Degree Of Alkyl Substitution Research Articles

Comparison of phytoremediation, bioaugmentation and natural attenuation for remediating saline soil contaminated by heavy crude oil

Bench-scale pot culture systems were used to investigate the effectiveness of phytoremediation (Testuca arundinacea), bioaugmentation (5 oil-degrading strains), and natural attenuation for remediating saline soil contaminated by Venezuela heavy crude oil. GC–MS was used to have a comparative characterization for chemical composition changes of aromatic groups (polycyclic aromatic hydrocarbons (PAHs), sulfur-containing heterocycles (SCHs) or polycyclic aromatic sulfur heterocycles (PASHs), and aromatic biomarkers) during different remediation processes. Bioaugmentation had faster startup (higher total petroleum hydrocarbon (TPH) removal efficiency on Day 30) while phytoremediation had higher TPH removal efficiency at the end of experiments (90days). The 90-day TPH removal efficiency of phytoremediation (64.0±1.6%) was significantly higher (p<0.05) than that of bioaugmentation (54.6±1.3%) which was significantly higher (p<0.05) than that of natural attenuation (20.7±2.8%). GC–MS analysis shows that the removal efficiencies of most of individual PAH and SCH compounds by phytoremediation were also significantly higher (p<0.05) than bioaugmentation which were significantly higher (p<0.05) than natural attenuation. GC–MS analysis also shows that the removal efficiencies of individual PAH and SCH compounds for all three treatments (phytoremediation, bioaugmentation, and natural attenuation) decreased with increases in ring number and degree of alkyl substitution. Overall, this study shows that phytoremediation with T. arundinacea and bioaugmentation with a halotolerant microbial consortium are two effective approaches for remediating saline soil contaminated by heavy crude oil.

Evaluation of dibutyrylchitin as new excipient for sustained drug release

Dibutyrylchitin (DBC), a lipophilic chitin diester, has been synthesized from chitin and butyric anhydride with methanesulfonic acid as catalyst. Exhaustive esterification of free alcoholic groups of chitin was assessed by FT-IR and 1H-NMR spectroscopy. High degree of alkyl substitution allowed DBC to acquire an almost completely lipophilic character. Tablets of paracetamol and metformin employing DBC as major excipient, in comparison with starch, microcrystalline cellulose, lactose and polyvinylpyrrolidone, were prepared and rates of drug release were checked by dissolution test assays. DBC released drug at a lower rate than that of the other tested materials. A comparison study of rate release of metformin from DBC tablets and from metformin-hydroxypropyl methylcellulose prolonged release oral formulation available on the market has been also curried out. Under the same conditions and in the presence of the same amount of loaded drug, DBC released 64% of metformin whereas hypromellose-based tablets released 87%.

Addressing fluorescence and liquid crystal behaviour in multi-mesogenic BODIPY materials

Two series of BODIPYs (4,4′-difluoro-3a,4a-diaza-s-indacene) bearing mesogenic substituents have been synthesised and their fluorescent and self-assembly behaviour has been observed and characterised. In order to induce a strong preference for liquid crystallinity, two mesogenic units have been attached to each fluorophore. The compounds were prepared by microwave-assisted Suzuki couplings in moderate yields. Two different, but related, mesogens were employed but only one series was found to exhibit the formation of liquid crystal mesophases. Monotropic nematic phases, observed by differential scanning calorimetry (DSC) and optical polarising microscopy (OPM), were adopted over varying temperature ranges depending on the degree of alkyl substitution on the fluorophore. Increasing alkyl substitution caused a gradual increase in fluorescence quantum yield due to rotational restriction of the BODIPY 8-phenyl ring, however, this also disrupted the formation of the nematic phase causing a reduction in the temperature range over which the mesophase was exhibited. The large, branched structure of the compounds prepared caused them to have a high viscosity in both the nematic and isotropic liquid phase. Due to the high viscosity, the Schlieren texture of the nematic phase could only be observed after several hours annealing.

ChemInform Abstract: Further Study on the Regioselectivity of (Dimethylvinylidene)carbene upon the Cycloaddition to the α,β-Unsaturated Double Bond of an Allylic Alcohol and the Normal Olefinic Group.

