Lev G. Bruk

Structure of Complexes Formed by Dissolution of Palladium Diacetate in Methanol and Chloroform. In Situ NMR Study

The behavior of palladium diacetate cyclic trimer [Pd(OAc)(2)](3) (1) upon its dissolution in methanol and wet chloroform was studied by (1)H and (13)C NMR including 2D-HSQC and 2D-DOSY techniques. Upon dissolution, trimer 1 reacts with methanol and is completely transformed first into the methoxo complex Pd(3)(μ-OMe)(OAc)(5) (2), which already at -18 °C undergoes a slow exchange of second bridging acetate ligand between the same palladium atoms to form the symmetric dimethoxo complex Pd(3)(μ-OMe)(2)(OAc)(4), the maximum relative concentration of which reaches 20-30 mol % of initial loading trimer 1. Along with the dimethoxo complex, both soluble and insoluble polynuclear palladium clusters are gradually formed at -18 °C, and their total amount reaches up to 60% of the starting Pd(2+) loading. The increase of temperature to 27 °C results in the reduction of palladium(II) to Pd metal by methanol, which is oxidized and transformed into formaldehyde hemiacetal and methyl formate. Upon dissolution in wet chloroform, trimer 1 is reversibly hydrolyzed to the hydroxo complex Pd(3)(μ-OH)(OAc)(5) (10) in ratio 1/10 ≈ 3/1. The temperature decrease and addition of acetic acid shift the equilibrium in this system toward trimer 1, and addition of water shifts it in the opposite direction. Addition of methanol to the equilibrium mixture of 1 and 10 results in the fast exchange of bridging acetate in trimer 1 by the μ-OMe group. Substitution of the μ-OH ligand by μ-OMe in 10 occurs in parallel but more slowly. Complex 2 formed in both cases is more stable in chloroform than in methanol.

Oxidative carbonylation of phenylacetylene catalyzed by Pd(II) and Cu(I): Experimental tests of forty-one computer-generated mechanistic hypotheses

Abstract We describe an experimental study of the reaction mechanism of phenylacetylene oxidative carbonylation to methyl ester of phenylpropiolic acid catalyzed by Pd(II) and Cu(I), PhCCH+CO+MeOH+2NaOAc+2CuCl 2 →PhCCCOOMe+2AcOH+2NaCl+2CuCl, which was closely guided by recent computational research on the generation of reaction mechanisms. Our initial mechanistic studies of this reaction were based on informal (non-computer-generated) mechanistic hypotheses. When experiments at 20°C and 1 atm led us to reject four of five mechanistic possibilities for the reaction, we turned to formulating new hypotheses with the aid of the computer programs ChemNet, which generated a reaction network consisting of 233 elementary steps, and MECHEM, which uncovered 41 simplest hypothetical pathways from within the reaction network. Our subsequent analysis of these 41 hypothetical mechanisms suggested a highly informative experiment based on the CH 3 OH/CH 3 OD kinetic isotope effect. The ratio between the rates of ester formation in nondeuterated and deuterated methanol was close to unity, suggesting that O–H bond scission occurs after the rate-limiting transmetalation step CuCCPh+PdCl 2 →ClPdCCPh+CuCl. This experiment led to rejecting 32 out of the 41 hypotheses. Four more mechanisms were rejected based on the results of preliminary experimental studies. Further work is needed to discriminate among the five remaining mechanisms.

Mechanistic study of acetylene carbonylation to anhydrides of dicarboxylic acids in solutions of palladium complexes

The mechanism of the reaction that produces maleic and succinic anhydrides from carbon monoxide and acetylene was studied in the catalytic PdBr2LiBrCH3CN system. The in situ study of reactivities of possible organic intermediates, the kinetic isotope effect study (for succinic and maleic anhydrides kHkD was estimated to be 1.05 ± 0.05 and 0.9 ± 0.1, respectively), the study of isotope exchange, and the oxygen effect on the process direction, revealed that maleic anhydride is most likely a key intermediate of the succinic anhydride formation. Maleic anhydride undergoes transformations through the mediation of a palladium hydride complex. This complex was detected in the catalytic solution at −40°C with 1H NMR (δ = −1.88 ppm, v12 = 70 Hz).

