Showing papers in "Collection of Czechoslovak Chemical Communications in 1997"

Dimethyl Sulfate Induced Nucleophilic Substitution of the [Bis(1,2-dicarbollido)-3-cobalt(1-)]ate Ion. Syntheses, Properties and Structures of Its 8,8'-μ-Sulfato, 8-Phenyland 8-Dioxane Derivatives

Abstract: Syntheses and structures of [8,8'-μ-(OSO2O)(1,2-C2B9H10)2-3-Co]-and [(8-Ph-1,2-C2B9H10)-3-Co-(1',2'-C2B9H11]- ions and of [(8-dioxane-1,2-C2B9H10)-3-Co-(1',2'-C2B9H11)] zwitterion are described. All compounds result on dimethyl sulfate induced nucleophilic B-substitution of the parent cobalta-carborane sandwich ion under acid catalysis. A general mechanism is proposed. Constitutions of all compounds are inferred from the mass and multinuclear NMR spectroscopy and all structures are established by X-ray diffraction. Selected NMR and X-ray data are presented.

MEASUREMENT AND CALCULATION OF THE RAMAN OPTICAL ACTIVITY OF α-PINENE AND trans-PINANE

Abstract: Raman scattering and Raman optical activity (ROA) spectra of α-pinene and trans -pinane were measured for both enantiomers. The experiment was performed on a newly built, fully computer-controlled backscattering incident circular polarization ROA instrument. High-quality spectra were obtained and interpreted on the basis of ab initio quantum chemical calculations. Excellent agreement between the calculated and observed spectra was observed in the mid IR region. An approximate distributed origin gauge model for calculation of the polarization tensors was proposed. The shapes of ROA spectra calculated with this model are similar to those obtained with conventional, computationally more demanding ROA computations.

Comparison of the Brillouin-Wigner Coupled Cluster Theory with the Standard Coupled Cluster Theory. Cancellation of Disconnected Terms in the Brillouin-Wigner Coupled Cluster Theory

Abstract: In our article (Hubac I., Neogrady P.: Phys. Rev. A 50 , 4558 (1994)) we have developed size-extensive Brillouin-Wigner coupled cluster (BWCC) theory. We have shown that the BWCC theory is fully equivalent to the standard coupled cluster theory. However, the BWCC theory does not obey the so-called linked cluster theorem. In this article we discuss the cancellation of disconnected terms in the BWCC theory.

Synthesis of N - and B -Substituted Derivatives of closo -Amino-undecahydro-dodecaborate(1-) Anion

Abstract: The synthesis of nitrogen and boron substituted derivatives of the 1-amino-closo-dodecaborate anion(1-) 1 is reported. Reasonable yields of the [R2NH-B12H11]- derivatives (R = C6H5CH2, 2-C10H7CH2, n-C16H33, n-C12H25) were obtained via conventional alkylation of 1 in aqueous propan-2-ol, starting from bulky primary alkylhalides. These [R2NH-B12H11]- derivatives were subsequently methylated by dimethyl sulfate under similar conditions. Reaction of 1 with palmitoyl chloride gave under anhydrous conditions the corresponding N-acyl derivative. Reaction of 1 with hydroxymethyl-18-crown-6 tosylate in THF in the presence of NaH led to the novel [(18-crown-6-CH2)2NH-B12H11]- anion, the Cs+ salt of which exhibits unusual solubility properties. A direct cyclization reaction of pentaethylene glycol ditosylate with 1 gave under similar conditions [(15-azacrown)-5-B12H11]-, the first known closo-borate anion with an attached aza-crown ring. These species exhibit potentially interesting complexation efficiency and solubility properties. Selective substitution of the boron cage by a bulky naphthyl substituent was achieved by palladium-mediated cross-coupling reaction between 1 and 1-BrMgC10H7. All derivatives were characterized by high-field 11B, 1H NMR and negative FAB mass spectrometry methods.

