Paolo Uguagliati

Carbonylation of ethanol promoted by palladium chloride

Abstract The reaction of ethanol with carbon monoxide at atmospheric pressure and mild temperatures in the presence of palladium chloride and lithium chloride produces ethyl chlorocarbonate, ethyl acetate, palladium metal, and an unidentified compound. Only diethyl carbonate is produced along with palladium metal in the absence of lithium chloride when sodium carbonate is added to the ethanol solution of palladium chloride saturated with carbon monoxide. A very labile palladium carbonyl intermediate complex can be isolated. A reaction mechanism is proposed.

Functionalized isocyanides as ligands. Synthesis of 2-(chloromethyl)- and 2-(iodomethyl)phenyl isocyanides and their transition-metal complexes

Preparation de 2-(XCH 2 ) C 6 H 4 NC (X=Cl ou I) par reaction en 4 etapes. Synthese des complexes de Pd, Pt, Cr, W et Do

Mechanisms of nucleophilic and electrophilic attack on carbon bonded palladium(II) and platinum(II) complexes

Abstract A systematic mechanistic study is reported for the formation of palladium(II) carbene complexes by nucleophilic attack of aromatic amines on isocyanide derivatives. The most prominent step of the reaction involves direct attack of the amine nitroge on the isocyanide carbon to give an intermediate which then is converted to the final carbene species by the agency of the entering amine itself which behaves as a bifunctional catalyst. The rate of the primary step is affected by the donor ability of the entering amine, by the electrophilic character of the isocyanide carbon, and by steric crowdiness around the reacting centers, with the solvent also playing an important role. The reaction system displays a high versatility through a proper choice of the substituents on the amine and isocyanide aromatic rings and of the ancillary ligands in the metal complex. A mechanistic study is also described of the cleavage of the platinum-carbon σ-bond by electrophilic attack by the proton on organoplatinum(II) complexes. The particular mechanism which is operative, viz. direct electrophilic attack at the metalcarbon bond or oxidative addition/reductive elimination, appears to be the result of many factors. These include electronic and steric properties of the cleaved group and of ancillary ligands, steric configuration of the substrate, nature of the electrophile and solvating ability of the medium.

Synthesis, characterization and X-ray structural determination of palladium(0)–olefin complexes containing pyridin-thioethers as ancillary ligands. Equilibria and rates of olefin and ligand exchange

The synthesis of Pd(0)–olefin complexes with pyridin-thioether ligands R′NSR is reported. X-ray structure determinations of selected species are described. The dynamic behavior was studied by variable-temperature 1H-NMR spectrometry. Equilibrium constants for olefin and chelate ligand exchange were determined by UV–vis spectrophotometry in chloroform at 25°C. The following metal–olefin stability order was observed: tetramethylethylenetetracarboxylate (tmetc)≈naphthoquinone (nq)<fumaronitrile (fn)≈maleic anhydride (ma)≪tetracyanoethylene (tcne). The ligand exchange equilibrium constants indicate that α-diimines and pyridin-thioethers affect the stability of the metal–bidentate ligand arrangement to a similar extent, as found in similar Pd(II) complexes. When the entering olefin is tmetc, the approach to equilibrium is slow so that both second-order rate constants k2 and k−2 could be determined along with their activation parameters for the reversible reaction of [Pd(η2-nq)(HNSiPr)] with tmetc. The results indicate an associative mechanism to be operative in these olefin exchange processes.

Reactions of 1,4-diazubutadienes with chloro-bridged palladium(II) and platinum(II) allyl derivatives

Abstract The 1,4-diazabutadienes (or α-diimines) RNC(R′)C(R″)NR, DAB, (R = p -C 6 H 4 OMe, R′ = R″ = H, DAB I ; R = p -C 6 H 4 OMe, R′ = H, R″ = Me, DAB II ; R = p -C 6 H 4 OMe, R′ = R″ = Me, DAB III ; R = t-Bu, R′ = R″ = H, DAB IV ) react with the allylic compounds [PdCl(η 3 -2-Y-C 3 H 4 )] 2 (Y = H: all; Y = Me: Meall) and [PtCl(C 3 H 5 )] 4 in the presence of NaClO 4 yielding the cationic complexes [M(η 3 -2-Y-C 3 H 4 )(DAB)]ClO 4 (M = Pd, I; M = Pt, II). The 1 H and 13 C NMR spectra indicate a σ,σ′-N,N′ chelation of DAB. In the complexes with the asymmetric ligand DAB II a fast syn-syn, anti-anti , exchange of the allylic protons occurs at room temperature in CDCl 3 . In acetonitrile a partial dissociation of DAB is observed, with the following order of stability constants; DAB III > DAB I and Pt ⪢ Pd. In the absence of NaClO 4 , equilibria are established involving the starting reactant [PdCl(η 3 -Meall)] 2 , binuclear species {[PdCl(η 3 -Meall)] 2 (DAB)}, III (in which DAB acts as a bidentate bridging ligand) and ionic complexes [Pd(η 3 -Meall)(DAB)] + [PdCl 2 (η 3 -Meall)] − , IV. These equilibria were studied in various solvents by variable temperature 1 H NMR spectra, electronic spectra, molecular weight and conductivity measurements. The complexes III can be isolated as solids and are the predominant species in concentrated solution only with DAB I and DAB IV , both having R′ = R″ = H. With DAB III , the predominant species in CDCl 3 at −40 °C is a complex of type IV. A similar compound, [Pd(η 3 -Meall)(bipy)][PdCl 2 (η 3 -Meall)], is also obtained in the reaction with 2,2′-bipyridine.

