Antonio Buonerba

Gold nanoparticles incarcerated in nanoporous syndiotactic polystyrene matrices as new and efficient catalysts for alcohol oxidations.

The controlled synthesis of gold nanoparticles (AuNPs), incarcerated in a semicrystalline nanoporous polymer matrix that consisted of a syndiotactic polystyrene-co-cis-1,4-polybutadiene multi-block copolymer is described. This catalyst was successfully tested in the oxidation of primary and secondary alcohols, in which we used dioxygen as the oxidant under mild conditions. Accordingly, (±)-1-phenylethanol was oxidised to acetophenone in high yields (96%) in 1 h, at 35 °C, whereas benzyl alcohol was quantitatively oxidised to benzaldehyde with a selectivity of 96% in 6 h. The specific rate constants calculated from the corresponding kinetic plots were among the highest found for polymer-incarcerated AuNPs. Similar values in terms of reactivity and selectivity were found in the oxidation of primary alcohols, such as cinnamyl alcohol and 2-thiophenemethanol, and secondary alcohols, such as indanol and α-tetralol. The remarkable catalytic properties of this system were attributed to the formation, under these reaction conditions, of the nanoporous ε crystalline form of syndiotactic polystyrene, which ensures facile and selective accessibility for the substrates to the gold catalyst incarcerated in the polymer matrix. Moreover, the polymeric crystalline domains produced reversible physical cross-links that resulted in reduced gold leaching and also allowed the recovery and reuse of the catalyst. A comparison of catalytic performance between AuNPs and annealed AuNPs suggested that multiple twinned defective nanoparticles of about 9 nm in diameter constituted the active catalyst in these oxidation reactions.

Novel iron(III) catalyst for the efficient and selective coupling of carbon dioxide and epoxides to form cyclic carbonates

“Re-cycling carbon dioxide with iron”. The synthesis of cyclic organic carbonates in high yield, stereo- and chemo-selectivity was accomplished through the coupling of carbon dioxide and epoxides, catalysed by a novel air-stable and easy-to-handle thioether-triphenolate iron(III) complex.

Highly Efficient Direct Aerobic Oxidative Esterification of Cinnamyl Alcohol with Alkyl Alcohols Catalysed by Gold Nanoparticles Incarcerated in a Nanoporous Polymer Matrix: A Tool for Investigating the Role of the Polymer Host

The selective aerobic oxidation of cinnamyl alcohol to cinnamaldehyde, as well as direct oxidative esterification of this alcohol with primary and secondary aliphatic alcohols, were achieved with high chemoselectivity by using gold nanoparticles supported in a nanoporous semicrystalline multi-block copolymer matrix, which consisted of syndiotactic polystyrene-co-cis-1,4-polybutadiene. The cascade reaction that leads to the alkyl cinnamates occurs through two oxidation steps: the selective oxidation of cinnamyl alcohol to cinnamaldehyde, followed by oxidation of the hemiacetal that results from the base-catalysed reaction of cinnamaldehyde with an aliphatic alcohol. The rate constants for the two steps were evaluated in the temperature range 10-45u2009°C. The cinnamyl alcohol oxidation is faster than the oxidative esterification of cinnamaldehyde with methanol, ethanol, 2-propanol, 1-butanol, 1-hexanol or 1-octanol. The rate constants of the latter reaction are pseudo-zero order with respect to the aliphatic alcohol and decrease as the bulkiness of the alcohol is increased. The activation energy (Ea) for the two oxidation steps was calculated for esterification of cinnamyl alcohol with 1-butanol (Ea = 57.8±11.5 and 62.7±16.7u2005kJu2009mol(-1) for the first and second step, respectively). The oxidative esterification of cinnamyl alcohol with 2-phenylethanol follows pseudo-first-order kinetics with respect to 2-phenylethanol and is faster than observed for other alcohols because of fast diffusion of the aromatic alcohol in the crystalline phase of the support. The kinetic investigation allowed us to assess the role of the polymer support in the determination of both high activity and selectivity in the title reaction.