Synthesis of Triferrocenylbenzenes by Tantalum(V)-Catalyzed Cyclotrimerization of Ethynylferrocene. The Crystal Structure of 1,3,5-Triferrocenylbenzene

Abstract: Exclusive, high-yield formation of the cyclotrimers of ethynylferrocene ( 1 ) was observed when TaCl 5 in benzene was employed as catalyst. Analysis of the resulting isomer mixture by means of HPLC and 1 H NMR revealed the presence of 1,3,5-triferrocenylbenzene ( sym -2 ) and 1,2,4-triferrocenylbenzene ( asym -2 ) in the ratio of 4 : 6. A small amount of pure sym -2 isomer was isolated, and its solid-state structure was established by single-crystal X-ray diffraction. The molecules of sym -2 possess a distorted up-up-down arrangement of the ferrocenyl groups attached to the benzene ring.

Template-directed syntheses of catenanes

Abstract: The art and science of introducing mechanical-interlocking at the molecular level in order to generate catenanes - molecules composed of two or more macrocyclic components - offers the opportunity of constructing a new range of molecular compounds possessing intriguing properties. However, the topological features displayed by catenanes has rendered the syntheses of such molecular compounds an extremely challenging task for synthetic chemists to address. Their early syntheses were based upon either statistical approaches - the threading of a small amount of a macrocycle on to an acyclic precursor as a chance event - or directed approaches, relying upon the temporary introduction of covalent bonds in the multistep synthesis of a so-called precatenane, followed by its conversion ultimately into a catenane. These approaches afforded catenanes in very low yields overall and only after following tedious and laborious synthetic procedures. Fortunately, however, with the advent of supramolecular chemistry, template-directed methods that allow us to self-assemble [n]catenanes much more efficiently have become available. Numerous successful template-directed syntheses have now emerged - some by chance and others by design. These methods have been based upon (i) metal coordinating, (ii) hydrogen bonding, (iii) solvophobic, and/or (iv) π-π stacking interactions which have been found to govern self-assembly processes to catenated compounds from appropriate precursors. Their relative simplicity, the high degree of control with which they can be employed, and the remarkable efficiency with which they proceed has already provided the opportunity to synthetic chemists to self-assemble a series of [n]catenanes, incorporating from two up to five mechanically-interlocked macrocyclic components.

Synthesis of Acyclic Nucleotide Analogues Derived from 6-Hetarylpurines via Cross-Coupling Reactions of 9-[2-(Diethoxyphosphonylmethoxy)ethyl]-6-iodopurine with Hetaryl Organometallic Reagents

Abstract: The title acyclic nucleotide analogues derived from 6-hetarylpurines were prepared by Pd(0)-catalysed cross-coupling reactions of 9-[2-(diethoxyphosphonylmethoxy)ethyl]-6-iodopurine ( 1 ) with hetarylorganometallics: (pyridin-2-yl)-, (imidazol-2-yl)- and (pyrrol-2-yl)zinc chlorides or (imidazol-5-yl)- stannanes, followed by deprotection in fair to good yields. The starting 6-iodopurine derivative 1 was prepared by iododeamination of the adenine derivative.

Microwave Activation of Homogeneous and Heterogeneous Catalytic Reactions

Abstract: Microwave heating was applied in homogeneous and in heterogeneous reactions and the results were compared from the point of view of activation of chemical reactions. Reactions including the addition of halo compounds to alkenes catalyzed by copper and ruthenium complexes in different solvents and NaY zeolite catalyzed alkylation of secondary amine in the absence of solvent were studied as model reactions to compare possibilities of microwave activation of reactants and catalysts. Rate enhancement of over one order of magnitude in homogeneous reactions was caused mainly by thermal dielectric heating effect which resulted from the effective coupling of microwaves to polar solvents. Activation of reactants and catalysts was very low if any. In heterogeneously catalyzed alkylation reactions highly efficient activation of zeolite catalyst was recorded. The results indicated that the best reaction conditions were in experiments when both activation of catalyst and performance of reaction were carried out under microwave conditions. Rate enhancement was most probably caused by "hot spots" or by "selective heating" of active sites. In both homogeneous and heterogeneous reactions non-thermal activation (specific effect) was excluded.

