UNT Libraries - 174 Matching Results

Search Results

Silene Stereochemistry

Description: The reaction of tert-butyllithium with chloromethylphenylvinylsilane at low temperatures in hexane gave a 48% yield of a mixture of the five isomers of 1,3-dimethyl-1,3-diphenyl-2,4-dineopentyl-1,3-disilacyclobutane, formed by the head-to-tail dimerization of both E- and Z-1-methyl-1-phenyl-2-neopentylsilenes, along with an acyclic dimer. These were separated and their stereochemistry was established by ('1)H- and ('13)C-NMR spectroscopy. The E- and Z-silenes were also trapped as their {4 + 2} cycloadducts with cyclopentadiene, 2,3-dimethyl-1,3-butadiene and anthracene, which also were separated and stereochemically characterized. A consistent mole ratio of 70:30 for the E- and Z-silene adducts is interpreted as evidence for stereochemical induction in the silene generation reaction. It is also suggested that the dimerization of the silenes to give the 1,3-disilacyclobutanes occurs by a nonstereospecific stepwise pathway. When E- or Z-1-methyl-1-phenyl-2-neopentylsilene was generated by the retro-Diels-Alder flow vacuum thermolysis of its corresponding cyclopentadiene or anthracene adduct at temperatures between 400 and 600(DEGREES)C and then trapped with 2,3-dimethyl-1,3-butadiene, the stereochemical distribution of the products is independent of the stereochemistry of the silene precursor, indicating that the silene is not configurationally stable towards cis-trans isomerization at these temperatures. Evidence that the intermolecular ene reaction and the {4 + 2} cycloaddition which occur with 2,3-dimethyl-1,3-butadiene are concerted is presented. When either the E- or Z-silene, generated by the sealed tube thermolysis of its anthracene adduct by 300(DEGREES)C, was trapped with trimethylmethoxysilene, the diastereomer obtained depended on the stereochemistry of the silene precursor, showing that the silene is configurationally stable towards cis-trans isomerization up to 300(DEGREES)C. The temperature dependence of the ratio of the two diastereomers obtained when the silene formed from the pure E- or Z-anthracene adduct was trapped at higher temperatures permitted the determination of an activation energy for the silene isomerization. The activation energies for the E- and Z- and Z- to E-silene isomerization are 45 (+OR-) ...
Date: August 1984
Creator: Lee, Myong Euy

Silenes and Silenoids in the Chemistry of Cyclopentadienylsilanes

Description: Evidence is presented that apparent silene products obtained from the metalation of cyclopentadienyldimethyl - chlorosilane either with tert-butyl1ithium or with methylenetriphenylphosphorane actually arise from the metalated starting material, a silenoid, rather than from a silafulvene intermediate. Trimethylmethoxysi1ane is shown to be an effective trap for dimethylsilafulvene. A new dimethylsilafulvene precursor, bis(dimethylmethoxysi1yl) cyclopentadiene, which gives high yields of dimethyldimethoxysi1ane and the silafulvene at temperatures as low as 240°C is reported.
Date: August 1986
Creator: Rozell, James M. (James Morris)

Sites of Reactivity During Ligand-Exchange Reactions in Octahedral Group VIB Metal Carbonyls

Description: The site of initial metal-carbonyl bond-breaking during ligand-exchange reactions in a series of octahedral metal carbonyls of the type (L2)M(CO)4 (M = Cr, Mo, W; L2 = diphos, phen, dipy) has been determined employing infrared spectroscopy and Fourier transform nuclear magnetic resonance spectroscopy. The results of this study reveal, for all metal carbonyl complexes of the type mentioned above, that loss of CO occurs exclusively at an axial position (cis to the bidentate ligand, I^)• The dynamic nature of the five-coordinate intermediates, such as (diphos)Mo(CO)3, (phen)M(CO)3 (M = Cr, Mo, W), and (dipy)Cr(CO)3, which are generated in solution upon CO dissociation, is reported and discussed. The results of this investigation confirm that these intermediates are fluxional on the time scale of CO-exchange process. A mechanism which describes the site of initial metal-carbonyl bond-breaking and the fluxionality of the five-coordinate intermediate during ligand-exchange reactions in the complexes (L2)M(CO)4 is proposed. A kinetic study of reactions of W(CO)6 with pseudo-halide anions (NCS-, NCO-, CN-) has been initiated. The results indicate that these reactions proceed via a bimolecular path, which involves initial attack of the pseudo-halide anion at a carbonyl carbon of W(CO)6,
Date: December 1979
Creator: Asali, Khalil Jamil

