Kinetics and mechanism of propene hydroformylation catalyzed by rhodium complexes with a diphosphite ligand

The kinetics of propene hydroformylation in the presence of the catalytic system Rh(acac)(CO)₂/nL (L = 2,2′-bis[(1,1′-diphenyl-2,2′-diyl)phosphito]-3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl, 0.5 < n < 20) in para-xylene at 90°C is reported. At n ≥ 2, the rate and regioselectivity of the process are independent of the L concentration. The reaction is of positive fractional order with respect to propene and hydrogen and of negative order with respect to CO. The molar ratio between the linear product and the branched product decreases with an increasing CO pressure and increases with an increasing H₂ pressure. The kinetic data are consistent with a process mechanism involving irreversible propene addition to the unsaturated hydride complex HRh(CO)L with the formation of the π-complex HRh(CO)L(C₃H₆). The insertion of coordinated propene into the H-Rh bond of this complex is reversible in the linear aldehyde formation route and is quasi-equilibrium in the branched isomer formation route. The conclusions as to the character of these reaction steps are corroborated by the compositions of the but-1-ene and but-2-ene hydro-formylation products.     

Potential surface topology of a butene—gold atom system in the ground state

Quantum chemical modeling of but-1-ene isomerization to cis-but-2-ene and trans-but-2-ene in the presence of a gold atom has been carried out in the framework of the density functional theory with an extended basis set, the PBE functional, and a pseudopotential with relativistic corrections included. Two possible mechanisms have been considered, viz., with the formation of an intermediate σ-complex with one C(sp²)—Au bond and with gold insertion into the C—H bond. In the former case, the calculated energy barriers to two isomerization stages are higher than 30 kcal mol⁻¹. In the latter case, the reaction involves three stages and proceeds via a metal hydride complex with low barriers. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1330–1337, July, 2008.     

Hydroperoxide Oxidation of Difficult-to-Oxidize Substrates: An Unprecedented C–C Bond Cleavage in Alkanes and the Oxidation of Molecular Nitrogen

In the V^(V)H₂O₂/AcOH system, C₅–C₂₀ n-alkanes, isooctane, and neohexane undergo oxidation to ketones and alcohols; the oxidation products of branched alkanes are indicative of a C–C bond cleavage in these substrates. A concept is developed, according to which the peroxo complexes of vanadium(V) are responsible for alkane oxidation. These complexes can transfer the oxygen atom or the O^+· radical cation to a substrate. The formation of nitrous oxide was found in the oxidation of molecular nitrogen in the H₂O₂/V^(V)/CF₃COOH system.     

Reactivity of Ionic Liquids: Reductive Effect of [C4C1im]BF4 to Form Particles of Red Amorphous Selenium and Bi2Se3 from Oxide Precursors

Temperature-induced change in reactivity of the frequently used ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄C₁im]BF₄) is presented as a prerequisite for the rational screening of reaction courses in material synthesis. [C₄C₁im]BF₄ becomes active with oxidic precursor compounds in reduction reaction at ϑ≥200 °C, even without the addition of an external reducing agent. The reaction mechanism of forming red amorphous selenium from SeO₂ is investigated as a model system and can be described similarly to the Riley oxidation. The reactive species but-1-ene, which is formed during the decomposition of [C₄C₁im]BF₄, reacts with SeO₂ and form but-3-en-2-one, water, and selenium. Elucidation of the mechanism was achieved by thermoanalytical investigations. The monotropic phase transition of selenium was analyzed by the differential scanning calorimetry. Beyond, the suitability of the single source oxide precursor Bi₂Se₃O₉ for the synthesis of Bi₂Se₃ particles was confirmed. Identification, characterization of formed solids succeeded by using light microscopy, XRD, SEM, and EDX.     

Kinetics and mechanism of the homogeneous oxidation of <Emphasis Type="Italic">n</Emphasis>-butenes to methyl ethyl ketone in a solution of Mo-V-phosphoric heteropoly acid in the presence of palladium pyridine-2,6-dicarboxylate

In catalytic two-step n-butene oxidation with dioxygen to methyl ethyl ketone, the first step is the oxidation of n-C₄H₈ with an aqueous solution of Mo-V-P heteropoly acid in the presence of Pd(II) complexes. The kinetics of n-butene oxidation with solutions of H₇PV₄Mo₈O₄₀ (HPA-4) in the presence of the Pd(II) dipicolinate complex (H₂O)PdII(dipic) (I), where dipic²⁻ is the tridentate ligand 2,6-NC₅H₃(COO⁻)₂, is studied. Calculation shows that, at the ratio dipic²⁻: Pd(II) = 1: 1, the ligand decreases the redox potential of the Pd(II)/Pd_met system from 0.92 to 0.73–0.77, due to which Pd(II) is stabilized in reduced solutions of HPA-4. The reaction is first-order with respect to n-C₄H₈. Its order with respect to Pd(II) is slightly below unity, and its order with respect to HPA-4 is relatively low (∼0.63). The activation energy of but-1-ene oxidation in the temperature range from 40 to 80°C is 49.0 kJ/mol, and that of the oxidation of but-2-ene is 55.6 kJ/mol. The mechanism of the reaction involving the cis-diaqua complex [(H₂O)₂Pd^II(Hdipic)]⁺, which forms reversibly from complex I, is proposed. The reaction rate is shown to increase with an increase in the HPA-4 concentration due to an increase in the acidity of the solution.     