Abstract Further study on the regioselective addition of vinylidenecarbene 1 to the α,β-unsaturated double bond of allylic alcohol and the normal olefinic group is described. This selectivity was found to be influenced by both of the allylic hydroxy group and the degree of alkyl substitution of the double bonds.

Contrasteric Regiocontrol in Rhodium-Catalyzed Hydrogenative Couplings of Nonsymmetric 1,3-Diynes to Ethyl Glyoxalate

To probe the origins of regioselection in rhodium-catalyzed hydrogenative couplings of 1,3-diynes to ethyl glyoxalate, a series of structurally related nonsymmetric 1,3-diynes, 1a−14a, were prepared and surveyed. Nonsymmetric 1,3-diynes 1a−8a possess different degrees of alkyl substitution at the propargylic carbons. Unusual contrasteric regioselectivity is observed, often with a complete preference for coupling at the diyne terminus proximal to the more substituted propargylic center. Couplings to diynes 9a−14a are meant to probe the effects of heteroatom substitution at the propargylic position vis-à-vis partitioning of regioisomeric coupling manifolds. Heteroatom substitution exerts only a modest influence on the regiochemistry of coupling, except in the case of (trimethylsilyl)methyl-substituted diynes 13a and 14a. The observed trends suggest that substituents at the diyne termini compete for stabilization of intermediate regioisomeric rhodium−alkyne π-complexes via hyperconjugative interactions, with regio-determining oxarhodacycle formation occurring from the more stable π-complex.

Conjugated Triblock Copolymers Containing Both Electron-Donor and Electron-Acceptor Blocks

We report a straightforward two-step synthesis toward conjugated DAD-type triblock copolymers with both electron-donor (D) and electron-acceptor (A) blocks. Cyano-substituted poly(phenylenevinylene) (CN−PPV) prepolymers with two bromide end groups hereby act as the central electron-acceptor building block and are generated under Yamamoto conditions. The dibromo prepolymers are finally coupled with monobromo-terminated regioregular poly(3-hexylthiophene) (P3HT), which is selected as the electron-donor block (D). The molecular weight of the different blocks, especially the acceptor block, can be controlled by adjusting the polymerization time and the substitution pattern (degree of alkyl substitution) of the monomers. Elemental analysis, NMR, DSC, and optical spectroscopy prove that the DAD conjugated triblock copolymers. Atomic force microscopy (AFM) images of one triblock copolymer (P3HT1−HCNPPV−P3HT1) exhibit the formation of regular nanosized mesostructures in thin films. The resulting conjugated triblock copolymer shows a distinctly different morphology as compared to a corresponding polymer blend. A covalent connection of donor (P3HT) and acceptor (CN−PPV) blocks is a favorable way to control the scale length of nanostructure formation.

Relation between Feed Quality and Coke Formation in a Three-Stage Atmospheric Residue Desulfurization (ARDS) Process

In this study, the effect of three different feedstocksnamely, straight-run atmospheric residue (AR), demetallized AR (DM-AR), and demetallized/desulfurized AR (DMDS-AR)on the coke formation on an industrial catalyst system consisting of a hydrodemetallization (HDM) catalyst (A), a hydrodemetallization/hydrodesulfurization (HDM/HDS) catalyst (B), and a hydrodesulfurization/hydrodenitrogenation (HDS/HDN) catalyst (C) were examined. The used catalysts, aged under the same operating conditions (catalyst A with atmospheric residue (AR), B with AR or demetallized atmospheric residue (DM-AR), C with AR or demetallized and desulfurized atmospheric residue (DMDS-AR)), were characterized by combining physical/chemical analyses with temperature-programmed oxidation/mass spectroscopy (TPO/MS) and solid-state 13C nuclear magnetic resonance (NMR). Special emphasis was made on finding a relationship between feed quality and the amount and nature of coke. The results revealed that hydrotreated feeds generate significantly more coke than straight-run AR. The higher propensity in coke formation using a demetallized feedstock is due to the bulky aromatic structure of the asphaltenes in DM-AR, along with a high H/C ratio of the feed (phase separation−precipitation−coke formation). In the case of DMDS-AR, coke generation is attributed to the nature of catalyst C (namely, high acidity and small pores), rather than to the quality of the feed. Concerning the coke properties, it was found that the coke generated from hydrotreated feeds is less aromatic than that from AR. Regarding other coke features, such as degree of condensation or degree of alkyl substitution, no obvious interplay between these structural parameters and the nature of feed or catalyst could be found.

