1,2- and 3,4-rich polyisoprene synthesized by Mo(VI)-based catalyst with phosphorus ligand

Polyisoprene with relatively high content of 1,2/3,4 structure was synthesized using a novel catalyst system composed of MoO₂Cl₂ supported by phosphorus ligand and Al(OPhCH₃)(i-Bu)₂ as co-catalyst. The effects of phosphites, phosphates and phosphoric acid as ligands were investigated in the coordination polymerization of isoprene in the chosen catalyst system. The studied ligands significantly affected the catalytic activity of the Mo–Al catalytic active center without significant effect on the stereoselectivity. Mo(VI)-based catalyst system was proved to be highly effective in the polymerization of isoprene even at low [Al]/[Mo] ratio (10), affording polyisoprene with 1,2- and 3,4-% structural units in the range of 44.6–52.5%, high molecular weights M_n ~ 10⁵, and relatively broad molecular weight distributions (M_w/M_n = 3.0–4.4). The effect of molar ratio of phosphorous ligand to Mo-catalyst on catalyst activity of isoprene polymerization was discussed, and the structures of Mo–phosphite complexes were preliminarily studied by IR.     

Asymmetric synthesis of amino acids via the catalytic reduction of acylaminoacrylic acid azlactone derivatives

Reductive aminolysis of 2-methyl-4-benzylidene-²-oxazolin-5-one upon treatment with a PdCl₂-S-phenylalanine ester (dimethylamide) catalytic system leads to the formation of the corresponding acylated dipeptide derivatives, with the R,S-configuration (diastereomer) predominating (DE 9–27%). The reaction stereoselectivity in dimethoxyethane increases sharply in the presence of triethylamine additive, and in the case of S-phenylalanine methyl ester reaches 47%. The stepwise mechanism for this process has been studied.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 811–818, April, 1990.     

Effects of Composition and Method of Preparation of Lead-Containing Oxide Systems on Their Catalytic Properties in the Oxidative Dimerization of Methane

The effects of the Pb-Sn-O composition, the method of precipitating the active components, and modification with alkaline earth metals on the physico-chemical and catalytic properties of lead-tin catalysts in oxidative dimerization of methane have been studied. The effect of the phases of lead orthostannate, Pb₂SnO₄, on the surface properties of the Pb-Sn-O system has been established. The most selective composition has a ratio PbO : SnO₂ = 80 : 20 mole % on -Al₂O₃ with divalent metal hydroxides. Introduction of alkaline earth metals into the Pb-Sn-O/-Al₂O₃ system leads to stabilization of lead in its 4-valent state with formation of metal orthoplumbates, M₂PbO₄, and to a decrease in selectivity with respect to C₂-hydrocarbons. The increased catalytic activity of the modified M-Pb-Sn-O/-Al₂O₃ samples (M = Ba, Sr, Ca, and Mg) is determined by the presence of tightly bound oxygen. Conclusions about the nature of the interaction of the reagents with the surface of the catalyst were reached.     

Solvent-free palladium-catalyzed addition of diaryl dichalcogenides to alkynes

Solvent-free palladium-catalyzed addition of diaryl disulfides and diselenides to terminal alkynes makes it possible to achieve high stereoselectivity and almost 100% yields in 10 min using only 0.1 mol.% catalyst. Both Pd(PPh₃)₄ and easily available Pd(OAc)₂ and PdCl₂ can be used in the reaction with an excess of triphenylphosphine. The catalyst and triphenylphosphine are readily recycled for repeated use. The study of the mechanism of the solvent-free catalytic reaction indicates that the process involves binuclear palladium complexes.     

A novel and efficient route to the construction of the 4-oxa-tricyclo[4.3.1.0]decan-2-one scaffold

A short and efficient route to the synthesis of 4-oxa-tricyclo[4.3.1.0]decan-2-one scaffold 12 in good yield is reported. Essential to the synthesis was the implementation of selective protection of the catechol system in xanthone 2 with Ph₂CCl₂ and MOM groups. Subsequently, a biomimetic tandem Claisen/Diels-Alder reaction occurred to produce the desired tricyclic scaffold 11a as a single isomer. A rationalization of the excellent region and stereoselectivity of this transformation was also proposed.     

