Andrea Vavasori

Carbon monoxide-ethylene copolymerization catalyzed by a Pd(AcO)2/dppp/TsOH1 system: the promoting effect of water and of the acid

Abstract For the title copolymerization the catalyst productivity (g-polymer/g-Pd · h) is significantly influenced by the presence of water and of the acid as it passes through a maximum upon increasing concentration of H2O and of TsOH. In the presence of 450 ppm of H2O, the maximum productivity is ca. 3.7 times higher than when the copolymerization is carried out in the presence of 1% of trimethylorthoformate, used as H2O scavenger in MeOH as solvent, at 90°C, under 45 atm of total pressure, employing the catalyst precursor in the molar ratio Pd/dppp/TsOH 1/1/2 ([Pd] 5.6 × 10−5 mol · 1−1). Under similar conditions, but under 60 atm of the two monomers, in the presence of 900 ppm of H2O and when employing an excess of the acid ( TsOH Pd 6.4) the productivity reaches a maximum of ca. 11500 g-polymer/g-Pd · h, which is 1.4 times higher than that obtained when the TsOH Pd ratio is 2 1 . The promoting effect of H2O is ascribed to the possibility that a higher concentration of active PdH species, which are proposed to initiate the catalytic process through the insertion of the olefin into a PdH bond, is achieved through the interaction of carbon monoxide with water on the metal center, via a reaction closely related to the water gas shift reaction. It is also proposed that the promoting effect of the acid is due to the reactivation of inactive Pd(0) species, which inevitably form under the reducing reaction conditions, with formation of active PdH species. When the copolymerization is carried out in the presence of benzoquinone (BQ), either under the reaction conditions in which the productivity reaches a maximum or under unfavorable conditions, that is, in the presence of low or relatively high concentrations of water, the productivity has an average value of ca. 7000 g-polymer/g-Pd · h. Since it was found by other research groups that in the presence of BQ the polymer takes origin mainly through the insertion of CO into a PdOCH3 species whose formation is favored in the presence of BQ, the findings presented above give further support to the suggestion that the promoting effect of H2O and of TsOH are due to the possibility that, when present in appropriate amounts, they favour the formation of PdH species which start the catalytic cycle.

Multistep electron transfer catalytic system for the oxidative carbonylation of phenol to diphenyl carbonate

Abstract The oxidative carbonylation of phenol to diphenyl carbonate is catalyzed by palladium salts in combination with a cocatalyst such as p -benzoquinone (BQ) or a salt of Co, Mn, Cu. The addition of a surfactant such as tetrabutylammonium bromide makes the catalytic system more efficient. The role of each component in the catalytic system is discussed. A catalytic cycle is proposed where, in the first step, diphenylcarbonate is formed from phenol and CO with concomitant reduction of Pd(II) to Pd(0) and formation of two protons. p -Benzoquinone, which is reduced to hydroquinone, in the presence of protons, reoxidizes Pd(0) to Pd(II) while the metal cocatalyst is reduced by hydroquinone which is reoxidized to p -benzoquinone. Oxygen and protons, arising from the last reaction, close the cycle with reoxidation of the reduced metal cocatalyst and formation of water.

The promoting effect of chelating ligands in the oxidative carbonylation of phenol to diphenyl carbonate catalyzed by Pd-Co-benzoquinone system

Abstract The system Pd(OAc) 2 /BQ/Co(acac) 3 (BQ=benzoquinone), in combination with tetrabutylammonium bromide (TBAB) as a surfactant agent and a chelating ligand such as 2,9-dimethyl-1,10-phenanthroline (dmphen) or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (dmdpphen), is an efficient catalyst for the oxidative carbonylation of phenol to diphenyl carbonate (DPC). The best results have been obtained using the system Pd(OAc) 2 /BQ/Co(acac) 3 /dmphen=1/30/8/5 (molar ratio) in which [Pd]=10 −3 mol l −1 and TBAB/Pd=60/1. This system gives the maximum productivity of 700 mol DPC/mol Pd h at 135°C and under P tot =60 atm (CO/O 2 =10/1 molar ratio). The role of each component of the catalytic system is discussed and a catalytic cycle is proposed.