The Characterization of Basic Petroleum Extracts by High-Resolution Mass Spectrometry and Simultaneous Orthogonal Acceleration Time-of-Flight−Magnet Scanning Tandem Mass Spectrometry

A hybrid double-focusing high-resolution magnetic sector−orthogonal acceleration time-of-flight (oa-TOF) tandem mass spectrometer has been used for the characterization of the basic compounds in a whole crude oil and its 400−414 °C distillation cut. Detailed compositional characterization is obtained from high-resolution experiments. Structural information is obtained from automated magnet scanning mass spectroscopy (MS/MS) experiments that are conducted by continuous oa-TOF data acquisition and simultaneous scanning of the magnet. Solid-phase weak cation exchange is used to remove the basic compounds from the hydrocarbon matrix. Samples are introduced into the mass spectrometer via a dynamic batch inlet system. Molecular ions are produced by charge exchange chemical ionization. The most-abundant compounds in the two basic extracts are due to the CnH2n+zN and CnH2n+zNS compound classes. Other compound classes of lower concentration are also detected (−NO, −SON, −NO2, −O, −SO, −O2, −SO2). The contour plot of the MS/MS data for the 400−414 °C extract has revealed two characteristic peak patterns: (i) a series of peaks parallel to the precursor ion series, containing information about the degree of alkyl substitution, and (ii) a series of peaks parallel to the MS-1 axis containing information about the aromatic nuclei. Despite the complexity of the data, a substantial structural similarity between the various basic nitrogen compounds is indicated by the two peak patterns. The important capability of the simultaneous MS/MS method to provide structural information about isobaric peak components is demonstrated. The chemical formulas of selected isobaric precursor ions are validated, and structures consistent with their product ion spectra are proposed.

Kinetics of reactions of chlorine atoms with a series of alkenes at 1 atm and 298 K: structure and reactivity

Chlorine atoms are important oxidants at dawn in the marine boundary layer where a variety of organics are also present, including alkenes. Using the relative rate technique, the kinetics of the gas phase reactions of atomic chlorine with a series of alkenes, relative to n-heptane as a reference, have been investigated at (298 ± 3) K and 1 atmosphere in either synthetic air or nitrogen. The rate constant for n-heptane, relative to n-butane whose rate constant was taken to be 2.18 × 10−10 cm3 molecule−1 s−1, was also measured and found to be (3.97 ± 0.27) × 10−10 cm3 molecule−1 s−1 (2s). Based on this value for the n-heptane reaction, the following absolute values for the rate constants, k (in units of 10−10 cm3 molecule−1 s−1) for the chlorine atom reactions were determined: propene, 2.64 ± 0.21; isobutene, 3.40 ± 0.28; 1-butene, 3.38 ± 0.48; cis-2-butene, 3.76 ± 0.84; trans-2-butene, 3.31 ± 0.47; 2-methyl-1-butene, 3.58 ± 0.40; 2-methyl-2-butene, 3.95 ± 0.32; 3-methyl-1-butene, 3.29 ± 0.36; 2-ethyl-1-butene, 3.89 ± 0.41; 1-pentene, 3.97 ± 0.36; 3-methyl-1-pentene, 3.85 ± 0.35; and cis-4-methyl-2-pentene, 4.11 ± 0.55 (±2s). The errors reflect those in our relative rate measurements but do not include the 10% error in the absolute value of the n-butane rate constant upon which these rate constants are ultimately based. A structure–reactivity scheme is presented that assumes that rate constants for addition of chorine atoms to the double bond, as well as that for abstraction of an allylic hydrogen atom, depend upon the degree of alkyl substitution at the double bond and allylic carbons. The surprising result is that the allylic hydrogen atoms react less rapidly with chlorine atoms than the analogous alkyl hydrogens in alkanes. The atmospheric implications for loss of alkenes in the marine boundary layer are discussed.