8-Hydroxyquinolates of trivalent metals as new catalysts for the reaction of CO2 with epoxides

Tris-(8-hydroxyquinolates) of aluminum(iii), chromium(iii), and iron(iii) in the presence of triphenylphosphine oxide efficiently catalyze the formation of alkylenecarbonates by the reaction of CO₂ with oxiranes, viz., propylene oxide, but-2-ene oxides, and isobutylene oxide. The catalytic system is active at 140—170 °C and a CO₂ pressure of 15—100 atm.     

Formation of palladium nanoparticles in olefin oxidation with iron(III) aqua ions in the presence of the Pd/ZrO2/SO4 metallic catalyst

The reactions of the Pd/ZrO₂/SO₄-catalyzed oxidation of ethylene, propene, and but-1-ene in a 0.1–1.5 M solution of perchloric acid with iron(III) aqua ions to carbonyl compounds, viz., acetaldehyde, acetone, and methyl ethyl ketone, respectively, were studied. The formation of palladium nanoparticles (5 nm) in solution on contact of the initial heterogeneous Pd/ZrO₂/SO₄ catalyst with perchloric acid was proved by transmission electron microscopy. The palladium nanoparticles are assumed to play the key role in olefin oxidation with the iron(III) aqua ions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 627–632, April, 2006.     

Generation of methylene by the liquid phase oxidation of isobutene with nitrous oxide

The application of nitrous oxide as an alternative oxidant provides new opportunities for selective oxidation of olefins. Here, we studied for the first time the thermal oxidation of isobutene with N₂O in the liquid phase. The study revealed that the oxidation proceeds via 1,3-dipolar cycloaddition of N₂O to the CC bond by two routes forming unstable 4,5-dihydro-[1,2,3]-oxadiazole intermediates. The main route (the contribution of 91%) includes the addition of the N₂O oxygen to the second carbon atom in olefin. In this case, the oxadiazole decomposes with the CC bond cleavage yielding acetone, methylene (:CH₂), and N₂. The methylene then readily reacts with isobutene and benzene (solvent). The minor route involves the addition of the N₂O oxygen to the first carbon atom and the oxadiazole decomposition with a hydrogen shift leading to isobutanal and N₂.The main distinctive feature of the studied reaction is the formation of methylene in high yield.     

N-Cα bond cleavage of the peptide backbone via hydrogen abstraction

The specific cleavage of N-C_α bonds on the peptide backbone to form the so-called ‘c’ and ‘z + 2’ products, which can be used for the rapid determination of protein amino-acid sequences, has been examined to clarify the mechanism(s) that occur during hydrogen abstraction induced by bombardment with 337-nm laser photons in matrix-assisted laser desorption/ionization (MALDI) method. Intramolecular hydrogen abstraction, which results from the hydrogen(s) on the C_α or C_β carbon, did not occur with a deuterium-labeled dodecapeptide. To confirm a proposition that intermolecular hydrogen abstraction occurs between the peptide and the MALDI matrix, a deuterium dodecapeptide embedded in a deuterium 2,5-dihydroxybenzoic acid matrix at a molar ratio of 1:7000 was analyzed. The resulting deuterium c product ions suggested that c ions form via intermolecular hydrogen abstraction, although the results obtained did not deny any other possibilities such as intramolecular transfer of labile hydrogen. A mechanism for the N-C_α bond cleavage has been proposed that the formation of hypervalent radical species and subsequent prompt bond cleavages occur. The proposed mechanism successfully rationalizes the formation of both the z + 2 and the c product ions.     

Homogeneous catalytic oxidation of light alkanes: C-C bond cleavage under mild conditions

The combined oxidation of CO and C₂–C₄ alkanes (associated petroleum gas and natural gas components) under the action of oxygen in trifluoroacetic acid solutions in the presence of rhodium and copper chlorides was accompanied by the oxidative degradation of C-C bonds in a hydrocarbon chain with the formation of carbonyl compounds, alcohols, and esters. For butane and isobutane, the reaction path with C-C bond cleavage was predominant. The buildup curves of isobutane oxidation products (both with the retention and with the degradation of the chain) were S-shaped and characterized by the same induction period; they did not pass through a maximum. A reaction scheme was proposed to reflect the main special features of the mechanism of transformations occurring in the O₂/Rh/Cu/Cl⁻ oxidation system.     