Carbonylation of Ethene in Methanol Catalysed by Cationic Phosphine Complexes of Pd(II): from Polyketones to Monocarbonylated Products

This review article describes the advances on the carbonylation of ethene in methanol since the discovery of highly active cationic cis-chelating diphosphine-Pd(II) catalysts for the production of perfectly alternating high molecular weight polyketones, which led to the recent development of efficient catalysts for the synthesis of monocarbonylated products. After dealing with the activation of the catalyst precursor and with mechanistic aspects of the catalytic cycles, the influence of the operating conditions, the nature of the counter anion, the acidity of the reaction medium and the nature of the ligand on both the rate and the selectivity is delineated. In particular, it is shown that the catalytic activity can be finely tuned by varying the electronic, bite angle and steric bulk properties of the ligand and that, within a series of related ligands, the selectivity mainly depends on the steric bulk.

Sorption and separation of Palladium, Platinum and Gold Chlorocomplexes by means of a dipicolinic acid polystyrene-based chelating resin

Abstract A chelating ion-exchange resin containing dipicolinic acid as functional group and based on microporous chloromethylated cross-linked polystyrene-divinylbenzene (2 %) copolymer has been prepared. Its sorption and desorption characteristics for Pd(II), Pt(II), Pt(IV) and Au(III) have been studied in aqueous chloride solutions under a number of experimental conditions, both in batch and in column, at room temperature and constant ionic strength ( μ = 1 mol/l, KCl/ HCl). In column operations at pH 6, Pd(II) can be separated from Pt(H) or Pt(IV) owing to the different rate of formation of the immobilized chelated species. From a mixture of Pd(II, Pt(IV) and Au(III) at pH 6, Pt(IV) flows unaffected, whereas Au(III) and Pd(II) are both retained and successively separated by selective elution. From the same mixture at pH ≤ 0 only Au(III) is sorbed by anionic exchange.

Synthesis of γ-ketocarboxylic acids via reduction of γ-keto-α-hydroxycarboxylic acids with carbon monoxide catalyzed by a PdHCl system

Abstract A PdHCl catalytic system is highly active in the synthesis of γ-ketoacids of type ArCOCH2CH2COOH via reduction with CO of the ketohydroxy acids ArCOCH2CHOHCOOH. Typical reaction conditions are: PCO: 20–30 atm; Pd/ substrate/H2O/HCl = 1/400–1000/800–3000/ 100–1000 (mol); temperature: 100–110°C; [Pd]: 10−3 to 10−2 M; solvent: dioxane; reaction time: 1–2 h. The reaction occurs in high yield only when the palladium precursor is used in combination with HCl and in the presence of H2O. Under the reaction conditions employed, the palladium(II) complex used as catalyst precursor decomposes to palladium metal. Pd/C is also highly active. It is proposed that the catalytic cycle proceeds through the following steps: (i) The chloride ArCOCH2CHClCOOH, which forms in situ from the starting substrate and HCl, undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH2COPh] moiety. (ii) Interaction of H2O and CO on the metal yields an intermediate having also a carbohydroxy ligand, (HOOC)-Pd-[CH(COOH)CH2COPh]. (iii) This intermediate, after β-hydride abstraction from the carbohydroxy ligand, gives off CO2 and reductive elimination gives product PhCOCH2CH2COOH. Alternatively, HCl may react with the intermediate proposed in step (i), yielding directly the product and a Pd(II) species, which is reduced by CO to a Pd(0) species, which starts another catalytic cycle.

Phosgene-free synthesis of 1,3-diphenylurea via catalyzed reductive carbonylation of nitrobenzene*

1,3-Diphenylurea (DPU) has been proposed as a synthetic intermediate for phosgene-free synthesis of methyl N-phenylcarbamate and phenyl isocyanate, which are easily obtained from the urea by reaction with methanol. Such an alternative route to synthesis of carbamates and isocyanates necessitates an improved phosgene-free synthesis of the corresponding urea. In this work, it is reported that Pd(II)-diphosphine catalyzed reductive carbonylation of nitrobenzene in acetic acid (AcOH)-methanol proceeds in high yield and selectivity as a one-step synthesis of DPU. We have found that the catalytic activity and selectivity of this process depends on solvent composition and on the bite angle of the diphosphine ligands. Under optimum reaction conditions, yields in excess of 90 molar % and near-quantitative selectivity can be achieved.