Solubility of petroleum hydrocarbons in oil/water systems

To elucidate mechanisms, two laboratory-scale experiments were correlated to understand and quantify how oil partitions into the aqueous phase. In the two experiments, free-phase petroleum was exposed to water in an effort to determine aqueous concentrations of various oil components. In the first investigation, an oil/water system was allowed to equilibrate for 16 days. The water column in the system was periodically sampled, and sample analysis was performed by GC-MS. After analyzing for naphthalene and various alkyl-substituted naphthalene compounds, the data was modeled and rate coefficients and the saturation concentrations were predicted. For naphthalene, the modeled saturation concentration was 1.4×10−6 mol/l , the rate coefficient was 0.239 h−1 and the predicted time to reach equilibrium was 19.3 h. For the alkyl-substituted compounds, there was an inverse correlation between both the rate coefficients and saturation concentrations and the degree of alkyl-substitution. In the second investigation, oil/water systems were allowed to equilibrate for 36 h. Various oil loadings (mass of oil/volume of water) were investigated. The mixing energy was twice that of the first experiment. After the 36 h, the water column was sampled and analyzed by GC-MS. There was a direct correlation between the measured total petroleum hydrocarbon (TPH) concentrations in the water and the oil loading. However, there was no such correlation between the naphthalenes and oil loading. It was concluded that the first experiment was a solubility phenomenon while the second experiment also included a colloidal phenomenon.

Role of asphaltene and resin in oil field emulsions

Asphaltenes and resins are the principal constituents of polar fractions of crude oils. These polar molecules combine with and/or adsorb on other organic and inorganic solids producing emulsion stabilizing films at oil–water interfaces, and contribute to the formation of tight emulsions which are difficult to process at the surface facilities. These emulsion-stabilizing organic constituents, namely the asphaltenes and resins, have been characterized in our laboratories after their separation from unstable, moderately stable and highly stable emulsions obtained from a carbonate reservoir in Saudi Arabia. These constituents were studied in detail in terms of their molecular parameters by proton and carbon-13 NMR and FT-IR spectroscopy and GC-MS under pyrolytic conditions. There is evidence that factors such as increased aromaticity, decreased alkyl side chain lengths, and reduced chain branching are associated with the formation of stable emulsions involving asphaltenes. Also, the presence of aromatic quaternary carbons (bridged as well as substituted) shows a direct correlation with the tendency of asphaltenes to form stable emulsions. NMR and IR spectroscopy also indicate that the condensed ring character of asphaltenes increases with increasing emulsion stability as indicated by parameters such as the percentage of bridged and substituted aromatic carbons, alkyl chain branching, and side chain length. The data indicate that the increasing ring condensation in asphaltenes and resins is associated with an increase in both the degree of alkyl substitution and alkyl branching on the aromatic ring and not with the alkyl side chain elongation. Also, the spectral data for asphaltenes and resins indicate very low concentration of OH and NH groups, which are often considered important for asphaltene aggregation through hydrogen bonding. These findings have been applied in developing molecular models for asphaltenes and resins from Arab Light crude oil.

Silanes and germanes as free-radical reducing agents: an ab initio study of hydrogen atom transfer from some trialkylsilanes and germanes to alkyl radicals

Ab initio molecular orbital calculations using a (valence) double-ζ pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation predict that hydrogen atoms, methyl, ethyl, isopropyl and tert-butyl radicals abstract hydrogen atom from silane, methylsilane, dimethylsilane, trimethylsilane, trisilylsilane and the analogous germanes via transition states in which the attacking and leaving radicals adopt colinear (or nearly so) arrangements. Except for reactions involving trisilylsilane which are predicted at the MP2/DZP level to involve transition states with Si–C distances of about 3.19 A, transition states which have (overall) Si–C and Ge–C separations of 3.12–3.15 and 3.24–3.26 A respectively are calculated; these distances appear to be independent of the number of methyl substituents on the group(IV) element, but appear to be slightly sensitive to the nature of the attacking radical, with marginally earlier transition states calculated as the degree of alkyl substitution on the attacking radical is increased. At the highest level of theory (QCISD/DZP//MP2/DZP), energy barriers (ΔE1‡) of 27–57 (Si) or 26–44 (Ge) kJ mol–1 are predicted for the forward reactions, while the reverse reactions (ΔE2‡) are calculated to require 85–134 (Si) or 102–138 (Ge) kJ mol–1. These values are marginally affected by the inclusion of zero-point vibrational energy correction. Importantly, QCISD and MP2 calculations appear to predict correctly the relative ordering of activation energies for alkyl radical reduction by silanes: tertiary < secondary < primary; SCF/DZP, AM1 and AM1 (CI = 2) calculations perform somewhat more poorly in their prediction of relative radical reactivity.