Solid State Diffusion Kinetics of Intermetallic Compound Formation in Composite Solder

Description: The Sn/Pb eutectic alloy system is the most widely used joining material in the electronics industry. In this application, the solder acts as both an electrical and mechanical connection within and among the different packaging levels in an electronic device. Recent advances in packaging technologies, however, driven by the desire for miniaturization and increased circuit speed, result in severe operating conditions for the solder connection. In an effort to improve its mechanical integrity, metallic or intermetallic particles have been added to eutectic Sn/Pb solder, and termed composite solders. It was the goal of this study to investigate the growth and morphology of the two intermetallic phases (Cu6Sn5 and Cu3Sn) that form between a Cu substrate and Sn/Pb solder under different aging and annealing conditions.
Date: May 1993
Creator: Sees, Jennifer A. (Jennifer Anne)

Solution Studies of the Structures and Stability of Mixed Lithium Alkoxide/Alkvllithium Aggregates

Description: New one- and two-dimensional NMR techniques were used to elucidate the solution structures of these complex mixtures. The system, lithium tert-butoxide/tert-butyllithium, was studied as a model system with O/Li ratios varying from 0/1 to 1/1. It was found that at low O/Li ratios, a single mixed tetrameric aggregate was formed. At higher O/Li ratios, mixed hexameric species were formed. Two other systems, lithium isopropoxide/iso-propyllithium and lithium n-propoxide/n-propyllithium were also studied at low O/Li ratios.
Date: December 1992
Creator: DeLong, George T. (George Thomas)

Solvent and Ionic Complexes of the Calix[6]arenes

Description: One of the more attractive attributes of calixarenes is their wide variety of possible conformations and hence cavity shapes. However, the flexibility that allows this long-range benefit gives rise to major synthetic challenges when working with the larger members of the family. O-alkylations have proven to be the most widely employed synthetic routes to "functionalization" of the calixarenes, and these have shown a dependence upon both solvent and the metal ions present. Surprisingly, there have been no structural data presented concerning the complexes between the simple unsubstituted calix[6]arenes and the metal ions of groups 1 and 2. The structures of four complexes, containing cesium, rubidium, and calcium are reported as determined by X-ray crystallography. The solution behavior of the complexes for both representative groups is also discussed, in particular with regard to conformational stabilization of the calix[6]arenes and the role of solvent upon this stabilization. These complexes are also investigated as starting materials for the selective functionalization of the calix[6]arenes.
Date: December 1997
Creator: Wolfgong, William J.

Some Studies Pertaining to the Biosynthesis and Metabolism of Asparagine and Lysine in Lactobacillus Arabinosus: I. B-Aspartylhydroxamic Acid: Its Action as a Feedback Inhibitor and a Repressor of Asparagine Synthetase in Lactobacillus Arabinosus II. Purification and Properties of Diaminopimelate Decarboxylase from Lactobacillus Arabinosus

Description: That Lactobacillus arabinosus 17-5, ATCC 8014, can supply its own requirement for the amino acid, lysine, is demonstrated by the fact that the organism is capable of growth in media devoid of lysine. Since the final biosynthetic step in lysine formation in all bacteria studied to date involves the decarboxylation of meso-dlaminopimelic acid (DAP) to produce lysine, it was of interest to determine whether an enzyme catalyzing such a reaction (DAP decarboxylase) is present in L. arabinosus.
Date: August 1969
Creator: Chen, Yueh Tsun

Spectrofluorometric and Solubility Studies of Polycyclic Aromatic Hydrocarbons in Hydrogen Bonded Binary Solvent Mixtures

Description: The purpose of this dissertation is to investigate the behavior of polycyclic aromatic hydrocarbons (PAHs) in binary solvent systems and determine and/or develop predictive mathematical expressions for describing solutions in which hydrogen-bonding occurs.
Date: May 1997
Creator: Powell, Joyce R., 1968-