New aspects of the carbonylation of allylpalladium complexes

Abstract The carbonylation of (η3-allyl)palladium(II) chloride dimer in the presence of an excess of ylide, such as Ph3PC(H)COR (R = Me or Ph) (Pd:ylide = 1:5) in MeOH or EtOH, at a CO pressure of 4 atm at room temperature occurs with reduction of the palladium(II) complex to palladium metal and with formation of the corresponding alkyl 3- butenoate with a high yield. The ylide does not give rise to any carbonylation product. When the carbonylation is carried out in the presence of PPh3 (Pd : PPh3 = 1 : 2–3), there is also formation of the unsaturated ester, although in lesser amount, together with [Pd3(PPh3)n(CO)3] (n = 3 or 4) or [Pd(PPh3)3(CO)] andtrans-[Pd(PPh3)2(COOR)Cl] (R = Me or Et). These products also form when the carbonylation is carried out in the presence of NEt3 or PrCOONa, in place of the ylide, and of PPh3. It has also been found that [Pd(PPh3)2Cl2] reacts in MeOH or EtOH at a CO pressure of 4 atm at ambient temperature in the presence of an excess of ylide to give the corresponding carbalkoxy complextrans-[Pd(PPh3)2(COOR)Cl]. These findings suggest that the ylide probably promotes formation of carbalkoxy species, as do NEt3 or PrCOONa because the ylide can behave as a base (pKa ⋍7). They are strong support for the suggestion that the carbonylation of (allyl)palladium complexes occurs via a (carbalkoxy)palladium species.

Quantum Chemical Investigation on Indole: Vibrational Force Field and Theoretical Determination of Its Aqueous pKa Value

Indole and its derivatives are molecules which play important roles in different fields, from biology to pharmacology. Here we report a thorough investigation on the anharmonic force fields of indole as well as the ab initio determinations of its gas phase basicity and aqueous pK(a) value. For the geometry optimizations, the calculations have been performed using both density functional (DFT) and second-order Møller-Plesset (MP2) levels of theory employing different basis sets. Anharmonic force fields have been obtained employing both the B3LYP and the B97-1 functionals and an hybrid approach: the best agreement to the experimental data has been determined employing the B3LYP functional combined with the recently developed N07D basis set (mean unsigned error, MUE, of 5.1 cm(-1) and a root-mean-square error, RMSE, of 7.2 cm(-1)). Gas phase basicity and proton affinity have been computed employing several computational schemes, namely the G3 and G4 Gaussian models, the complete basis set (CBS) extrapolation methods of Petersson and co-workers, several DFT calculations, and different hybrid extrapolation schemes based on combining single-point energy calculations performed at MP2 as well as at coupled cluster level of theory with single, double and perturbative triple excitations, CCSD(T). Regarding the aqueous pK(a) computations, two implicit solvation models (SMD and SM8) have been employed to determine the free energy of solvation and the corresponding pKa value.

Acid Catalyzed Direct-Amidation–Dehydrocyclization of 2-Hydroxy-acetophenones to Benzoxazoles by a One-Pot Sustainable Synthesis

A series of 2-methyl-benzoxazoles have been synthesized starting from 2-hydroxy-acetophenones via a one-pot three steps reaction. Hydroxylamonium salt has been used as amidation agent. The reaction occurs with different anions, but the best results is achieved with hydroxylamonium hydrchloride. Despite the number of consecutive stages, the reaction is highly selective. Mild reaction conditions and various solvents can be used, but trifluoroacetic acid is the preferred. Almost, complete recovery of the trifluoroacetic acid can be achieved by vacuum distillation. The role of trifluoroacetic acid, as well as, of the hydroxylamonium salt suggests a cooperative effect leading to high selective formation of 2-methyl-benzoxazoles.Graphical AbstractOne-pot TFA catalyzed synthesis of benzoxazoles starting from 2-hydroxyacetophenones.