Poly(lactide-co-ε-caprolactone) copolymers prepared using bis-thioetherphenolate group 4 metal complexes: synthesis, characterization and morphology

Titanium and zirconium complexes 1–3 (1 = (t-BuOS)2Ti(O-i-Pr)2; 2 = (t-BuOS)2Zr(O-t-Bu)2; 3 = (CumOS)2Zr(O-t-Bu)2) supported by two phenolate bidentate ligands (t-BuOS-H = 4,6-di-tert-butyl-2-phenylsulfanylphenol and CumOS-H = 4,6-di-cumyl-2-phenylsulfanylphenol) promoted the copolymerization of L-lactide with e-caprolactone. The reactivity displayed by the two monomers during the copolymerization experiments and the microstructure disclosed by 13C NMR analysis indicated a gradient distribution of the two monomers along the polymer chain. Copolymers with high e-caprolactone content showed a large scale formation of crystalline spherulites prone to perfection of the crystallinity upon thermal annealing at 50 °C. Differently L-lactide rich copolymers revealed a thin film morphology consisting of small rigid domains of L-lactide segments of about 15 nm embedded in a soft matrix of the counterpart. Copolymers with comparable mole fractions of the two monomers were entirely amorphous.

Glycidol: an Hydroxyl-Containing Epoxide Playing the Double Role of Substrate and Catalyst for CO2 Cycloaddition Reactions.

Glycidol is converted into glycerol carbonate (GC) by coupling with CO2 in the presence of tetrabutylammonium bromide (TBAB) under mild reaction conditions (T=60u2009°C, PCO2 =1u2005MPa) in excellent yields (99u2009%) and short reaction time (t=3u2005h). The unusual reactivity of this substrate compared to other epoxides, such as propylene oxide, under the same reaction conditions is clearly related to the presence of a hydroxyl functionality on the oxirane ring. Density functional theory calculations (DFT) supported by 1 Hu2005NMR experiments reveal that the unique behavior of this substrate is a result of the formation of intermolecular hydrogen bonds into a dimeric structure, activating this molecule to nucleophilic attack, and allowing the formation of GC. Furthermore, the glycidol/TBAB catalytic system acts as an efficient organocatalyst for the cycloaddition of CO2 to various oxiranes.

Stereorigid OSSO-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of rac-Lactide

The synthesis and characterization of a series of group 4 metal complexes of general formula {OSSOX}M(OR)2 (X = R = tBu, M = Zr (1); X = cumyl, M = Zr, R = tBu (2); X = cumyl, M = Ti, R = iPr (4); X = cumyl, M = Hf, R = tBu (5)) and {OSSOX}2Zr (X = Cl (3)) supported by o-phenylene-bridged bis(phenolato) ligands (OSSOtBu-H = 6,6-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-di-tert-butyphenol); OSSOCum-H = 6,6-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenol); OSSOCl-H = 6,6-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-dichlorophenol)) are described herein. Complexes 1-5 were readily obtained by σ-bond metathesis reactions between the proligand and the appropriate homoleptic metal precursor. The reaction with OSSOCl yielded the bis-ligand complex{OSSOCl}2Zr (3) regardless of the OSSOCl-H/Zr(OtBu)4 molar ratio or experimental conditions. All complexes were characterized in solution using NMR spectroscopy and, in the case of 2, by single-crystal X-ray diffraction experiments. These complexes show a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands; zirconium and hafnium complexes 1-3 and 5 are configurationally stable, whereas titanium complex 4 is fluxional in solution at room temperature. The complexes tested in the ring-opening polymerization (ROP) of racemic-lactide showed, except in the case of 3, moderate rates and good levels of polymerization control. Upon addition of an exogenous alcohol (isopropyl alcohol or tert-butyl alcohol) efficient binary catalytic systems were achieved. Polymerizations were well-controlled, as testified by the linear growth of the molecular weight as polymerization proceeded, narrow polydispersity indices, and molecular weights close to those expected on the basis of added alcohol amounts. Experimental and theoretical evidence is provided that ROP reactions operate according to an activated monomer mechanism.