Control of mesophase pitch properties by supercritical fluid extraction

Supercritical fluid (SCF) extraction has been used to fractionate an isotropic petroleum pitch and produce a number of pitch fractions, many of which contain 100% mesophase. Toluene was used as the supercritical solvent, and experiments were carried out in a region of liquid-liquid equilibrium that exists for mixtures of pitch and toluene above the critical pressure of toluene. A central composite design was used to investigate the effects of the operating variables of temperature, solubility parameter and solvent-to-pitch ( S P ) ratio on the yield and softening point of the produced mesophase pitch fractions. Temperatures, pressures and S P ratios from 310 to 360 °C, 45 to 155 bar and 2.5 to 4.0, respectively, were investigated. Using the developed statistical models, the effect of operating variables on softening point and mesophase yield can be quantitatively predicted. Chemical analyses of the mesophases by elemental analysis and DRIFT indicate that the extraction process can also be used to alter the chemical composition (e.g. the degree of alkyl substitution) of the produced mesophases. The flexibility of SCF extraction for processing pitches is illustrated by the fact that one can change the operating temperature and pressure and still maintain a constant mesophase yield or softening point.

Metal Atoms in the Synthesis of Metal Clusters, VIII.On the Reaction of Sterically Demanding Cyclopentadiene Ligands with Cobalt Atoms: Synthesis, Crystal Structure, Spectroscopic Behavior, and Reactivity of Di-, Tri- and Tetranuclear Hydrido Clusters of Cobalt

The metal-vapor reactions of Co atoms with 1,3-tBu2CpH (1a), 1,3-tBu2CpH (1a), 1,2,4-tBu3CpH (1b) and EtMe4CpH (1c) are described. With 1a the two mononuclear complexes [(η5-tBu2Cp) (η4-tBu2-1,3-cyclopentadiene)Co] (2a) and [(η5-tBu2Cp)2Co] (3a) were isolated, together with the dinuclear cobalt cluster [{(η5-tBu2Cp)Co}2H3] (4a) and trace amounts of the tetranuclear cluster [{(η5-tBu2Cp)CoH}4] (5a). The molecular structures of 3a and 4a were determined by X-ray diffraction. Reaction of 1b with Co atoms afforded a single product, the dinuclear cluster [{(η5-tBu3Cp)Co}2H3] (4b), whose molecular structure was determined by single-crystal X-ray diffraction. Both, 4a and 4b exhibit extremely short Co-Co distances [2.244(1) (4a) and 2.242(1) A (4b)], as found for the Me5Cp analog [{(η5-Me5Cp)Co}2H3] (4c). Reaction of an isomeric mixture of Me4EtCpH (1c) with Co atoms furnished the mononuclear sandwich [ (η5-Me4EtCp)(η4-Me4Et-1,3- cyclopentadiene)Co] (2b), the trinuclear hydridocobalt cluster [{(η5-Me4EtCp)Co}3H4] (6a) and the tetranuclear hydridocobalt cluster [{(η5-Me4EtCp)Co}4H4] (5c). The molecular structure of 5c was determined by X-ray crystallography and revealed a tetrahedral arrangement of the cobalt atoms. The electrochemical behavior of the dinuclear complexes 4a-4c was studied by cyclic voltammetry. Reversible redox couples were found for all three compounds, with a correlation between the degree of alkyl substitution and their respective cathodic shifts. Compounds 4a and 4b react with CO to yield the mononuclear and dinuclear complexes [ (η5- CpR)Co(CO)2] (R1,3-tBu2, 1,2,4-tBu3) (7a and 7b) as well as [{(η5-CpR)CO}2(CO)2] (8a and 8b). Reaction of the trinuclear hydridocobalt cluster 6a and its Me5Cp analog 6b with AgBF4 in the presence of PEt3 yielded the heteronuclear clusters [{η5-Me4CPR)Co}3AgP(Et)3H4]+[BF4]− (REt, Me) (9a and 9b). 9a was structurally characterized by X-ray crystallography.