Spectroscopic and Physical Effects of Highly Polar Groups

Description: Since the development of the understanding that the electron distribution within a molecule is chiefly responsible for its properties and behavior, factors influencing this charge distribution have been of interest to scientists. The chemical reactivity of a molecule, the physical properties, and to a large extent, structure and geometry, are all functions of the electron distribution. This study examines the issue of electronic structure from two points of view, each of them focussing on a specific component within the molecules studied. In the present work, the effects of the highly polar carbonyl group on spectroscopic parameters and physical behavior are investigated. An additional area of study is the effect of fluorine substitution on the energy levels of some halogenated ethylenes. The specific parameters examined are the ionization potentials, the absorption frequencies, and the energies of a class of excited states known as molecular Rydberg states. It was during the study of these halogenated ethylenes that the observations leading to the carbonyl compound investigations were made, so that the two areas examined are connected both experimentally and chemically.
Date: December 1981
Creator: Schander, Judith Turner

Stereochemical Studies of Nitrosamines: The Induced Circular Dichroism of Achiral Nitrosasmines

Description: The induced circular dichroism (ICD) of several chiral nitrosamines and various chiral reagents has been investigated. The interaction is attributed to a 1:1 hydrogen bonded complex between the NO group of the nitrosamine and the hydroxyl groups of alcohols and polyols, or the amino group of amines. Only those chiral reagents possessing large differences in size of the groups about the hydrogen bonding site contributed to CD anomalies. The acyclic 2-octanols did not give observable Cotton effects, presumably due to the similarity in size of the methyl and methylene groups and rotational freedom of the acyclic system. The signs of the Cotton effects could be correlated with the absolute configuration of the sterically hindered alcohols and amines. Only the alpha, axial hydrogens of conformationally biased, heterocyclic nitrosamines were found to undergo selective hydrogen-deuterium exchange, suggesting that this feature is critical for nitrosamine carcinogenicity.
Date: August 1980
Creator: Fribush, Howard M.

The Stereochemistry of Silenes and Alpha-Lithio Silanes

Description: When E- or Z-l-methyl-l-phenyl-2-neopentylsilene was generated by the retro-Diels-Alder vacuum-sealed tube thermolysis of its corresponding anthracene adduct, in the presence of various alkoxysilanes, only one diastereomeric adduct was formed in each case, showing that the reactions are stereospecific. An x-ray crystal structure of the methoxytriphenylsilane adduct of the E-silene confirmed its relative configuration as (R,S) or (S,R). This demonstrated that the addition of alkoxysilanes to silenes is stereospecific and syn. The relative configurations of similar alkoxysilane and alkoxystannane adducts to E- and Z-l-methyl-l-phenyl-2-neopentylsilene were assigned based on a combination of xray structures and *3C NMR data. A strong, nonbonded oxygen-metal interaction is apparent in all of those compounds studied. Treatment of the alkoxystannane adducts with alkyl lithium reagents results in tin-lithium exchange in some cases. The results indicate that the resulting <x-lithio alkoxysilanes are not configurationally stable in either THF or hydrocarbon solvents. The reaction of tert butyl lithium with a-trimethylsilylvinylmethylphenylchlorosilane in hydrocarbon solvents yields E- and Z-l-methyl-l-phenyl-2-neopentyl-2-trimethylsilylsilene. In the absence of any traps these silenes undergo a novel tert butyl lithium catalyzed rearrangement to 2-phenyl-3-trimethylsilyl-5,5-dimethyl-2-silahex-3-ene. These silenes were also trapped as their [4+2] cycloadducts with anthracene. The Z-isomer of the anthracene adduct was separated and its stereochemistry confirmed by an x-ray crystal structure. The anthracene adducts of both E- and Z-l-methyl-1-phenyl-2-neopentyl-2-trimethylsilylsilene undergo a facile, stereospecific decomposition at temperatures as low as 190°C to regenerate their respective silenes, the mildest stereospecific route to a silene yet reported. The E- and Z-silenes react stereospecifically with methanol under vacuum-sealed tube conditions. The stereochemistry of the addition is syn and a common mechanism is proposed for the addition of alcohols and the addition of alkoxysilanes to silenes.
Date: May 1987
Creator: Bates, Tim Frank