Thieno[3,2- b ]benzofuran - Synthesis and Reactions

Abstract: Thieno[3,2-b]benzofuran was synthesized starting from benzo[b]furan-3(2H)-one or benzo[b]furan-2-carbaldehyde. Electrophilic substitution reactions such as bromination, formylation, acetylation or nitration, take place in position 2. An electron donating group in position 2 directs further electrophilic substitution into positions 3 and 6, whereas compounds with an electron acceptor in position 2 are substituted exclusively in position 6. Metallation with butyllithium took place in position 2. The 1H and 13C NMR signals of the title compound were fully assigned.

Washing of Pulp Fibre Bed

Abstract: The mechanism of the displacement washing of unbleached kraft pulp was investigated. The flow of the wash liquid through a pulp bed was described by the dispersion model using one dimensionless parameter known as the Peclet number. To characterize the displacement washing, the wash yield, dispersion coefficient, and bed efficiency, as well as time parameters such as the mean residence time of lignin, and space time were evaluated. Results obtained for kraft pulp were compared with those for static bed of glass spheres.

1-Carba-arachno-pentaborane(10) Derivatives

Abstract: The formation of organo substituted 1-carba- arachno -pentaborane(10) derivatives is shown to proceed in high yield via in situ generated 1,1,1-tris(diethylboryl)propane ( 2 ) from diethyl(propyn-1-yl)borane ( 1 ) by hydroboration with an excess of diethylborane (hydride bath). In the hydride bath, exchange reactions between 2 and other geminal bis(diethylboryl)alkanes take place until the carbaborane skeleton is formed. If tris(diethylboryl)methane is used under the same conditions, the corresponding 1-carba- arachno -pentaborane(10) derivatives 11 and 12 are formed in mixture with other unknown boranes or carboranes. 11 B and 13 C NMR data are presented to allow for straightforward identification of the 1-carba- arachno -pentaboranes(10).

ansa-vanadocene complexes : syntheses, structures and ligand exchange reactions

Abstract: Tetramethylethano-bridged vanadocene dichlorides (CH 3 ) 4 C 2 (3-R-C 5 H 3 ) 2 VCl 2 with R = H, Me, t -Bu are obtained by reacting the corresponding ligand MgCl salts with vanadium(III) acetylacetonato compounds, followed by oxidation with PCl 3 or AgCl. Smooth ligand exchange reactions of these ansa -vanadocene dichlorides afford acetylacetonato cations and binaphtholate complexes; from these, the dichlorides are regenerated by exposure to MgCl 2 in THF solution. Separation of meso and rac isomers (as well as enantiomer separation) is achieved via corresponding binaphtholate complexes; conversion to the vanadium(III) diisonitrile cations (CH 3 ) 4 C 2 (3-R-C 5 H 3 ) 2 V(CN t -Bu) 2 + allows an assessment of diastereoisomer ratios by 1 H NMR. Lewis-acidic bistriflate derivatives of these ansa -vanadocene complexes are obtained from the dichlorides by reaction with silver triflate.

Glycosylation of Triterpene Alcohols and Acids of the Lupane and A-Secolupane Series

Abstract: A series of 3- and 28-glucosides and glucosyl esters of betulinic acid ( 1a ), 28-hydroxy-3,4-secolupa-4(23),20(29)-dien-3-oic acid ( 22a ), dimethyl ester of 28-hydroxy-2,3-secolup-20(29)-en-2,3-dioic acid ( 43a ), their 20(29)-dihydro derivatives ( 1b , 22b , 43b ) and several other triterpenes of the lupane ( 12a , 12b ) and 3,4-secolupane series ( 18a , 18b , 32a ) has been prepared by reaction of tetra- O -acetyl-α-D-glucopyranosyl bromide in acetonitrile in the presence of mercury(II) cyanide and subsequent deacetylation of the obtained tetra- O -acetyl-β-D-glucopyranosyl derivatives. In several cases attempted glucosylation in the presence of silver silicate afforded predominantly the corresponding 1,2-orthoacetates of α-D-glucopyranose.