Liquid-phase noncatalytic oxidation of monoterpenoids with nitrous oxide

A series of monoterpenoids differing in the number of double bonds and the pattern of their substitution were tested in the liquid-phase noncatalytic oxidation with nitrous oxide (N₂O). The structure of olefins has a significant effect on the oxidation route. In the case of terpenoids containing 1,1-disubstituted double bond, nor-carbonyl compounds are formed with high selectivity. Dedicated to the memory of Academician N. N. Vorozhtsov on the 100th anniversary of his birth. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1193–1197, June, 2007.     

Radical low-temperature oxidation of dibenzyl sulfide with the triphenylbismuth—tert-butyl hydroperoxide system

The reaction of Ph₃Bi with Bu^tOOH in a mole ratio of 1: 3 in hydrocarbon solvents affords the η²-peroxo complex of triphenylbismuth Ph₃Bi(η²O₂) which oxidizes the C-H bonds of the methylene group of dibenzyl sulfide. The reaction proceeds via the radical mechanism with the formation of intermediate unstable sulfur-containing hydroperoxide. Its decomposition is accompanied by the C-S bond cleavage, resulting in benzaldehyde. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1183–1185, June, 2008.     

Effect of NO, SO2, and O2 on the conversion of nitrous oxide on iron-containing zeolite catalysts

Conditions were found for facilitation of the conversion of nitrous oxide in the presence of Fe-containing zeolite catalysts by oxidants (NO, SO₂, and O₂). The results were interpreted in the framework of a mechanism involving decomposition of N₂O. The effect of NO_x on the reduction of nitrous oxide by C₃-C₄ alkanes was established. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 4, pp. 241–245, July–August, 2006.     

Reactivity of fluorinated sulfur-containing heterocycles towards nucleophilic and oxidizing reagents

The reaction of 5,5-bis(trifluoromethyl)-6-thia-bicyclo[2.2.1]hept-2-ene, 5,5-bis(trifluoromethyl)-6-thia-bicyclo[2.2.2]oct-2-ene and 2,2-bis(trifluoromethyl)-3,6-dihydro-4,5-dimethyl-2H-thiopyran with i-C₃H₇MgCl leads to the formation of ring opening products as the result of nucleophilic attack of the Grignard reagent on the sulfur atom. According to DFT calculations the reactivity of the sulphur-containing substrate correlates with the strain energy of the heterocycle. The oxidation of 3-thia-4,4-bis(trifluoromethyl)tricyclo[5.2.1.0^2,5]non-7-ene by hydrogen peroxide in hexafluoro-iso-propanol solvent resulted in formation of the corresponding sulfoxide however, the reaction with m-chloroperoxybenzoic acid produced the product of exhaustive oxidation of sulfur and the double bond. In sharp contrast, the oxidation of 5,5-bis(trifluoromethyl)-6-thia-bicyclo[2.2.1]hept-2-ene and 5,5-bis(trifluoromethyl)-6-thia-bicyclo[2.2.2]oct-2-ene by MCPBA (2d, 25 °C) proceeds with the preservation of the double bond, leading to the selective formation of the corresponding sulfones.     

Hydrolysis of the uranyl ion in sodium nitrate medium

Studies on Nickel Oxide Mixed Catalysts. VIII. Catalytic Properties of NiO? Al₂O₃/SiO₂ Catalysts Catalytic properties of NiO? Al₂O₃/SiO₂ catalysts prepared by precipitation-deposition and impregnation have been investigated in dimerization of ethene and isomerization of but-1-ene. It was found that the catalytic activity is mainly determined by the interaction between the catalyst components where a X-ray amorphous nickel alumolayersilicate is formed. The dimerization of ethene proceeds by participation of coordinatively unsaturated nickel ions with aluminum ions in the neighbourhood. The catalytic activity in the isomerization of but-1-ene depends on the surface acidic properties of the catalyst.     

Charakterisierung und katalytische Aktivität von Ni2+-ausgetauschten X- und Y-Zeolithen. II. Die Reduzierbarkeit des Ni2+ durch niedere Olefine und die Dimerisierungsaktivität der Ni2+-ausgetauschten Zeolithe

Characterization and Catalytic Activity of Ni²⁺ -X and -Y Zeolites. II. Reducibility of Ni²⁺ by Low Olefines and the Dimerization Activity of the Ni²⁺ -Zeolites The reducibility of Ni²⁺ in X and Y zeolites by hydrogen, but-1-ene, propene, and ethene is compared. The degree of reduction was determined after isothermal reduction and reoxidation by the TPR method. At 673 K on X zeolites the reducibility decreases in the order: H₂ > but-1-ene, propene > ethene. On Y zeolites an inversion takes place: but-1-ene, propene > H₂, ethene. The mechanism of reduction by olefins should be determined by an intermediate splitting off of a hydride ion as a reducing species. Such a mechanism explains the higher degree of reduction in the more acid Y zeolites. Assuming low valent nickel as an active center in ethen dimerization the induction period results from the reduction of Ni²⁺ ions.     