Alkyl Substituent Effect in the Deprotonation of Unsymmetrical Ketones

The effect of various degrees of alkyl substitution on the relative rates of deprotonation from the two distinct sites in several unsymmetrical ketones in the gas phase is examined. The infrared multiple photon activation of an appropriately deuterium-labeled alkoxide ion generates the ion−molecule complex for the half-reaction of the bimolecular proton transfer process between an alkyl anion and an unsymmetrical ketone with one deprotonation site selectively deuterated. The resulting products are enolate ions generated by the removal of either a deuteron or a proton and, thus, are distinguishable by mass. The measurement of the enolate ion product ratios, along with an independent measurement of the kinetic isotope effect, allowed the kinetic effect of the alkyl environment on the relative proton transfer rates to be determined. The primary and secondary isotope effects are also estimated from the enolate ion product ratios. By examining the magnitude of the kinetic alkyl effect, the primary isotope effe...

Calculation of the Effects of Branching and Conjugation on Intrinsic Barriers for H Atom Transfer Reactions Involving Hydrocarbons

This paper describes the use of the semiempirical molecular orbital (MO) theoretical methods (AMI and MNDO-PM3) to calculate barriers for a series of H atom transfer identity reactions involving alkyl, alkenyl, arylalkyl, and hydroaryl systems. Transition state (TS) energies were calculated for a series of known H abstractions and show to correlate linearly with experimental TS energies. Barriers for H abstraction reactions decrease with the degree of alkyl substitution at the radical site, and increase with the degree of conjugation. Barriers for transfer of a {beta}-hydrogen from a radical to an unsaturated hydrocarbon (radical hydrogen transfer or RHT) were also calculated. The results show that methyl group substitutions at the radical site lower the barrier while substitutions at the site {beta} to the radical, the position from which the H originates, raise the barrier. The barriers for RHT reactions involving conjugated systems increase with increasing radical delocalization and correlate linearly with the strength of the donor radical {beta}-C-H bond. RHT barriers are estimated to range from E{sub a} = 17-20 kcal/mol for benzene-plus-cyclohexadiene to E{sub a} = 26-29 kcal/mol for anthracene-plus-9-hydroanthracene. 61 refs., 5 figs., 4 tabs.

The Effect of the Degree of Substitution on Some Textile Properties of Jute and Cotton Fibres

An investigation is reported in which the effects of the degree of alkyl substitution on percentage elongation, bundle strength and moisture regain were studied. A remarkahle increase in percentage elongation was achieved with partially substituted jute fibre within a 1–2-hour reaction time

Bromo[ 18F]fluorination of cyclohexenes: a method for the preparation of [ 18F]fluorocyclohexanes

In an earlier report, we described an olefin halofluorination process that was used to label monosubstituted alkenes with fluorine-18 at the no-carrier-added level. In this report, we describe further studies on the radiohalofluorination of more highly substituted and more reactive alkenes, such as the cyclohexenes 1,4-diisopropylcyclohexene (DIPC), 1-methylcyclohexene (MeC) and pregnenolone (3β-hydroxy-5-pregnen-20-one). DIPC, MeC and pregnenolone were labeled with fluorine-18 in 30%, 26% and 35% radiochemical yields, respectively, within 15 min at 0 °C under no-carrier-added conditions. A comparison of the conditions required to obtain optimal yields for fluorine-18 radiohalofluorination with these trisubstituted cyclic alkene systems vs. the monosubstituted alkenes studied earlier, suggests that alkene systems with different degrees of alkyl substitution (and hence different reactivity towards electrophiles) will each require careful optimization of the reaction conditions.