Cobaltacarboranes with Intramolecular Monophosphorus Bridges 8,8'-μ-Me2P(1,2-C2B9H10)2-3-Co, 6,6'-μ-Me2P(1,7-C2B9H10)2-2-Co and the Respective Non-Bridged Trimethylphosphine Derivatives (8-Me3P-1,2-C2B9H10)-3-Co-(1,2-C2B9H11) and (6-Me3P-1,7-C2B9H10)-2-Co-(1,7-C2B9H11)

Abstract: The first two cobaltacarboranes with intramolecular phoshorus bridges and two related non-bridged trimethylphosphine-cobaltacarborane derivatives are described and characterized by mass spectrometry and multinuclear NMR; all four compounds are zwitterions: 8,8'-μ-Me2P(1,2-C2B9H10)2-3-Co (1a), 6,6'-μ-Me2P(1,7-C2B9H10)2-2-Co (2a), (8-Me3P-1,2-C2B9H10)-3-Co-(1,2-C2B9H11) (1b) and (6-Me3P-1,7-C2B9H10)-2-Co-(1,7-C2B9H11) (2b). X-Ray structures of the first two and of the last compound has been established.

Glucose Hydrogenation in a Trickle-Bed Reactor

Abstract: Catalytic hydrogenation of 40% aqueous solutions of D-glucose to D-glucitol was studied in a high-pressure trickle-bed reactor. The reactions were performed on a supported nickel catalyst at temperatures ranging from 115 to 165 °C and in the pressure range 0.5 to 10 MPa. The order of the reaction with respect to hydrogen is 0.65 and apparent activation energy 23.8-48.5 kJ mol-1, the latter depending on initial molar glucose concentration and density and viscosity of the solution. The influence of external diffusion is necessary to take into account for scaling-up the process.

Synthesis and Characterization of Palladium Complexes Containing 3,3'-Annelated 2,2'-Bipyridines and 3,3'-Annelated 2,2'-Biquinolines

Abstract: The synthesis and characterization of a number of PdCl 2 (N⁀N) and Pd(N⁀N)(fn) complexes (N⁀N = 3,3'-annelated 2,2'-bipyridines and 3,3'-annelated 2,2'-biquinolines, fn = fumaronitrile) have been described. Despite the variation in dihedral angle and distance between the nitrogen lone pairs in these series of ligands, they are all capable of forming stable chelates. An investigation was made as to whether the 3,3'-trimethylene-2,2'-biquinoline (3-bq) and 3,3'-tetramethylene-2,2'-biquinoline (4-bq) ligands could serve as C 2 -chiral auxiliaries. The ligand 3-bq in Pd(3-bq)(fn) turned out to be conformationally rigid on the NMR time scale, but the diastereomers appeared to be indistinguishable, as could be concluded from the 1 H NMR spectra. However, Pd(4-bq)(fn) exists in two diastereomeric forms and their ratio (57 : 43) could be determined from 1 H NMR spectra. Assignment of the major and minor isomers was done with a 2D NOESY experiment. Epimerization of the diastereomers occurred on the laboratory time scale which was deduced from a spin saturation transfer experiment and also from the observation that diastereomerically enriched crystalline material of Pd(4-bq)(fn) at low temperature showed a ratio of 80 : 20 which deteriorated to a ratio of 57 : 43. From the spin saturation transfer experiment, the thermodynamic parameters for the inversion could be determined (∆H ≠ = 29.1 kJ mol -1 and ∆S ≠ = -136.3 J K -1 mol -1 ). This large negative entropy of activation favours an epimerization process via inversion of the coordinated 4-bq ligand over a mechanism via alkene dissociation/recoordination. The structures of the complexes have been studied in solution and in the solid state and the following X-ray crystal structures have been determined: (i) [dichloropalladium(II)](3,3'-trimethylene-2,2'-bipyridine-κ 2 N ), monoclinic crystals, space group P2 1 / n , with a = 8.4230(7) A, b = 16.425(2) A, c = 9.5836(7) A, β = 106.316(7)°, Z = 4, final R = 0.033 for 2 309 reflections, (ii) [3,3'-bis(methoxycarbonyl)-2,2'-bipyridine-κ 2 N ][dichloropalladium(II)], triclinic crystals, space group P1 , with a = 7.340(4) A, b = 10.012(3) A, c = 11.954(3) A, α = 98.10(2)°, β = 104.03(3)°, γ = 107.33(5)°, Z = 2, final R = 0.039 for 4 273 reflections, (iii) [chloromethylpalladium(II)](4,5-diazafluorene-κ 2 N ), orthorhombic crystals, space group Pmcn , with a = 6.713(1) A, b = 10.120(1) A, c = 17.187(3) A, Z = 4, final R = 0.048 for 949 reflections.