Oxidative carbonylation of alkynes in self-oscillating mode. Effect of the nature of substrates on the dynamic behavior of reaction system

The oxidative carbonylation of alkynes in the oscillation mode was studied. The influence of the nature of substrates, alkynes and alcohols, on the pattern of oscillations was considered. The role of oxidants, I2 and H2O2, in this process was demonstrated. The reaction network of the process was formulated, and four hypothetical mechanisms were selected.

Mechanisms of ≡CH bond activation in oxidative carbonylations of alkynes catalyzed by palladium complexes

Abstract Three types of catalytic system based on palladium complexes were studied in the synthesis of alkynylcarboxylic acid esters by oxidative carbonylation of monosubstituted alkynes. Three mechanisms were proposed for activation of the ≡CH bond in alkynes and the formation of RC≡C[Pd]X, a key intermediate: (i) electrophilic substitution of H+ by Cu(I), (ii) electrophilic substitution of H+ by I+, and (iii) oxidative addition of the CH bond in alkynes to palladium.

Computer-assisted generation of chemical reaction networks. Applications in mechanistic studies of organometallic catalysis

The Chem Net computer program designed for mechanistic studies of organic reactions occurring in solutions of metal complexes and on the surface of heterogeneous catalysts is proposed. The use of the program is illustrated by applications to catalyzed homogeneous hydrocarboxylation and hydroformylation of ethylene and heterogeneous hydrogenolysis of ethane.

New catalytic systems for oxidative carbonylation of acetylene to maleic anhydride

A classification of polyfunctional catalytic systems based on discrimination of the main component (the catalyst participating in all stages of the formation of the product of catalytic reaction) and elucidating the functions of additional components of a catalytic system is suggested. The role of additional components in a number of new palladium-based catalytic systems used in the synthesis of maleic anhydride by oxidative carbonylation of acetylene was studied. It was established that the functions of Co and Fe phthalocyanine complexes (PcCo and Pc*Fe, respectively) in the mechanism of the process are different.

Mechanism of oxidative carbonylation of phenylacetylene and methylacetylene. Generation and experimental discrimination of hypotheses

Formal and informal methods for advancing hypotheses on mechanisms were used in a study of the oxidative carbonylation of phenylacetylene to methyl phenylpropiolate catalyzed by the PdCl2−CuCl−CuCl2 system. The hypotheses remaining after discrimination and consistent with all experimental data include the steps of formation of the CuI alkynyl complex, transfer of the phenylethynyl group from CuI to PdII, insertion of carbon monoxide into a Pd−C or Pd−OMe bond of the PdII σ-alkynyl complex. Comparison of the formal and informal methods for advancing hypotheses confirmed a higher effeciency of the first method.

The Mechanism of Low-Temperature Oxidation of Carbon Monoxide by Oxygen over the PdCl2–CuCl2/γ-Al2O3 Nanocatalyst

The state of palladium and copper on the surface of the PdCl2–CuCl2/γ-Al2O3 nanocatalyst for the low-temperature oxidation of CO by molecular oxygen was studied by various spectroscopic techniques. Using X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), freshly prepared samples of the catalyst were studied. The same samples were also evaluated after interaction with CO, O2, and H2O vapor in various combinations. It was shown that copper exists in the form of Cu2Cl(OH)3 (paratacamite) nanophase on the surface of the catalyst. No palladium-containing crystalline phases were identified. Palladium coordination initially is comprised of four chlorine atoms. It was shown by XAS that this catalyst is not capable of oxidizing CO at room temperature in the absence of H2O and O2 over 12 h. Copper(II) and palladium(II) are reduced to Cu(I) and Pd(I,0) species, respectively, in the presence of CO and H2O vapor (without O2). It was found by DRIFTS that both linear (2114 cm−1, 1990 cm−1) and bridging (1928 cm−1) forms of coordinated CO were formed upon adsorption onto the catalyst surface. Moreover, the formation of CO2 was detected upon the interaction of the coordinated CO with oxygen. The kinetics of CO oxidation was studied at 18–38 °C at an atmospheric pressure for CO, O2, N2, and H2O (gas) mixtures in a flow reactor (steady state conditions).

Synthesis and structure of a product of interaction of acetonitrile with hydrogen bromide, [H2N=C(Me)−NH−C(Me)Br2]Br

A mixture of solid products was obtained upon absorption of dry HBr by MeCN. One of the products, [H2N=C(Me)−NH−C(Me)Br2]Br, was isolated as white single crystals and characterized by X-ray diffraction analysis.