parent polymer, unless a sufficient amount of pF was added to trigger N-methylation of the backbone. In contrast, glycidol and sugar-functionalized BPEIs exhibited lower toxicity but similar (if not higher) transfection efficiency as compared to unmodified BPEI.

Broad Spectrum Macromolecular Antimicrobials with Biofilm Disruption Capability and In Vivo Efficacy

In this study, antimicrobial polymers are synthesized by the organocatalytic ring-opening polymerization of an eight-membered heterocyclic carbonate monomer that is subsequently quaternized with methyl iodide. These polymers demonstrate activity against clinically relevant Gram-positive Staphylococcus epidermidis and Staphylococcus aureus, Gram-negative Escherichia coli and Pseudomonas aeruginosa, and fungus Candida albicans with fast killing kinetics. Importantly, the polymer efficiently inhibits biofilm growth and lyses existing biofilm, leading to a reduction in biomass and cell viability. In addition, the macromolecular antimicrobial is less likely to induce resistance as it acts via a membrane-lytic mechanism. The polymer is not cytotoxic toward mammalian cells with LD50 of 99.0 ± 11.6 mg kg-1 in mice through i.v. injection. In an S. aureus blood stream infection mouse model, the polymer removes bacteria from the blood more rapidly than the antibiotic Augmentin. At the effective dose, the polymer treatment does not damage liver and kidney tissues or functions. In addition, blood electrolyte balance remains unchanged after the treatment. The low cost of starting materials, ease of synthesis, nontoxicity, broad spectrum activity with fast killing kinetics, and in vivo antimicrobial activity make these macromolecular antimicrobials ideal candidates for prevention of sepsis and treatment of infections.

Melt-processable dynamic-covalent poly(hemiaminal) organogels as scaffolds for UV-induced polymerization.

Hemiaminal poly(ethylene glycol) (PEG)-based organogels are formulated in polymerizable solvents. The dynamic-covalent nature of the solvent-H-bonded hemiaminal crosslinks, together with the modification of the crosslinking density of the organogels allows for temperature-dependent viscoelastic control. The shape of uncured gels can be permanently retained by templated UV-curing of the solvent, offering great promise for complex manufacturing, printing, sealants, and materials repair.

Organocatalyzed synthesis of fluorinated poly(aryl thioethers)

The preparation of high-performance fluorinated poly(aryl thioethers) has received little attention compared to the corresponding poly(aryl ethers), despite the excellent physical properties displayed by many polysulfides. Herein, we report a highly efficient route to fluorinated poly(aryl thioethers) via an organocatalyzed nucleophilic aromatic substitution of silyl-protected dithiols. This approach requires low catalyst loadings, proceeds rapidly at room temperature, and is effective for many different perfluorinated or highly activated aryl monomers. Computational investigations of the reaction mechanism reveal an unexpected, concerted SNAr mechanism, with the organocatalyst playing a critical, dual-activation role in facilitating the process. Not only does this remarkable reactivity enable rapid access to fluorinated poly(aryl thioethers), but also opens new avenues for the processing, fabrication, and functionalization of fluorinated materials with easy removal of the volatile catalyst and TMSF byproducts.Fluorinated poly(aryl thioethers), unlike their poly(aryl ethers) counterparts, have received little attention despite excellent physical properties displayed by many polysulfides. Here the authors show a highly efficient route to fluorinated poly(aryl thioethers) via an organocatalyzed nucleophilic aromatic substitution of silyl-protected dithiols.

Erratum: Organocatalyzed synthesis of fluorinated poly(aryl thioethers)

A correction to this article has been published and is linked from the HTML version of this article.