Effects of calcination and support on supported manganese catalysts for the catalytic oxidation of toluene as a model of VOCs

The conventional impregnation method was used to prepare 15 wt% Mn-supported catalysts, which were applied to the catalytic oxidation of volatile organic compounds (VOCs; toluene, benzene, and o-xylene). The effects of calcination temperatures in the range of 500–900 °C and supports (γ-Al₂O₃, SiO₂, and TiO₂) on the property and performance of 15 wt% Mn-supported catalysts were investigated. Their physicochemical characteristics were analyzed by the BET, XRD, NH₃–TPD, H₂–TPR, and XPS. The calcination temperature greatly affected the crystalline structure and O1s _D (defect oxides)/O1s _L (lattice oxides) area ratio of the 15 wt% Mn/γ-Al₂O₃ (15 Mn/Al) catalyst. The order of the O1s _D/O1s _L area ratios of the 15 Mn/Al catalysts with respect to calcination temperature was 900 > 500 > 700 °C, which was in good agreement with that observed for the catalytic activity. In addition, the activity order of the 15 wt% Mn-supported catalysts with respect to the type of support was γ-Al₂O₃ > SiO₂ > TiO₂. The 15 wt% Mn/Al catalyst, which had a higher O1s _D/O1s _L area ratio, showed better activity than the 15 wt% Mn/SiO₂ (15 Mn/Si) and 15 wt% Mn/TiO₂ (15 Mn/Ti) catalysts. Defect oxides played a significant role in the catalytic oxidation of VOCs. The catalytic activity with respect to the type of VOC decreased in the order of benzene > toluene > o-xylene.     

Ethylene–propylene copolymerization initiated with solubilized Ziegler–Natta macromolecular complexes. I. Determination of kinetic parameters

Copolymerization of ethylene with propylene (E–P) was initiated with soluble polystyrene –butadiene–Li/TiCl₄ complexes in toluene leading to vinylic-olefinic based block copolymers. The activity of the system was measured at a constant ratio r = Li/Ti for different feed compositions. From measurements of the amount of copolymers produced, a direct determination of the efficiency of the catalytic system was made. This efficiency was found to be close to that obtained for homopolymerizations of C₂H₄ and C₃H₆ initiated with the same catalytic complex (? 80% with respect to the concentration of “carbon-titanium” bonds). Both the measured values of the efficiency and the determination of the molar masses of the polyolefinic block are consistent with a living character of the system involved in the E–P copolymerization. In such conditions well-defined linear [styrene]-b-[ethylene-co-propylene] copolymers were obtained. Furthermore, ¹³C NMR analyses allowed determination of the comonomer reactivity ratios, the values of which indicate an E–P random copolymerization. From both these values and those of the absolute rate constants of propagation determined for the homopolymerization of C₂H₄ and C₃H₆ initiated with the same catalytic system, the absolute rate constants of cross-propagation were deduced.     

A quantum-mechanical study of charge transfer in redox catalysis: Decomposition of hydrogen peroxide on copper ammoniates and iron porphyrin

Reactions of redox type at a catalyst are initiated by transfer of a charge q, with accompanying change in the electronic structure of the reagent. It is shown that quantum chemistry provides estimates of q sufficient for many qualitative and semiquantitative purposes, provided that the interaction is one of stationary coordination, or nearly so. The semiempirical approximation in the MO LCAO method is used with self-consistent charges in calculations for the decomposition of H₂O₂ on copper ammoniates. The results indicate indirectly that the reaction has a radical mechanism (q 1 e in stationary coordination). An approximate analytic formula for q is derived for a catalyst containing a large conjugated system (chelate or enzyme), in which a major part is played by the redox (catalytic) capacity of the system, which is defined as the derivative of the charge with respect to the Fermi level and which is easily calculated or determined empirically. This concept explains the high catalytic activity of such systems in redox processes. Detailed calculations are presented for the decomposition of H₂O₂ on iron porphyrin.     

