Novel solvent-controlled chemoselective palladium/copper(II) halide catalyzed oligomerization of <Emphasis Type="Italic">tert</Emphasis>-butyl acetylene

Solvent-controlled chemoselective palladium-catalyzed oligomerization of tert-butyl acetylene is reported in this paper. The reaction was carried out smoothly in benzene/n-BuOH binary solvent system. When unpolar aprotic benzene was the preponderating component in the binary system, a cyclotrimerization process occurred to produce 1,3,5-tri-tert-butylbenzene via a mechanism of three acetylene molecules, inserted step by step, forming σ-butadienyl-Pd and σ-hexatrienyl-Pd intermediates. While when the polar, protic and strong coordinating component n-BuOH, which aids Cu(II) to cleave the C-Pd σ-bonds and solvate Pd(II), Cu(II) cations, halo anion, σ-butadienyl-Pd intermediate, etc., was increased to a certain extent in the binary solvent system, the reaction proceeded readily via a n-BuOH-promoted mechanism to give (3Z,5Z)-2,2,7,7-tetramethyl-3,6-dichloro-3,5-octadiene or (3Z,5Z)-2,2,7,7-tetramethyl-3,6-dibromo-3,5-octadiene, respectively. Possible weak hydrogen bonds and n-π weak force between n-BuOH (electron pair donor (EPD)) and tert-butyl acetylene (and σ-butadienyl-Pd intermediate, electron pair acceptor (EPA)) in the latter process were also in favor of the n-BuOH promoted pathway. Meanwhile, the coupling product 2,2,7,7-tetramethyl-3,5-octadiyne was exclusively obtained when the reaction was conducted in singular polar H₂O. Influences of the solvent, catalysts, as well as possible mechanism were discussed in this paper.     

The elimination of metal(I) hydride in the mass spectra of some metal(II) compounds

Metal(I) hydrides are eliminated as neutral species in the electron impact ionization mass spectra of copper(II) and palladium(II) complexes of ethylene-N,N′-3-benzoylprop-2-en-2-amine. Deuterium labelling shows that the hydrogen atom of the metal(I) hydride is derived predominantly from the ethylene bridge both for ion source reactions and for metastable ion transitions. Evidence supporting the proposed rationalization for elimination of metal(I) hydride is provided by the observation of an analogous reaction in the mass spectrum of (ethylene-N,N′-salicylaldiminato)copper(II). The mass spectrum of ethylene-d₄-N,N′-3-benzoylprop-2-en-2-amine shows an unusual rearrangement to give [C₇H₅D₂]⁺ ions involving a formal phenyl-to-methylene transfer.     

Reactions of bis(hexamethylbenzene)iron(0) with carbon monoxide and with unsaturated hydrocarbons

Thermal decomposition of bis(hexamethylbenzene)iron(0) in the presence of carbon monoxide yields a novel carbonyl iron complex, [C₆(CH₃)₆]Fe(CO)₂. The cyclohexadiene complex [C₆(CH₃)₆]Fe(C₆H₈) is obtained from reaction of bis(hexamethylbenzene)iron(0) with either 1,3-cyclohexadiene or benzene, and the yield is much greater in the presence of hydrogen gas. Interaction of bis-(hexamethylbenzene)iron(0) with 2-butyne induces a catalytic cyclotrimerization to give more hexamethylbenzene. Kinetic and isotope distribution studies indicate that the primary step in these reactions is not a direct loss of one ring ligand, but rather an insertion of the iron center into one of the ligand methyl CH bonds, leading to a benzyl hydride complex species. Mechanisms for the subsequent reactions of this iron hydride species are proposed.     

溶剂对钯催化的叔丁基乙炔低聚反应化学选择性的调控作用

报道了溶剂对钯催化的叔丁基乙炔低聚反应化学选择性的调控作用. 反应可在苯-正丁醇双组分溶剂体系中顺利进行, 当双组分溶剂体系中苯占优势比例时, 反应发生递次的三分子炔烃顺式插入, 经由顺式s-烯钯中间体生成环三聚产物1,3,5-三叔丁基苯; 而当双组分溶剂中正丁醇组分上升至一定比例, 反应选择性生成(3Z,5Z)-2,2,7,7-四甲基- 3,6-二氯-3,5-辛二烯或(3Z,5Z)-2,2,7,7-四甲基-3,6-二溴-3,5-辛二烯, 这是由于正丁醇可显著加快C—Pd σ键的断裂, 并与叔丁基乙炔、σ-烯钯中间体形成弱氢键作用力, 同时也与Pd(II)和Cu(II)等离子存在配位效应. 在强极性质子溶剂H₂O中, 反应生成偶联双炔: 2,2,7,7-四甲基-3,5-辛二炔. 文中就反应溶剂体系、钯铜催化剂及反应可能机理等分别进行了探讨.     