A return to the plesek reaction and some useful variations. carbon-substituted methyl and phenyl derivatives of 5,6-dicarba-nido-decaborane(12), nido- 5,6-c2b8h12

Abstract: A modification of the Plesek reaction has been employed to improve the yield in the synthesis of the parent nido ten-vertex dicarbaborane 5,6-C2B8H12, and to synthesize some of its 5- and 6-substituted derivatives. The method is based on the oxidation of the anions [7,8-R1R2-nido-5,6-C2B9H10]- with acidic aqueous of [Fe(H2O)6]Cl3 to produce the series of substituted dicarbaboranes 5,6-R1R2-nido-5,6-C2B8H10, where R1, R2 = H, H; H, Me; Me, H; Me, Me; H, Ph; Ph, H, and Ph, Ph. The characteristic reaction mode is an oxidative elimination of one boron vertex that is adjacent to the carbon vertex in the open pentagonal face of the starting eleven-vertex nido dicarbaborane anion. The products are characterized by mass spectrometry and 1H and 11B NMR spectroscopy including [11B-11B]-COSY and 1H-{11B(selective)} measurements that permit complete assignments of all the observed resonances to individual cluster {BH} units.

Deactivation of Polymer-Supported Palladium Catalystsin the Hydrogenation of 4-Nitrotoluene

Abstract: The deactivation of palladium catalysts supported on commercially available sulfonated polystyrene-divinylbenzene resins (DOWEX) in the hydrogenation of 4-nitrotoluene was studied Catalysts were prepared from 4 and 8 mole % crosslinked polymers, with particle size between 004 and 080 mm and palladium contents ranging from 025 to 1 wt% (dry state) The catalytic tests were carried out in a batch reactor vigorously shaken, at 30-50 °C, and 025 and 05 MPa The analysis of the conversion curves showed that three factors are mainly responsible for the relatively high deactivation rates: (i) the concentration of hydrogen in the neighbourhood of palladium crystallites, (ii) temperature, (iii) the nanomorphology of the support Some practical advice concerning the employment of polymeric hydrogenation catalysts and relevant operational conditions can be achieved from the results presented herein

Some Reactions of the Products of Reactions of 1,4-Bis(trimethylsilyl)-1,3-butadiyne with Titanoceneand Zirconocene

Abstract: Bis(η 5 -cyclopentadienyl)-2,4-bis(trimethylsilylethynyl)-3,5-bis(trimethylsilyl)titanacyclopenta-2,4-diene ( 1 ) and the seven-membered zirconacyclocumulene bis(η 5 -cyclopentadienyl)-2,4,7-tris(trimethylsilyl)-3-(trimethylsilylethynyl)zirconacyclohepta-2,4,5,6-tetraene ( 2 ) react with sulfur monochloride to yield an identical product, 2,4-bis(trimethylsilylethynyl)-3,5-bis(trimethylsilyl)thiophene ( 3 ). A one-to-one mixture of the homo- and heterobimetallic bis-σ,π-acetylide bridged complexes Cp 2 Ti(μ-η 1 :η 2 -CCSiMe 3 )M(μ-eta 1 :η 2 -CCSiMe 3 )Cp 2 (M = Zr ( 4 ), M = Ti ( 5 )) is formed in the reaction of zirconacyclocumulene 2 with three equivalents of Cp 2 Ti(η 2 -Me 3 SiCCSiMe 3 ). The reaction proceeds most likely via alkyne elimination, a C-C-single bond activation, and the cleavage of the starting diyne takes place.