Platinum complexes of malonate-derived monodentate phosphines and their application in the highly chemo- and regioselective hydroformylation of styrene

Neutral complexes of the formula PtCl₂L₂ (where L = diethyl 2-diphenylphosphino-malonate (1), diethyl 2-methyl-2-diphenylphosphinomalonate (2), dibenzyl 2-diphenylphosphinomalonate (3), 1,3-dihydroxy-2-methyl-2-diphenylphosphinopropane (4)) were prepared. These proved to be precursors to active catalysts for the hydroformylation of styrene. The platinum-containing catalytic systems prepared from ligand 4 provided the highest activity, while the platinum compounds prepared from other ligands all showed similar levels of reactivity to each other. The matching of high chemo- and regioselectivities were observed in most cases. Surprisingly, the complexes were practically inactive in imidazolium-type ionic liquids. ³¹P NMR studies on the PtCl₂L₂ complexes revealed that the stereoselectivity of the cis/trans geometrical isomers is strongly dependent on the structure of the ligand.     

Synthesis of (E)-alkenes via hydroindation of CC in InCl3-NaBH4 system

In InCl₃-NaBH₄-MeCN system, terminal aryl alkynes could couple with aryl iodides and bromides to give disubstituted alkenes via hydroindation of CC. In the similar way, (E)-alkenylsilanes were synthesized via reduction of alkynylsilanes in tetrahydrofuran (THF) in high yields. The processes showed high regio- and stereoselectivity.     

Pd-catalyzed selective tandem arylation-alkylation of 1,1-dihalo-1-alkenes with aryl- and alkylzinc derivatives to produce α-alkyl-substituted styrene derivatives

Trans-selective monoarylation of 1,1-dibromo- and 1,1-dichloro-1-alkenes (1) can be achieved in >80% yields and in ≥98-99% stereoselectivity with arylzinc bromides in the presence of a catalytic amount of Cl₂Pd(DPEphos) or Cl₂Pd(dppb), the former permitting cleaner and higher yielding reactions. Although THF is a generally satisfactory solvent, ether and toluene are superior to THF in some cases. The second substitution of (Z)-α-bromostyrenes (3) with alkylzincs in the presence of 2 mol% of Pd(^tBu₃P)₂ proceeds to give the corresponding 2 in >90% yields and in ≥98-99% stereoselectivity. Although somewhat less satisfactory, the use of Cl₂Pd(DPEphos) permits a one-pot tandem arylation-alkylation.     

Highly selective silylformylation of internal and functionalised alkynes with a cationic dirhodium(II) complex catalyst

The tetracationic complex [Rh₂(MeCN)₂(Naft)₄](BF₄)₄ (Naft = μ-1,8-naphthyridine) was found to be an efficient catalyst for the silylformylation of internal and functionalised alkynes to yield useful synthetic intermediates. The complex exhibits an unprecedented chemoselectivity towards alkyne silylformylation instead of simple hydrosilylation, as well as a good stereoselectivity. The catalytic efficiency of the complex is markedly superior compared to that of previously reported catalysts such as [] or Rh₄(CO)₁₂; incidentally, the performance of the latter catalyst was found to vary dramatically with its shelf-life, which indicates that the catalyst evolves with ageing towards other species, most notably higher nuclearity rhodium carbonyl clusters, which are more chemoselective towards silylformylation. Preliminary results on the determination of the catalytically active species in the case of complex [Rh₂(MeCN)₂(Naft)₄](BF₄)₄ indicate that the complex is reduced in situ to a dirhodium(I) species which maintains the dimeric, lantern-shaped structure.     

1,10-Phenanthroline catalysis of the chromium(VI) oxidation of alcohols: a kinetic and mechanistic study

Several primary, secondary and tertiary alcohols are oxidised in acidic solutions using Cr^VIas an oxygen donor and 1,10-phenanthroline (Phen) as catalyst. A large primary kinetic isotope effect, kie(k _H/k _D 10) suggests a linear transition state in the noncatalytic system, whereas a moderate kinetic isotope effect observedin the catalytic pathway can be associated with a cyclic transition state. The k _D2O/k _H2Oratio does not seem to indicate any significant solvent isotope effect. Cr^III inhibition, reflected in the firstorder plots, can be explained by exchange reactions. PhCH₂OH reacts ca. 7 times faster than MeOH. Moderately negative ^* values indicate ahydride abstraction mechanism. Imperfect correlation is observed in the LFER between catalytic and noncatalytic reaction systems.Enhanced reactivity has been observed in the presence of 2,2-bipyridyl, whereas the catalytic activity is insignificant with picolinicacid, imidazole and oxalic acid. Consideration of the kinetic behaviour of catalytic and noncatalytic reaction pathways provides valuable mechanistic insight into the systems studied.     