Stereoselective Solid-State NaBH₄ Reduction of 1-Methylpentacyclo[5.4.0.0²,⁶.0³,¹⁰,0⁵,⁹]undecane-8, 11-Dione, Synthesis and Chemistry of Strained Alkenes, and Chemical and Microbial Synthesis of Racemic and Optically Active (S)-4-Hydroxy-2-Cyclohexenone

Description: Part I. Reduction of the 1-methylpentacyclo [5.4.0.0²,⁶.0³,¹⁰,0⁵,⁹]undecane-8, 11-dione (9) with solid NaBH₄ resulted in highly stereoselective reduction of both C=O groups in the substrate, thereby affording the corresponding endo-8, endo-11-diol (11a). The configuration of 11a was established unequivocally by converting 11a into the corresponding cyclic thiocarbonate ester, 12. Part II. Z-1,2-Di(1'-adamantyl)ethene (14) was synthesized with a high degree of stereoselectively in four steps (Scheme 9 in Chapter 2). E-1,2-di(1'-adamantyl)ethene (15) was synthesized by iodine promoted isomerization of 14. Both structures were established unequivocally via single-crystal X-ray structural analysis. E-1-(exo-8'-Pentacyclo[5.4.0.0²,⁶.0³,¹⁰,0⁵,⁹]undecyl)-2-phenylethylene (16a) was synthesized, and its structure was established via analysis of its 1H, 13C, and 2D COSY NMR spectra. Part III. Reactions of electrophiles, i.e.,:CCl_2, PhSCl, and Br_2, to Z- and E-1,2-di(1'-adamantyl)ethenes (14 and 15, respectively) are described (Scheme 5, 8, 10, and 13 in Chapter 3). Structures of the corresponding products were established unequivocally via analysis of their respective one- and two-dimensional NMR spectra and/or single-crystal X-ray structural analysis. Part IV. An improved asymmetric synthesis of optically active (S)-4-hydroxy-2-cyclohexenone 1 (64%ee, determined via Mosher's method) has been developed (Scheme 5 in Chapter 4). The key step in this synthesis involves the baker's yeast reduction of 13. The absolute configuration of the major product, (S)-1, was established unequivocally via single-crystal X-ray structural analysis of a precursor. The optical purity of the major product 14a (80%de, 67%ee) was established via careful integration of relevant gated-decoupled 13C NMR spectra.
Date: August 1995
Creator: Xing, Dongxia

Structural Elucidation of tert-Butyllithium/Lithium Alkoxide and Lithium Hydride/Lithium Alkoxide Mixed Aggregates

Description: The effects of lithium alkoxides on the rates of reactions and on the structures of a series of tert-butyllithium/lithium alkoxide mixed aggregates were studied, where the alkoxides were iso-butoxide, tert-butoxide and menthoxide. It was found that their effects depend not only on their amount present, but also on their steric bulk. The tert-butyllithium/lithium alkoxide mixed aggregates were exposed to UV light or heat to form lithium hydride/lithium alkoxide mixed aggregates. The aggregation states were assigned from either 13C-6Li coupling or a new technique based on the relative intensity of NMR peaks using different nuclei. The compounds formed depend upon the method of formation and the alkoxide. The unique properties of the lithium hydride/lithium alkoxide mixed aggregates are their high solubility in hydrocarbon solutions, very reactive bases, showing 6Li-1H couplings, and having only one hydride ion per aggregate. Their formation, reactivity, solubility, and aggregation states were found to depend on the size of lithium alkoxides. X-ray crystal structures of lithium tert-butoxide and lithium menthoxide were also studied and found to be hexameric.
Date: December 1997
Creator: Nguyen, Hanh D.