Nickel(0) and Palladium(0) Complexes with 1,3,5-Triaza-7-phosphaadamantane. Catalysis of Buta-1,3-diene Oligomerization or Telomerization in an AqueousBiphasic System

Abstract: New homoleptic nickel(0) and palladium(0) complexes with a water-soluble ligand, 1,3,5-triaza-7-phosphaadamantane, were prepared and characterized by 1H, 13C, and 31P NMR spectra. The complexes, together with the known analogous Ni(0) and Pd(0) complexes with tris(hydroxymethyl)phosphine, were found to be catalysts for buta-1,3-diene oligomerization or telomerization with water in an aqueous biphasic system without a cosolvent or a modifier. Tetrakis[tris(hydroxymethyl)phosphine]nickel (7) preferentially catalyzes oligomerization (both linear and cyclic) in the first example of a nickel-catalyzed buta-1,3-diene oligomerization in an aqueous biphasic system. Palladium complexes give telomers or linear oligomers in quantitative yields. In the case of the triazaphosphaadamantane complex 4, high selectivity to octadienyl ethers (87%) was observed. High values of metal leaching into the product phase in these reactions suggest an easy extraction of starting or intermediate metal complexes caused by the fact that both monomer and products are good ligands for the metal complexes in this particular case.

Production and Properties of Polypropylene Track Membranes

Abstract: This paper describes results of the method of manufacturing the polypropylene track membranes produced by physicochemical treatment under irradiation of accelerated heavy ions of polymer films. The developed method allows to produce membranes with pores of 0.1-0.2 μm in diameter and more, with an improved structural and physicochemical properties. Polypropylene track membranes of a novel sample are characterized by high homogeneity of pore sizes in magnitude, considerable mechanical strength, advanced thermal stability and resistance to oxidation in aggressive media. It opens new fields for their usage.

Adsorption and diffusion in hzsm-5 zeolite studied by an oscillating microbalance

Abstract: A novel microbalance technique has been used to study diffusion and adsorption in a commercial HZSM-5 zeolite. This new technique uses an inertial microbalance TEOM (Tapered Element Oscillating Microbalance) to measure mass changes in the zeolite bed. Time resolution as short as 0.1 s, a flow-through design where all the probe molecules see the zeolite bed and high sensitivity allowing zeolite loadings down to a few milligrams are the three most important properties of the TEOM. The probe molecules studied were o-xylene, p-xylene and toluene which were introduced at 303, 373 or 473 K and at partial pressures in the range of 0.2-10 kPa. The inverse characteristic uptakes (D/L2), corrected (D0/L2) and steady-state (Dss/L2) diffusion times are reported. The thermodynamic correction used for D0/L2 calculations almost eliminated the concentration dependence of the diffusivities. The Dss/L2 values were found to be rather unaffected by both temperature (373-473 K) and concentration, suggesting a certain degree of unification for diffusivities. o-Xylene uptake rates in the TEOM were found to be significantly higher than in a gravimetric microbalance under identical conditions, probably as a result of additional mass transfer resistance other than intracrystalline diffusion caused by poor contact between the gas phase and the zeolite in a conventional gravimetric microbalance.

Transport of adefovir (pmea) in human t-lymphoblastoid cells

Abstract: The uptake of [ 3 H]PMEA by human T-lymphoblastoid cells (CCRF-CEM) apparently proceeds by fluid-phase endocytosis. The transport kinetics was shown to be nonconcentrative and nonsaturable. The uptake takes place even at a low temperature (4 °C), is strictly dependent on the intracellular level of ATP, is not substantially affected by cell suspension density and is not competitively inhibited by other PME derivatives.

