PdCl2-catalyzed hydrogenolysis of a C-O bond in monoaryl sulfates by sodium phosphinate in an aqueous alkaline medium

The hydrogenolysis of the C-O bond in monoaryl sulfates by the action of an excess of NaH₂PO₂ in the presence of catalytic amounts of PdCl₂ and KOH is studied. The reaction proceeds chemoselectively with complete ester conversion to the corresponding arenes.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 605–606, March, 1993.     

Studies on high chemical reactivity of nano-NaH

A comparison between the initial reaction rates of nanometric and commercial NaH has been studied in four test reactions: 1) hydrogenolysis of chlorobenzene; 2) selective reduction of cinnamaldehyde to cinnamyl alcohol; 3) metallation of dimethyl sulfoxide; and 4) catalytic hydrogenation of olefins. The experimental results indicate that when NaH is used as a chemical reagent in the first three reactions, the initial reaction rates of nano-NaH is 230, 120 and 110 times higher than those of the commercial ones respectively, and it is in agreement with the difference in specific surface areas between these two forms of NaH. When NaH is used as a catalyst component together with Cp₂TiCl₂ in the fourth reaction, catalyst with nano-NaH gives extremely high activity in the hydrogenation of olefins, while the one with commercial NaH gives no activity at all even if a large amount of the commercial NaH is used to make the total surface area equivalent to that of nano-NaH. Thus, it is evident that although large specific surface area is important for nano-NaH to be used as a catalyst component, high surface energy with surface defects seems to be more important. The large specific surface and the activated surface of nano-NaH with high surface energy should be the main factors for their extremely high chemical reactivity, while whether the former or the latter one plays a leading role depends on the type of reactions involved. __________ Translated from Chinese Journal of Applied Chemistry, 2007, 24(1): 21–24 [译自: 应用化学]     

Ethane hydrogenolysis on silica-supported tantalum hydride. Quantum-chemical study

The structures of the (H₄Si₂O₅)O₂Ta(R, RR"R, RR) clusters (R, R", R = H, Me, Et, Pr; R = CH₂), which model the basic structural units of the catalytic cycle of ethane hydrogenolysis on silica-supported tantalum hydride, were studied by the density functional theory (B3LYP) and the perturbation theory (MP2). The possible structure of active sites was proposed based on comparison of experimental results with calculated data. Ethane hydrogenolysis and metathesis proceed by a mechanism involving the formation of ethylene -complexes and carbenium derivatives of tantalum as intermediates.     

A Useful,Reliable and Safer Protocol for Hydrogenation and the Hydrogenolysis of O‐Benzyl Groups: The In Situ Preparation of an Active Pd0/C Catalyst with Well‐Defined Properties

A simple, highly reproducible protocol for the hydrogenation of alkenes and alkynes and for the hydrogenolysis of O‐benzyl ethers has been developed. The method features the in situ preparation of an active Pd⁰/C catalyst from Pd(OAc)₂ and charcoal, in methanol. The mild reaction conditions (25 °C) and low catalyst loading required (0.025 mol %), as well as the absence of contamination of the product by palladium residues (<4 ppb), make this a sustainable, useful process for organic chemists. Alternatively, the protocol can be carried out under microwave activation, to shorten the reaction times, with cyclohexene as the hydrogen source.     

碱助剂促进Ni基催化剂氢解山梨醇制备二元醇

采用逐步湿浸渍的方法制备了一系列含有不同载体和碱促进剂的Ni基催化剂用于生物质基平台化合物山梨醇的氢解反应. 通过反应对载体和碱促进剂进行了筛选和组分含量的优化,碱性促进剂的引入不仅增强了催化剂的碱性,而且通过Ni²⁺和碱促进剂的强相互作用提高了Ni在催化剂上的分散性;10%Ni/10%La₂O₃/ZrO₂表现出了非常高的氢解活性和较好的二元醇(乙二醇和1,2丙二醇)选择性,金属Ni和碱促进剂La₂O₃之间的协同作用机理对于山梨醇选择性氢解制备二元醇影响显著. 在优化的反应条件下,山梨醇达到100%的转化并且有超过48%的二元醇产率. 研究中对催化剂进行了XRD、BET、H₂-TPR和CO₂-TPD表征,用于分析催化剂结构性能. 通过对山梨醇氢解以及中间产物动力学曲线的研究,得出多元醇氢解活性与所含羟基数正相关,产物的最终分布是氢解动力学平衡的最终结果.     

Hydrogenolysis of cycloalkanes on a tantalum hydride complex supported on silica and insight into the deactivation pathway by the combined use of 1D solid-state NMR and EXAFS spectroscopies

Hydrogenolysis of cyclic alkanes is catalysed by [(triple bond)SiO)(2)Ta-H] (1) at 160 degrees C and leads to lower alkanes and cyclic alkanes including cyclopentane. The turnover number is correlated with the number of carbon atoms of the cyclic alkanes, and therefore while cycloheptane is readily transformed, cyclopentane does not give any product (<1 %). The mechanism of ring contraction probably involves carbene de-insertion as a key carbon-carbon bond-cleavage step. The reluctance of cyclopentane to undergo hydrogenolysis was further studied: under the reaction conditions cyclopentane reacts with 1 to give the corresponding cyclopentyl derivative [(triple bond)SiO)(2)Ta-C(5)H(9)] (13), which evolves towards cyclopentadienyl derivative [(triple bond)SiO)(2)Ta(C(5)H(5))] (14) according to both solid-state NMR and EXAFS spectroscopies. This latter complex is inactive in the hydrogenolysis of alkanes, and therefore the formation of cyclopentane in the hydrogenolysis of various cyclic alkanes is probably responsible for the de-activation of the catalyst by formation of cyclopentadienyl complexes.     

Effects of low hydrogen partial pressures on propylene polymerization by Cp*TiMe2(μ-Me)B(C6F5)3

Polymerization of propylene by Cp*TiMe₂(μ-Me)B(C₆F₅)₃ in the presence of increasing partial pressures of H₂ results in ever decreasing polymer molecular weights, which is consistent with the hydrogenolytic chain transfer processes involving metal–polymer bonds in many heterogeneous and homogeneous systems. However, catalytic activities are not significantly increased as the extent of hydrogenolysis increases, unlike metallocene catalyst systems in which the H₂ reacts primarily with dormant catalytic sites containing propylene 2,1-insertion products. It was shown previously that monocyclopentadienyl systems do not become seriously deactivated following 2,1 insertions, and thus hydrogenolysis does not result in enhanced activities. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4386–4389, 1999     

Molecular hydrides of samarium and europium LnH2(THF)2 (Ln=Sm or Eu): Synthesis and properties

The molecular hydrides Ln¹¹H₂(THF)₂ (Ln=Sm or Eu) were prepared by hydrogenolysis of the naphthalene complexes of divalent samarium and europium C₁₀H₈Ln(THF)₂ (Ln=Sm or Eu, respectively) as well as of the stilbene derivative of samarium(II) (PhCHCHPh)Sm(DME)₂ in THF at room temperature under atmospheric pressure. The resulting complexes were characterized by the data of microanalysis, IR spectroscopy, and magnetic susceptibility. Chemical properties of the complexes were studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 947–945, May, 2000.