Hydrogenation of Cinnamaldehyde on Ir/γ-Al2O3 Catalysts. Influence of the Surface Acidity

The oxidation of methane has been studied in a flow system as a function of the chemical composition of zeolite catalyst using nitrous oxide as oxidant. It is concluded that methanol is a primary oxidation product which may undergo further oxidation to formaldehyde and to carbon oxides. However, it may also undergo conversion over the acidic catalyst to higher hydrocarbons. Reaction with nitrous oxide resulted in the production of carbon oxides, methanol, formaldehyde, C₂ - C₄ , C₅ - C₇ nonaromatics, and aromatics. The effect of Fe₂ O₃ and Al₂ O₃ , with or without, over HZSM5 on products was studied.     

Hydrosilylation of non-1-ene with phenylsilane in the presence of yttrium and lutetium bisguanidinate hydride complexes

Some regularities of hydrosilylation of non-1-ene with phenylsilane catalyzed by yttrium and lutetium bisguanidinate hydride complexes {[(Me₃Si)₂NC(NR)₂]₂Ln(μ-H)}₂ (Ln = Y, Lu; R = Prⁱ, Cy (Cy is cyclohexyl)) have been studied. The addition of PhSiH₃ to the double bond of non-1-ene in the presence of a {[(Me₃Si)₂NC(NPrⁱ)₂]₂Y(μ-H)}₂ complex has the first order in olefin and zero order in phenylsilane. This indicates that the insertion of non-1-ene into the Ln-H bond is a rate determining stage of the process, whereas the metathesis of the Ln-C σ-bond upon the action of phenylsilane proceeds rapidly. The first example of successive double alkylation of phenylsilane with an olefin catalyzed by a rare-earth metal complex with the formation of tertiary silane has been discovered.     

Polyfluorinated hydrazones in organic synthesis

The oxidation of 1,1,1,5,5,5-hexafluoro-4-trifluoromethylpentane-2,3-dione bishydrazone (1) with SeO₂, HgO, and Hg(OAc)₂ was studied. The use of selenium dioxide leads to 1,1,1,5,5,5-hexafluoro-4-trifluoromethylpent-2-ene (2) and C₁₂H₄F₁₈Se₂. 1,1,1,5,5,5-Hexafluoro-4-trifluoromethylpent-2-yne (4) and C₆H₂F₉HgC₆H₂F₉ were obtained by the oxidation of bishydrazone1 with mercuric oxide. The oxidation of compound1 with mercuric acetate in diglyme affords 1,1,1,5,5,5-hexafluoro-4-trifluoromethylpenta-2,3-diene (6). Alkyne4 is isomerized to allene6 at 130 °C. For Part 3, see Ref. 1 Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 677–680, April, 1998.     

Novel C<Subscript>β</Subscript>–C<Subscript>γ</Subscript> Bond Cleavages of Tryptophan-Containing Peptide Radical Cations

In this study, we observed unprecedented cleavages of the C_β–C_γ bonds of tryptophan residue side chains in a series of hydrogen-deficient tryptophan-containing peptide radical cations (M^•+) during low-energy collision-induced dissociation (CID). We used CID experiments and theoretical density functional theory (DFT) calculations to study the mechanism of this bond cleavage, which forms [M – 116]⁺ ions. The formation of an α-carbon radical intermediate at the tryptophan residue for the subsequent C_β–C_γ bond cleavage is analogous to that occurring at leucine residues, producing the same product ions; this hypothesis was supported by the identical product ion spectra of [LGGGH – 43]⁺ and [WGGGH – 116]⁺, obtained from the CID of [LGGGH]^•+ and [WGGGH]^•+, respectively. Elimination of the neutral 116-Da radical requires inevitable dehydrogenation of the indole nitrogen atom, leaving the radical centered formally on the indole nitrogen atom ([Ind]^•-2), in agreement with the CID data for [WGGGH]^•+ and [W_1-CH3GGGH]^•+; replacing the tryptophan residue with a 1-methyltryptophan residue results in a change of the base peak from that arising from a neutral radical loss (116 Da) to that arising from a molecule loss (131 Da), both originating from C_β–C_γ bond cleavage. Hydrogen atom transfer or proton transfer to the γ-carbon atom of the tryptophan residue weakens the C_β–C_γ bond and, therefore, decreases the dissociation energy barrier dramatically.