Proton-Sponge-Initiated Reactions of 6,8-Dicarba-arachno-nonaborane(13) and 6,7-Dicarba-arachno-nonaborane(13) with Methyl Propynoate: One-Step Syntheses of the 6-(MeOOCCH2)-5,6,7-Tricarba-arachno-decaborane(12) and 6-(MeOOCCH2)-5,6,10-Tricarba-nido-decaborane(10) Tricarbaboranes

Abstract: The reaction of arachno-6,8-C2B7H13 (1) with methyl propynoate in the presence of only 0.1 equivalent of 1,8-bis(dimethylamino)naphthalene (Proton Sponge®, PS) results in the one-step formation of the neutral tricarbaborane 6-(MeOOCCH2)-arachno-5,6,7-C3B7H12 (2). Similarly, only 0.1 equivalent of PS initiates the reaction of the isomeric arachno-6,7-C2B7H13 (3) carborane with methyl propynoate to yield the new tricarbaborane, 6-(MeOOCCH2)-nido-5,6,10-C3B7H10 (4). These results suggest a cyclic reaction sequence involving initial deprotonation of the starting dicarbaborane by PS, followed by reaction of the resulting anion with methyl propynoate to produce a new anion. Protonation of this second anion by another equivalent of the starting dicarbaborane then produces the final neutral tricarbaborane product and the cycle begins again. While 4 is unreactive toward NaH or KH, it readily added n-BuLi to give, following acidification with HCl-Et2O, a new product tentatively identified as 6-(MeOOCCH2)-Bu-arachno-5,6,10-C3B7H11 (6). The proposed structures of 4 and 6 are strongly supported by ab initio/IGLO nuclear shielding calculations.

Crystal Structures of Titanium(III) Bis(acetylide) Tweezer Complexes with Alkali Metal Cations

Abstract: Crystal structures of the Ti(III) tweezer complexes [(C 5 HMe 4 ) 2 Ti(σ-C≡CSiMe 3 ) 2 ] - [Li(THF) 2 ] + ( 2a ), [(C 5 HMe 4 ) 2 Ti(σ-C≡CSiMe 3 ) 2 ] - Na + ( 3 ) and [(C 5 HMe 4 ) 2 Ti(σ-C≡CSiMe 3 ) 2 ] - Cs + ( 5 ) have been determined. In all of them the alkali metal cation is placed away from the Ti-C α1 -C α2 plane at the distance: Li + 0.511 A, Na + 1.023 A and Cs + 0.521 A. The reason for the deviation of Li + in 2a is the asymmetrical orientation of the THF ligands in the [Li(THF) 2 ] + cation with respect to the Ti-C α1 -C α2 plane, which seems to release the steric congestion between the THF ligands and the trimethylsilyl groups. In 3 and 5 , the molecules form polymer chains with a weak intermolecular bonding interaction between the cations and one of the C 5 HMe 4 ligands of the neighbouring molecule in a sandwich manner.

Chemometric Analysis of Substituent Effects. XI. Solvent Effects on Dissociation of 2,6-Disubstituted Benzoic Acids