Nb(iPrNPMe2)3Fe–PMe3: A potential high reactivity heterobimetallic catalyst for acetylene cycloadditions

Alkynes cycloaddition reactions are powerful tools for constructing cyclic molecules with optimal atom efficiency, but these reactions cannot proceed at ambient temperature without transition-metal catalysts. In this work, a heterobimetallic complex featuring an Nb–Fe triple bond, Nb(ⁱPrNPMe₂)₃Fe–PMe₃, has been evaluated as the potential catalyst for acetylene cycloaddition, using density functional theory. The calculated results show that the singlet-state (i.e. ground-state) Nb(ⁱPrNPMe₂)₃Fe–PMe₃ can be applied to benzene synthesis, but is not suitable for cyclobutadiene. Benzene can be obtained easily at room temperature and is the unique product on the singlet potential surface. The irradiation of infrared-red light can drive the excitation of singlet Nb(ⁱPrNPMe₂)₃Fe–PMe₃ to its triplet state. Both benzene and cyclobutadiene can be formed on the triplet reaction potential surface due to their low energy barriers. Therefore, Nb(ⁱPrNPMe₂)₃Fe–PMe₃ is a potential high reactivity heterobimetallic catalyst for the cyclotrimerization of alkynes. In the reaction process, the catalytic active site of Nb(ⁱPrNPMe₂)₃Fe–PMe₃ moves from niobium to iron.     

Synthesis and Crystal Structure of Mercury(II) Metal Crown Ether with N‐Heterocyclic Carbene Linkage

The mercury(II) metal crown ether ( 2a ) was obtained in high yield by reaction of the carbene precursor 1,2‐bis[N‐(1‐naphthylmethylene)imidazoliumethoxy]benzene dihexafluorophosphate ( 1 ) and Hg(OAc)₂. Addition of NaI to the acetone solution of 2a resulted in precipitation of pale yellow solid 2b . The structures of 2a and 2b were determined by single‐crystal X‐ray diffractometry. Both molecules display a helical conformation with a torsional cycle. The mercury atom in complex 2a is tricoordinated by two intramolecular carbene carbon atoms and an acetate oxygen atom. The mercury atom in complex 2b is tetracoordinated by two intramolecular carbene carbon atoms and two cis‐iodine atoms.     

Prominent enhancing effects of substituents on the strength of π···σ‐hole tetrel bond

The potential applications of tetrel bonds involving π‐molecules in crystal materials and biological systems have prompted a theoretical investigation of the strength of π···σ‐hole tetrel bond in the systems with acetylene and its derivatives of CH₃, AuPH₃, Li, and Na as well as benzene as the π electron donors. A weak tetrel bond (ΔE < 15 kJ/mol) is found between acetylene and tetrel donor molecule TH₃F (T = C, Si, Ge, Sn, and Pb). All substituents strengthen the π tetrel bond, but the electron‐donating sodium atoms have the largest enhancing effect and the interaction energy is up to about 24 kJ/mol in C₂Na₂‐CH₃F. The electron‐donating ability of the AuPH₃ fragment is intermediate between the methyl group and alkali metal atom. The origin of the stability of the π tetrel‐bonded complex is dependent on the nature of the tetrel donor and acceptor molecules and can be regulated by the substituents.     

Reaction of 2,2,2-trichlorobenzo[d]-1,3,2-dioxaphosphole-5-carbonylchloride with phenylacetylene: predominant formation of 2-(2-chloro-2-phenylethenyl)-2,2-dichlorobenzo[d]-1,3,2-dioxaphosphole-5-carbonylchloride

The reaction of 2,2,2-trichlorobenzo[d]-1,3,2-dioxaphosphole-5-carbonylchloride with phenylacetylene in benzene (80 °C) afforded 2-(2-chloro-2-phenylethenyl)-2,2-dichlorobenzo[d]-1,3,2-dioxaphosphole-5-carbonylchloride (yield >95%) as a result of the electrophilic cis-addition of the phosphorus(v) derivative at the triple bond of acetylene with retention of coordination of the P atom. Hydrolysis of this compound affords predominantly 2-hydroxy-5-(hydroxycarbonyl)phenyl (2-chloro-2-phenylethenyl)phosphonate. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 377–379, February, 2006.     