Palladium(II) Allyl Complexes with Nitrogen-Sulfur Bidentate Ligands.Substituent Effects in the Mechanism of Allyl Amination.

The reactivity of palladium(II) allyl complexes containing the nitrogen–sulfur bidentate ligand N–SR (N–SR=2-(phenylthiomethyl)pyridine, 2-(phenylthiomethyl)-6-methylpyridine, 2-(tert-butylthiomethyl)pyridine) was studied in CHCl3 in the presence of the activated olefin fumaronitrile (fn). The stepwise mechanism involves a fast pre-equilibrium in which the N–SR ligand is displaced by the amine with formation of an inert bis-amino allyl species and concomitant rate-determining bimolecular attack of the amine on the coordinated allyl moiety to give the allylamine and the olefin-stabilized Pd(0) complexes [Pd(η2-fn)(N–SR)]. The influence of the substituents at the allyl fragment and at the nitrogen–sulfur ligand is rationalized together with the fluxional behavior in solution.

Binding ability of 2,6-bis(methylthiomethyl)pyridine with proton, palladium(II) and copper(II) in aqueous solutions

Abstract The acidity constant of the tridentate ligand 2,6-bis(methylthiomethyl)pyridine (L) and formation constants of its Pd(II) and Cu(II) complexes [PdLTu]2+ and [CuL(H2O)]2+ have been determined in aqueous solutions by potentiometric and spectrophotometric techniques. The acidity constant as determined by potentiometry is log K=4.04±0.04 (4.01±0.02 by spectrophotometry), whereas the formation constants for the Pd(II) and Cu(II) species are log K=28.92±0.09 and 4.6±0.1 (4.41±0.04), respectively. Some preliminary results on the high selectivity for Pd(II) over Cu(II) of a macroporous polystyrene-divinylbenzene resin bearing the same chelating group are also reported.

Novel palladium(II) allyl complexes with nitrogen-sulfur donor bidentate ligands. Mechanism of allyl amination of [Pd(η3-allyl)-(N-SR)]ClO4 (allyl = C3H5; N-SR = C5H4N-2-CH2SR, R = C6H5, C2H5) in the presence of activated olefins. X-ray structure determination and fluxional behavior

Abstract The reactions of [Pd(η3-C3H5)(C5H4-N-2-CH2SR) (R = C6H5, C2H5) with amines in the presence of fumaronitrile (in), involving the formation of allylamines, were studied kinetically in CHCl4, by UV-Vis techniques. The proposed stepwise mechanism involves a fast pre-equilibrium in which the mixed N-SR ligand is displaced by the amine, giving an inert bis-amino allyl species. The concomitant rate-determining bimolecular attack by the amine yields the allylamine and the corresponding Pd(O) complexes [Pdη3-fn) (C3H54N-2-CH2SR)]. The solid state structure of complex was determined by X-ray crystallography. A preliminary 1H-NMR study of the fluxional manifestations was carried out. The apparent rotation of the allyl moiety is operative at the lowest temperature increasing the temperature promotes the breaking and rearangement of the Pd-S bond and, finally, the η′-η′-η′ rearrangement of the allyl fragment.

Palladium(0)–olefin complexes with potentially terdentate nitrogen–sulfur ligands. The role of the chelate in the olefin exchange path

Abstract The synthesis and the reactivity of Pd(0) olefin complexes [Pd(η 2 -olefin)(SNS)] and [Pd(η 2 -olefin)(NSN)] containing potentially terdentate nitrogen–sulfur ligands were studied. The presence of a potentially coordinating atom in the environment of the metal, influences strongly the fluxional behavior in solution but not the overall reactivity with respect to olefin exchange and thermodynamic stability which is very close to that of the corresponding bidentate nitrogen–sulfur complexes. The intimate mechanism of olefin exchange also involves a path promoted by the third dangling coordinating atom which induces olefin dissociation and stabilizes the ensuing Pd(0) three-coordinated species.

The marked influence of steric and electronic properties of ancillary pyridylthioether ligands on the rate of allene insertion into the palladium–carbon bond

Abstract Neutral methyl- and acyl-palladium chloro complexes containing pyridylthioether ancillary ligands (R′NSR) (R′=H, Me, Cl; R=Me, Et, i-Pr, t-Bu, Ph) have been synthesised and characterised by elemental analysis and spectroscopic methods. The reactivity of these complexes toward allene (allene=DMA=1,1-dimethylpropadiene; TMA=1,1,3,3-tetramethylpropadiene) insertion into the palladium–carbon bond has been studied by 1H-NMR and UV–vis techniques. The rate of reaction appears to be strongly influenced by the steric and electronic properties of the ancillary ligand. The distortion induced by the substituent R′ in position 6 of the pyridine ring on the main coordination plane of the substrate (allowed by sulphur sp3 hybridisation) renders the substrate itself more prone to nucleophilic attack by the allene. The rate of allene insertion can further be enhanced by lowering the basicity of the chelating atoms in the NS moiety which results in an increase of electrophilicity of the palladium core, so that the rate constants measured in the case of the complexes containing the ligand 6-chloro-2-phenylthiomethylpyridine (ClNSPh) are by far the greatest observed so far for similar reactions. Furthermore, on the basis of the indications emerging from the exhaustive study on the behaviour of all the related pyridylthioether methyl complexes, an associative asynchronous bond making mechanism for the rate determining nucleophilic attack by allene is proposed.