Neue Synthesen für homonukleare Cobalthydrid-Clusterverbindungen / New Synthetic Routes to Homonuclear Cobalt Hydride Clusters

Several homonuclear cobalt hydride clusters are accessible by reacting the complexes (η5-CpR)Co(η2-ethene)2 (R = H5, Me5) 3, 4 with (C2H5)2AlH followed by alcoholysis of the crude reaction mixtures. Depending on the degree of alkyl substitution, different nuclearities of the resulting clusters have been observed. In the case of the pentamethyl substituted derivative (η5-Cp*)Co(η2-ethene) 4 a mixture of di-, tri- and tetranuclear cobalt hydride clusters namely [(η5-Cp*)Co]2H3 1, [(η5-Cp*)Co]3H4 2 and [(η5-Cp*)Co]4H4 7 were form­ed and separated by repeated crystallization. When reacting the parent compound (η5-Cp)Co(η2-ethene) 3 with (C2H5)2AlH and quenching the reaction mixture with alcohol at low temperature, the tetranuclear cobalt cluster [(η5-Cp)Co]4H4 5 is the main product. Aside from this, trace amounts of the cobalt alkylidine cluster [(η5-Cp)Co]4μ-[C(CH3)]H 6 were formed as detected by mass spectrometry. For the first time a direct synthesis of [(η5-Cp**)Co]3H4 10 and [(η5-Cp**)Co]4H4 11 was achieved by reaction of cobalt atoms with tetramethyl-ethyl-1,3-cyclopentadiene (mixture of isomers) in methylcyclohexane at -120 °C. However, the main product of this metal atom reaction is the mononuclear cobalt sandwich compound (η5-Cp**)Co(η4-tetramethyl-ethyl-1,3-cyclopentadiene) 9 as a mixture of isomers. The trinuclear cobalt alkylidine cluster [(η5-Cp*)Co]3μ3-[C(CH3)]2 8 is formed when reacting (η5-CpR)Co(η2-ethene)2 3 with equimolar amounts of di-isobutyl-aluminium- hydride or the borabicyclus 9-BBN.

Studies on the Spin-Lattice Relaxation Times of the Carbons of the Investigational Antileukemic Drug and Enzyme Inhibitor MethylglyoxalBis(Amidino-Hydrazone) ['MethylglyoxalBIS(Guanylhydrazone)'] and Its Dialkylglyoxal Analogs

Abstract The first study on the 13C relaxation times of bis(amidinohydrazones) is reported. The spin-lattice relaxation times (T1) of the carbons of the free bases of methylglyoxal bis(amidinohydrazone) (MGBG) and of four dialkylglyoxal analogs thereof were determined with the aid of the inversion recovery method and using dimethyl sulfoxide as the solvent. In the series of compounds studied, one of the side chains was always a methyl group, while the other one was altered (hydrogen, methyl, ethyl, propyl, butyl). Remarkable differences were found to exist between the T1 values of the various carbons within each molecule. The T1 values were in the range 1.5 - 2 s for methyl carbons, 0.16 - 1.9 s for carbons of longer alkyl groups, 4.3 - 7.0 s for unprotonated carbons of the glyoxal moiety, 0.57 s for the protonated glyoxal carbon of MGBG, and 2.6 - 3.1 s for guanidino carbons. The bulk of the differences are explainable by assuming that the major relaxation mechanism for the protonated carbons is dipolar relaxation. In alkyl side chains, the T1 values increased in a very regular fashion down the chain. This effect made possible the assignment of two previously unassigned carbon resonances of the butyl group of BMGBG. T1 studies thus offer a facile and reliable method for the assignment of side-chain carbon resonances of bis(amidinohydrazones). Further, T1 measurements were found to offer a very good method for the individual assignment of the glyoxal carbons of unsymmetrical congeners, whose assignment has so far constituted a problem. The method, based on the finding that the one of the carbons bonded to the shorter alkyl chain has a longer relaxation time than does the other one, made possible the unambiguous assignment of several previously unassigned carbon resonances. The results obtained also offer a reliable method for unambiguously distinguishing between the resonances of glyoxal carbons and guanidino carbons that have been difficult to distinguish from each other because the separation of their chemical shifts is often extremely small. Correlations observed between T1 values and the degree of alkyl substitution in the molecule are discussed, as are also possible relaxation mechanisms. Somewhat unexpectedly, the results obtained suggest that dipolar relaxation through the hydrogens of neighboring carbon atoms may to a significant extent contribute to the relaxation of some unprotonated carbons in bis(amidinohydrazones).