C—H...π interactions in cocrystals of bis(trimethylsilyl)acetylene and diphenylacetylene with benzene

We present here the crystal structures of two acetylene derivatives cocrystallized with benzene, namely bis(trimethylsilyl)acetylene benzene solvate, C₈H₁₈Si₂·C₆H₆, (I), and diphenylacetylene benzene solvate, C₁₄H₁₀·C₆H₆, (II). In (I), both molecules belong to the symmetry point group C_2h and are located about special positions with site symmetry 2/m. In (II), both molecules show crystallographic inversion symmetry. In both structures, there are C—H...π contacts between aromatic H atoms and the π‐electrons of the triple bond. In addition to these, in (II) there are C—H...π contacts between aromatic H atoms and the π‐electron cloud of the benzene molecules.     

A Cocatalyst that Stabilizes a Hydride Intermediate during Photocatalytic Hydrogen Evolution over a Rhodium‐Doped TiO2 Nanosheet

The hydrogen evolution reaction using semiconductor photocatalysts has been significantly improved by cocatalyst loading. However, there are still many speculations regarding the actual role of the cocatalyst. Now a photocatalytic hydrogen evolution reaction pathway is reported on a cocatalyst site using TiO₂ nanosheets doped with Rh at Ti sites as one‐atom cocatalysts. A hydride species adsorbed on the one‐atom Rh dopant cocatalyst site was confirmed experimentally as the intermediate state for hydrogen evolution, which was consistent with the results of density functional theory (DFT) calculations. In this system, the role of the cocatalyst in photocatalytic hydrogen evolution is related to the withdrawal of photo‐excited electrons and stabilization of the hydride intermediate species; the presence of oxygen vacancies induced by Rh facilitate the withdrawal of electrons and stabilization of the hydride.     

A reinvestigation of the gas phase reaction of boron atoms, B(Pj)/B(Pj) with acetylene,

The reaction dynamics of ground state boron atoms, B(²P_j), with acetylene, was reinvestigated and combined with novel electronic structure calculations. Our study suggests that the boron atom adds to the carbon–carbon triple bond of the acetylene molecule to yield initially a cyclic intermediate undergoing two successive hydrogen atom migrations to form ultimately an intermediate i3. The latter was found to decompose predominantly to the c-BC₂H(X²A′) isomer plus atomic hydrogen via a tight exit transition state. To a minor amount, an isomerization of i3–i4 prior to a hydrogen atom ejection forming the linear structure, HBCC(X¹Σ⁺), has to be taken into account. Since the c-BC₂H(X²A′) and HBCC(X¹Σ⁺) isomers are separated by an isomerization barrier to ring closure of only 3 kJ mol⁻¹, internally excited HBCC(X¹Σ⁺) products can isomerize to the c-BC₂H(X²A′) structure and vice versa.     

Tris(trimethylsilyl)silane promoted radical reaction and electron-transfer reaction in benzotrifluoride

Tris(trimethylsilyl)silane (TTMSS) promoted free radical reaction in benzotrifluoride (BTF) was investigated. Compared to same reaction using environmentally less desirable tri-n-butyltin hydride (TBTH) in benzene, less quantity of BTF than that of benzene can be used because of slower hydrogen atom transfer from TTMSS than that from TBTH toward primary alkyl radicals. Also, electron-transfer reactions promoted by tris(p-bromophenyl)aminium hexachloroantimonate (TBPA) and FeCl₃ were conducted in BTF. Then, TBPA was found to be effective in BTF comparably to that in methylene chloride. In addition, an interesting observation that FeCl₃ promoted reaction was accelerated by the addition of imidazolium salt was made. All the results suggest that BTF is a tolerable solvent for free radical reaction with TTMSS and electron-transfer reactions using TBPA as well as FeCl₃.     

Acetylene cyclotrimerization by early second-row transition metals in the gas phase. A theoretical study

The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined.     

Some peculiarities of interactions in the Hf2Fe−H2 system at low temperatures and high pressures

Hydride formation was studied in the Hf₂Fe−H₂ system at hydrogen pressure of up to 2000 atm in a temperature range from 195 to 295 K. Hydride phases of different compositions were studied by the X-ray diffraction method. The hydrogenation reaction in the system can take two pathways to form two stable hydride phases depending on the conditions of initial hydrogenation. Absorption of hydrogens at a pressure of about 2000 atm yields a hydride which contains two H atoms per metal atom. Models of the arrangement of hydrogen atoms in the crystal lattice of hydride phases were suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 33–36, January, 1997.     