Abstract: Eleven symmetrically 2,6-disubstituted benzoic acids (with the following substituents: OCH 3 , OC 2 H 5 , OC 3 H 7 , OCH(CH 3 ) 2 , OC 4 H 9 , CH 3 , F, Cl, Br, I, and NO 2 ) have been synthesized and their dissociation constants measured potentiometrically in methanol, ethanol, propan-1-ol, propan-2-ol, butan-2-ol, acetone, dimethyl sulfoxide, dimethylformamide, acetonitrile, pyridine, and 1,2-dichloroethane. The experimental data obtained have been analyzed from the point of view of solvent effects on acidity of the individual derivatives. Different behaviour found with benzoic acid and the disubstituted derivatives in protic solvents is due to changes in solvation. The different character of solvation of benzoic acid and the disubstituted derivatives depends on the type of substitution, being manifested only in 2,6-disubstituted benzoic acids. The graphical analysis has shown a distinct trend in the increase of magnitude of deviation of the point of benzoic acid in the series: propan-2-ol, butan-2-ol, propan-1-ol, ethanol, methanol. This order correlates with the steric demands of carbon chain of the alcohols used. The abnormal behaviour of benzoic acid in the dissociation in these alcohols as compared with that of its 2,6-disubstituted derivatives is due to the different extent of solvation of the reaction centre caused by steric hindrance. Against the expectation, benzoic acid appears to be a weaker acid in protic solvents, whereas its alkoxy derivatives are stronger acids. The solvation also minimizes the inductive effect of alkoxy groups in the symmetrically 2,6-disubstituted derivatives. In aprotic solvents the acidity of 2,6-dialkoxybenzoic acids is also increased, in this case as a result of sterically forced deviation of the reaction centre and/or the substituents out of the plane of benzene ring.

Pyrolysis of Hexadecane

Abstract: The kinetics of thermal decomposition of hexadecane was studied in a flow tubular reactor from stainless steel. The experiments were performed in the temperature range of 700 to 780 °C for the mass ratio of steam to hydrocarbon 3 : 1. The hexadecane pyrolysis took place according to the first-order reaction with a frequency factor of 3.5 . 10 9 s -1 and an activation energy of 162 kJ mol -1 . In the pyrolysis products there were above all 1-alkenes. From alkanes, methane and ethane and less propane were formed in a higher degree. The prevailing compounds are ethene and propene whose amount increases with increasing temperature and residence time. The content of 1-alkenes higher than 1-pentene decreases with increasing conversion which gives evidence of their decomposition owing to their lower stability in comparison with the lighter 1-alkenes. The formation of dienes (1,3-butadiene and propadiene) and benzene also confirmed the course of secondary reactions. The observed higher formation of hydrogen results from the reaction of steam with coke deposited on the walls of the reactor or with hydrocarbon radicals. The evidence of the coke formation is given also by the presence of carbon oxides whose amount grew with the pyrolysis severity. The high content of 1-hexene in comparison with the other higher 1-alkenes is probably caused by the isomerization of alkyl radicals by 1,5-hydrogen transfer.

An Effective Synthesis of 1,3-Oxathiolanes

Abstract: 2-Alkyl- or 2,2-dialkyl-1,3-oxathiolanes can be effectively prepared from aldehydes or ketones and 2-mercaptoethanol, with triisopropylsilyl triflate as a catalyst. The reaction is over within minutes and, despite the fact that water is nor removed during the reaction, the yields of products are high.

Phenyl-Induced 3,1,11-Rearrangement in the Synthesis of Mixed Pyrrolyl/Dicarbollide Cobaltadicarbaboranes

Abstract: Mixed pyrrolide cobaltadicarbollides have been prepared from the reaction involving potassium pyrrolide, anhydrous CoCl 2 and the [7,8-(C 6 H 5 ) 2 - nido -7,8-C 2 B 9 H 10 ] - anion in dimethoxyethane. An alternative and higher-yield route consists in the reaction between 1,2-(C 6 H 5 ) 2 - closo -1,2-C 2 B 10 H 10 and potassium pyrrolide in the presence of anhydrous CoCl 2 . As confirmed by the X-ray diffraction analysis of [3-(η 5 -NC 4 H 4 )-1,11-(C 6 H 5 ) 2 - closo -3,1,11-CoC 2 B 9 H 9 ], the phenyl rings were found to enhance the rearrangement of the metallacarborane cluster carbons to produce the 3,1,11-isomer instead of the expected 3,1,2-derivative. As suggested by 1 H and 13 C NMR spectroscopy, the electron-withdrawing properties and steric requirements of the phenyl rings facilitate the rearrangement.