Brigitte Bibal

Hydrogen-bonding organocatalysts for ring-opening polymerization

In the ring-opening polymerization (ROP) of cyclic esters and carbonates, hydrogen-bonding organocatalysis offers an interesting alternative to metal-based and enzymatic catalysis to access biocompatible and biodegradable polymers. The design of catalysts, strategies of activation and mechanistic elucidations are highlighted. Recent developments with functionalized monomers for applications in nanomaterials and biomedicine have opened perspectives to broaden the scope of future catalytic systems. In the field of sustainable chemistry, hydrogen-bonding structures devoted to ROP have become a full-fledged class of catalysts.

(Thio)Amidoindoles and (Thio)Amidobenzimidazoles: An Investigation of Their Hydrogen-Bonding and Organocatalytic Properties in the Ring-Opening Polymerization of Lactide

The mechanism of the ring-opening polymerization (ROP) of lactide catalyzed by two partner hydrogen-bonding organocatalysts was explored. New amidoindoles 4 a,c, thioamidoindoles 4 b,d, amidobenzimidazoles 5 a,c, and thioamidobenzimidazoles 5 b,c were synthesized and used as activators of the monomer. In the solid state and in solution, compounds 4 and 5 showed a propensity for self-association, which was evaluated. (Thio)Amides 4 and 5 do catalyze the ROP of lactide in the presence of a cocatalyst, tertiary amine 3 a or 3 b, which activates the growing polymer chain through hydrogen-bonding. Reactions were conducted in 2-24 h at 20 degrees C; conversion yields ranged between 22 and 100 %. A detailed study of the intermolecular interactions undertaken between the participating species showed that, as expected, simultaneous weak hydrogen bonds do exist to activate the reagents. Moreover, interactions have been revealed between the partner catalysts 4/5+3. ROP catalyzed by these partner activators is thus governed by multiple dynamic equilibria. The latter should be judiciously adjusted to fine-tune the catalytic properties of (thio)amides and organocatalysts, more generally.

Ring-Opening Polymerization of L-Lactide Efficiently Triggered by an Amido-Indole. X-ray Structure of a Complex between L-Lactide and the Hydrogen-Bonding Organocatalyst

N-(3,5-Bis(trifluoromethyl)phenyl)-1H-indole-2-carboxamide 1e is an efficient hydrogen-bonding organocatalyst for the ring-opening polymerization of l-lactide. This new catalytic species does control the dispersity (1.08) and molecular weight (3460 g/mol vs 3064 in theory) of the poly(l-lactides) prepared in 2 h. (1)H NMR analysis showed that compound 1e complexes l-lactide in CDCl(3) through the two available NH groups (amide and indole). In particular, the catalytic species appeared to be mainly the H-bonding donor amide (1e in extended conformation, alone or dimer (1e)(2)) and, to a lesser extend, the dual H-bonding amido-indole (1e in its the pinched conformation). The first X-ray structure of the complex between a H-bonding organocatalyst and l-lactide also revealed a tight H-bonded network between the dimer (1e)(2) and l-lactide.

Ring-opening polymerization of lactones using supramolecular organocatalysts under simple conditions

Ring-opening polymerizations of δ-valerolactone (δ-VL) and e-caprolactone (e-CL) were catalyzed by a metal-free system composed of two H-bonding components, a phenol derivative to activate the monomer, and DBU, which enhanced the nucleophilicity of the initiator and the propagating chain. Compared to other H-bonding systems for the ROP of lactones, phenol + DBU catalysts had the practical advantages of being commercially available and inexpensive, efficient at room temperature and under simple experimental conditions that avoid drying of reactants and the use of a glove-box. In addition, the obtained polyesters had a narrow dispersion of molar masses which were controlled by the concentration ratio of monomer versus initiator. Moreover, the initiation of the polymerization by DBU and residual water molecules (despite no specific drying of reagents) was shown to be very minor under the experimental conditions. No initiation by the phenol catalysts was observed. Block copolyesters PVL-PLA and PCL-PLA were also prepared using these conditions.

Polyphosphorylated Triphenylenes: Synthesis, Crystal Structure, and Selective Catechol Recognition.