A Mechanistic Study of the Utilization of arachno‐Diruthenaborane [(Cp*RuCO)2B2H6] as an Active Alkyne‐Cyclotrimerization Catalyst

The reaction of nido‐[1,2‐(Cp*RuH)₂B₃H₇] ( 1 a , Cp*=η⁵‐C₅Me₅) with [Mo(CO)₃(CH₃CN)₃] under mild conditions yields the new metallaborane arachno‐[(Cp*RuCO)₂B₂H₆] ( 2 ). Compound 2 catalyzes the cyclotrimerization of a variety of internal‐ and terminal alkynes to yield mixtures of 1,3,5‐ and 1,2,4‐substituted benzenes. The reactivities of nido‐ 1 a and arachno‐ 2 with alkynes demonstrates that a change in geometry from nido to arachno drives a change in the reaction from alkyne‐insertion to catalytic cyclotrimerization, respectively. Density functional calculations have been used to evaluate the reaction pathways of the cyclotrimerization of alkynes catalyzed by compound 2 . The reaction involves the formation of a ruthenacyclic intermediate and the subsequent alkyne‐insertion step is initiated by a [2+2] cycloaddition between this intermediate and an alkyne. The experimental and quantum‐chemical results also show that the stability of the metallacyclic intermediate is strongly dependent on the nature of the substituents that are present on the alkyne.     

Role of hydrogen enrichment on acetylene emission during benzene oxidation

A zero-dimensional model (perfectly-stirred reactor) in conjunction with CHEMKIN II and a scheme resulting from the merging of validated kinetic schemes for the oxidation of benzene were used to investigate the effect of hydrogen addition on the formation-depletion of C₂H₂, which is known as a soot precursor. The current modeling study treats the dependence of acetylene amounts on hydrogen percentage in the fuel mixture, and defines the key reaction mechanisms responsible for the observed reduction in C₂H₂ and consequently in polycyclic aromatic hydrocarbons and soot amounts induced by the hydrogen additive. The main objective of this work was to obtain fundamental understanding of the mechanisms, through which the hydrogen affects the acetylene yields. It was found that, at high temperatures hydrogen/benzene fuel mixtures displayed lower acetylene concentrations compared to the pure benzene fuel, whereas opposite trends were observed at low reaction temperatures.     

Mechanism of alkene isomerization by bifunctional ruthenium catalyst: A theoretical study

The molecular mechanism of the isomerization of 1-pentene to form (E)-2-pentene catalyzed by the bifunctional ruthenium catalyst has been investigated using density functional theory calculations. The reaction is likely to proceed through the following steps: 1) the β-H elimination to generate the ruthenium hydride intermediate; 2) the reductive elimination of the hydride intermediate to generate the nitrogen-protonated allyl intermediate; 3) the transportation of the hydrogen by the dihedral rotation with Ru–P bond acting as axis; 4) the oxidative addition to afford another hydride complex; 5) the reductive elimination of the hydride intermediate to form the C₂-C₃ π-coordinated agostic intermediate; 6) the coordination of the nitrogen to the ruthenium center to give the final product. The rate-determining step is the oxidative addition step (the process of the hydrogen moves to ruthenium center from the nitrogen atom) with the free energy of 31.2 kcal/mol in the acetone solvent. And the N-heterocyclic ligand in the catalyst mainly functions in the two aspects: affords an important internal-basic center (nitrogen atom) and works as a transporter of hydrogen. Our results would be helpful for experimentalists to design more effective bifunctional catalysts for isomerization of a variety of heterofunctionalized alkene derivatives.     

Supramolecular interactions in the 2,6‐bis(benzimidazol‐2‐yl)pyridine–terephthalic acid–water (2/1/4) cocrystal

In the title compound, 2C₁₉H₁₃N₅·C₈H₆O₄·4H₂O, the terephthalic acid molecule lies on a crystallographic inversion centre and the H atoms of one water molecule exhibit disorder. The maximum deviation of any atom from the mean plane through the C and N atoms of the 2,6‐bis(benzimidazol‐2‐yl)pyridine molecule is only 0.161 (4) Å. In the crystal structure, the water molecules play an important role in linking the other molecules via hydrogen bonding. The structure forms a three‐dimensional framework via strong intermolecular hydrogen bonding. In addition, there are π–π stacking interactions between the imidazole, pyridine and benzene rings.     

Synthesis and Structure of Alkali Bis(phosphinimino)methanides

The reaction of CH₂(PPh₂NSiMe₃)₂ with n‐butyllithium or potassium hydride in THF leads, after the recrystallization from toluene or n‐heptane/diglyme, to the corresponding alkali derivatives [Li(THF)][CH(PPh₂NSiMe₃)₂] ( 1 ), K[CH(PPh₂NSiMe₃)₂] ( 2 ), and [K(digylme)][CH(PPh₂NSiMe₃)₂] ( 3 ). Upon coordination to the metal center the ligand forms a six membered metallacycle in which both phosphinimine nitrogen atoms bind to the metal atom.     

Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4)

The reactions of the indenyl radicals with acetylene (C₂H₂) and vinylacetylene (C₄H₄) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉H₇^.) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆H₅^.) and acetylene forming phenylacetylene (C₆H₅CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol⁻¹) and slow, contrary to the exoergic (−38 kJ mol⁻¹) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.