Oxygenation of Dimethylsulfoxide and Copper-Catalyzed Autoxidation of Substituted Benzoins

The catalytic activity of autoxidized copper (I) in the oxidation of para-disubstituted benzoins in dimethylsulfoxide by O₂ was studied both kinetically and by product analysis. Stoichiometry (1), accounts for more than 80% of the reaction. The catlytic oxydation was followed by monitering th consumption of O₂ manometrically by a fully automatic apparatus. (X = H, CH₃, OCH₃, C₆H₅ CI) A redox shuttle mechanism is proposed, where the rate-determining step is the autoxidation of Cu(I) followed by a rapid oxidation of the substrate by an oxocupric species. The redox stiochiometry (I) corresponds to that found for external monooxy-genases (or mixed-function oxidases), and the significance of our results with respect to analogous catalytic systems is discussed.     

Highly active asymmetric Diels-Alder reactions catalyzed by C2-symmetric bipyrrolidines: catalyst recycling in water medium and insight into the catalytic mode

A new class of C₂-symmetric 3,3′-dialkoxy-2,2′-bipyrrolidines have been designed and synthesized for asymmetric organocatalytic Diels-Alder reactions of α,β-unsaturated aldehydes. The remarkable rate-accelerating effect for the cycloaddition reaction has been observed in aqueous medium. The catalyst 1c·2HClO₄ can be recovered and reused several times by simple extraction without significant loss of catalytic activity and stereoselectivity. The catalytic mode has been demonstrated by DFT calculation, NMR, and X-ray crystallographic studies for diiminium intermediate.     

Catalytic Properties of the Fe2O3–Bi2O3 System in Ammonia Oxidation to Nitrogen Oxides

The catalytic properties of the Fe₂O₃–Bi₂O₃ system in ammonia oxidation are studied at 1073 K. The effect of phase composition on the physicochemical and catalytic properties of the system is determined. The catalytic properties of individual components of the system (-Fe₂O₃, Bi₂Fe₄O₉, BiFeO₃, and -Bi₂O₃) are studied.     

Structural organization of phosphorus titanate oxides prepared by the alkoxo method and their catalytic activity in ethylene glycol oxyethylation

The relationship of the structural organization and acid-base properties of the surface of phosphorus titanate oxides prepared from tetra-n-butoxytitanium and phosphorous esters with the catalytic activity and selectivity of these materials in ethylene glycol oxyethylation was studied. Single-phase phosphorus-containing oxides synthesized from 2-diethylamido-4-methyl-1,3,2-dioxophosphorinane and diphenyl(methano)phosphocane have strong surface aprotic acid sites and exhibit high catalytic activity with respect to oxyethylation and a record-breaking selectivity in the formation of the lower homolog, diethylene glycol. The last-mentioned fact is a consequence of the sieve effect exerted by the homogeneous porous structure of supermicropores (8—10 ) of the oxides. An increase in the concentration of the strong acid sites (130 kJ mol^–1) on the oxide surface enhances the catalytic activity. On the basis of the temperature programmed desorption of ammonia and CO₂ and kinetic measurements, a concerted acid-base mechanism was proposed for the catalytic addition of ethylene oxide to ethylene glycol on the phosphorus-titanate surface.     

Immobilization of transition metal ion complexes and their catalytic activity towards the activation of hydrogen peroxide

Transition metal ion-imidazole complexes have been immobilized on silica, silica–alumina (25%Al₂O₃), and alumina supports by adsorption and functionalization methods. The catalytic activity of these supported complexes in the decomposition of H₂O₂ has been studied. The reaction exhibits first-order kinetics with respect to [H₂O₂] and the quantity of catalyst. The rate of reaction decreases as [H₂O₂]₀ increases. The order of catalytic reactivity is strongly dependent on the type of metal ion, support, and the immobilization method. The complex anchored via adsorption exhibited a higher activity compared to the corresponding complex anchored via functionalization of the surface. The reaction proceed via formation of the peroxo-intermediate, which has an inhibiting effect on the reaction rate. The reaction is enthalpy-controlled as is concluded from the isokinetic relationship. A mechanism is proposed involving the generation of HO₂ radicals from the peroxo-intermediate in the rate-determining step.