Designed as a multivalent hydrogen bond acceptor, new receptors, Discopus 1a,b, were built from a triphenylene core surrounded by six (diaryl)phosphinate groups. An efficient synthesis was developed to prepare these elaborated structures in a high overall yield. The X-ray structure of receptor 1b showed strong cooperative hydrogen bonds with two water molecules and intermolecular CH-pi contacts. In chloroform, Discopus 1a,b displayed recognition properties toward dihydroxybenzenes, selectively forming complexes with catechol derivatives 4a-c in a 1:2 (host:guest) stoichiometry. According to NMR and microcalorimetry titrations, association constants were found in the 30-2837 M(-1) range, which were larger than those reported for curvated catechol receptors (14-120 M(-1)). Interestingly, Discopus present two distinct catechol binding sites. Weak hydrogen bonding between host phosphinates and guest hydroxyl groups was shown by infrared spectroscopy and (31)P NMR. Molecular dynamics simulations and recognition experiments suggested that a stronger hydrogen bond assisted by a pi-interaction between the Discopus core and one catechol molecule could exist within the 1:2 complex.

Activation of carbonyl bonds by quaternary ammoniums and a (Na+:crown-ether) complex: investigation of the ring-opening polymerization of cyclic esters

Quaternary ammoniums associated with bis(trifluoromethane)sulfonimide (NTf2) or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) counterions were readily prepared from commercially available tertiary amine, amidine, guanidine and pyridine. As predicted by molecular modelling, these ammonium salts proved to be efficient multiple H-bond donor catalysts in the ring-opening polymerization of cyclic esters (lactide, δ-valerolactone and e-caprolactone). In addition, a sodium(I) complex of [15-c-5] crown-ether paired with NTf2 or BARF was shown to activate the cyclic monomers through a cation–dipole interaction. These simple and non-protonated ionic structures appeared as attractive alternative organocatalysts to classical H-bond donors for the activation of carbonyl bonds.

Controlled bulk polymerization of L-lactide and lactones by dual activation with organo-catalytic systems

The acid–base catalytic system based on N,N-dimethyl-4-aminopyridine (DMAP) and a protic acid that has already been revealed to be efficient for the ring-opening polymerization (ROP) of L-lactide in solution at room temperature was tested for the same polymerization in bulk at 100 °C. As observed in solution, the presence of the DMAP·HX (X = Cl, CH3SO3, CF3SO3) salt enhanced yields. Linear and star-like polylactides with 3 and 4 branches were prepared. Polylactides were thus easily prepared reaching high molar masses (up to 75000 g mol−1 for linear PLLA and 140000 g mol−1 for star-like PLLA) with good control in less than 1 h. In all cases, the appearance of transesterification reactions was shown to occur only at very high yield. The ROP of lactones (e-caprolactone and δ-valerolactone) was also investigated with the same catalytic systems in bulk conditions. In contrast to lactide polymerization, only the DMAP/DMAP·HOTf allowed lactone polymerization with a slower rate. However, the control over the molar masses remained very good. Block copolymers were also synthesized.

A Hydrosoluble Triphenylene That Preferentially Binds Acetylcholine, Epibatidine, and Nicotine

Synthesis and binding properties of a new hydrosoluble triphenylene 1b are reported. Selective recognition of acetylcholine (ACh) against other aliphatic ammoniums is achieved by this flat receptor, which also forms complexes with epibatidine and nicotine. Ionic pairing and hydrophobic effects between host 1b and ACh are studied by infrared spectroscopy.

A simple ionic triphenylene receptor for catecholamines, serotonin and D-glucosamine in buffered water

The combination of hydrophobic effects and ionic pairing within a triphenylene-based receptor were exploited for the binding of biological phenylethylamines, serotonin and D-glucosamine in phosphate buffered water.

α-Halogenoacetanilides as hydrogen-bonding organocatalysts that activate carbonyl bonds: fluorine versus chlorine and bromine

α-Halogenoacetanilides (X=F, Cl, Br) were examined as H-bonding organocatalysts designed for the double activation of CO bonds through NH and CH donor groups. Depending on the halide substituents, the double H-bond involved a nonconventional CH⋅⋅⋅O interaction with either a HCXn (n=1-2, X=Cl, Br) or a HCAr bond (X=F), as shown in the solid-state crystal structures and by molecular modeling. In addition, the catalytic properties of α-halogenoacetanilides were evaluated in the ring-opening polymerization of lactide, in the presence of a tertiary amine as cocatalyst. The α-dichloro- and α-dibromoacetanilides containing electron-deficient aromatic groups afforded the most attractive double H-bonding properties towards CO bonds, with a NH⋅⋅⋅O⋅⋅⋅HCX2 interaction.