Nanostructured ethylene–styrene copolymers

A judicious choice of a polyinsertion catalyst and monomer feed composition allows the one-pot synthesis of ethylene–styrene copolymers with an unprecedented structure, containing an isotactic polystyrene (iPS) block joined to an isotactic ethylene-alt-styrene sequence (iP(E-alt-S)). Both segments in the native polymer give rise to crystallinity. The characterization of the materials has been performed through different techniques. In particular AFM analysis of samples obtained by spin coating shows the presence of either circular nano-domains with about a 30 nm diameter due to self-assembling of the copolymer chains or bicontinuous nanostructured phases depending on the block lengths.

Novel nanostructured semicrystalline ionomers by chemoselective sulfonation of multiblock copolymers of syndiotactic polystyrene with polybutadiene

Novel semi-crystalline sulfonated copolymers (sPS(B-SA)) have been synthesized by sulfonation of multiblock copolymers of syndiotactic polystyrene with cis-1,4-polybutadiene (sPSB), using a two-stage solution process comprising the addition of thiolacetic acid to the butadiene units, followed by the in situ oxidation of the thioacetyl moieties with performic acid. The sulfonation process is quantitative, chemoselective, cheap and more environmentally benign than similar methods previously reported; in addition it allows achieving high ion exchange capacity values (up to 4.48 equiv. per kg). The sPS(B-SA)s are crystalline, preserving the native polymorphic behavior of the crystalline syndiotactic polystyrene segments. Thin films of sPS(B-SA) samples show a phase separated morphology at the nanometer scale evidenced by tapping mode and tunneling current atomic force microscopy; proton conductive regions embedded in a hard-hydrophobic matrix of syndiotactic polystyrene were actually observed.

Olefin-styrene copolymers

In this review are reported some of the most relevant achievements in the chemistry of the ethylene–styrene copolymerization and in the characterization of the copolymer materials. Focus is put on the relationship between the structure of the catalyst and that of the obtained copolymer. On the other hand, the wide variety of copolymer architecture is related to the properties of the material and to the potential utility.

Polymerization of ethylene and propylene promoted by group 4 metal complexes bearing thioetherphenolate ligands

The synthesis of four new group 4 metal complexes 1–4 (1 = (t-BuOS)2TiCl2; 2 = (CumOS)2TiCl2; 3 = (t-BuOS)2Zr(CH2Ph)2; 4 = (CumOS)2Zr(CH2Ph)2) bearing two bidentate thioetherphenolate ligands (t-BuOS-H = 4,6-di-tert-butyl-2-phenylsulfanylphenol; CumOS-H = 4,6-bis-(α,α-dimethylbenzyl)-2-phenylsulfanylphenol) has been accomplished. These complexes exhibit fluxional behaviour in solution and this was revealed by VT 1H NMR and supported by density functional theory (DFT) calculations. All these complexes are active catalysts in ethylene polymerization, producing linear polyethylene. Notably, the zirconium complex 3 displays, under proper reaction conditions, very high activity (1422 kgPE molcat−1 bar−1 h−1), which compares well with that of the most active post-metallocene catalysts. Furthermore, propylene polymerization catalyzed by the titanium complex 1 yields atactic polypropylene, whereas the zirconium complexes 3 and 4 selectively produce oligopropylene with Schultz–Flory distribution. NMR analysis of the unsaturated chain ends in the latter samples provides evidence of a regioselective propagation reaction with a large preference for 1,2-monomer insertion. DFT calculations allowed the modelling of the elementary reaction steps, namely, the chain propagation reaction, β-hydrogen elimination and transfer, highlighting the importance of the flexibility and steric hindrance of the ancillary ligands in determining the high activity of the title catalysts.