Studies of Nitrogen-containing Compounds Having Pyrethroid-like Bioactivity

Description: During recent years most of the successful developments in pyrethroids have been primarily concerned with structural or compositional variations. As a part of our continuing interest in pyrethroid insecticides, nitrogen-containing compounds having pyrethroid-like structures were synthesized. Seven prolinate compounds, N-(substituted)-phenyl-prolinates and N-carbobenzoxy-prolinates were coupled with known pyrethroid alcohols. These structural variations which "locked in" a specific conformation between the nitrogen and chiral a-carbon in the acid moiety of fluvalinate were studied to determine the influence of certain conformations on insecticidal toxicity. The toxicity data for the prolinate compounds showed intermediate mortality against nonresistant cockroaches. It was concluded that the conformation imposed by the proline ring portion of the esters was probably close to the favored conformation for interaction of fluvalinate-like pyrethroids with the insect receptor site. A second series of nitrogen-containing compounds, twenty-five carbamate esters resulting from the condensation of N-isopropyl-(substituted)-anilines and N-alkyl-(substituted)-benzylamines with appropriate pyrethroid alcohols were studied for insecticidal activity. These studies were conducted on pyrethroid-susceptible houseflies. Some of the carbamate esters exhibited high toxicity when synergized by piperonyl butoxide. For example, the toxicity ( LD 50 ) of O-a-cyano-3-phenoxyfaenzyl-N-a,a-dimethyl-4-bromo-benzyl carbamate was 0.012 ug/g, which is significantly greater than that reported for the potent pyrethroid, fenvalerate. Correlations of insecticidal activity with respect to structure and conformational factors of the carbamate esters have been made. The N-isopropyl substituent decreases insecticidal activity in the N-benzyl-derived compounds, while the N-isopropyl substituent enhances activity in the N-phenyl-derived compounds. Certain substituents on the phenyl ring of both analogs greatly affect insecticidal potency of the carbamate esters. Also, some alkyl substituents (especially, a,cx-dimethyl and a-cyclopropyl groups) on the benzylic carbon of the benzylamine series enhance toxicity. The a,a-dimethyl branching of the N-benzyl carbamate approximates the steric shape given by the gemdimethyl group for conventional cyclopropane ring-containing pyrethroids. The N-benzyl compounds are significantly synergized by ...
Date: August 1989
Creator: Lee, Jimmy Jing-Ming, 1955-

Studies of Solvent Displacement from Solvated Metal Carbonyl Complexes of Chromium, Molybdenum, and Tungsten

Description: Flash photolysis techniques were applied to studies of solvent displacement by Lewis bases (L) from solvated metal carbonyl complexes of Cr, Mo, and W. On the basis of extensive studies of the reaction rate laws, activation parameters , and linear-free-energy-relationships, it was concluded that the mechanisms of solvent displacement reactions depend on the electronic and steric properties of the solvents and L, as well as the identities of the metal atoms. The strengths of solvent-metal bonding interactions, varying from ca. 7 to 16 kcal/mol, and the bonding "modes" of solvents to metals are sensitive to the structures of the solvent molecules and the identities of the metal centers. The results indicate dissociative desolvation pathways for many arene solvents in (solvent)Cr(CO)_5 (solvent = benzene, fluorobenzene, toluene, etc.) complexes, and are consistent with competitive interchange and dissociative pathways for (n-heptane)M(CO)_5. Different types of (arene)-Cr(CO)_5 interactions were suggested for chlorobenzene (CB) vs. fluorobenzene and other non-halogenated arenes, i.e. via σ-halogen-Cr bond formation in the CB solvate vs. π-arene-Cr bond formation through "isolated" double bonds in solvates of the other arenes. The data also indicate the increasing importance of interchange pathways for solvent displacement from the solvates of Mo and W vs. that of Cr.
Date: August 1990
Creator: Zhang, Shulin

Studies on Human Plasma Lecithin:Cholesterol Acyltransferase: Physical and Chemical Characterization and Coupled Spectrophotometric Enzyme Assay

Description: The physico-chemical properties of lecithin:cholesterol acyltransferase were investigated. The amino acid composition analysis showed a relatively high content of glutamic acid, aspartic acid, glycine and leucine. The spectrophotometric titration of phenolic groups in the enzyme showed a large increase in absorbance at 295 nm with an apparent pK of about 12.0. The largest change in molar ellipticity at 222 nm was also observed above pH 11. Circular dichroism studies revealed that human lecithin:cholesterol acyltransferase has a relatively high content of β-pleated sheet structure (48%) with 20% α-helix, and 32% remaining structure. Human lecithin:cholesterol acyltransferase has a high extinction coefficient at neutral pH. Microsequencing of the amino terminal residues of the enzyme revealed a hydrophobic character. Inactivation of lecithin:cholesterol acyltransferase activity was observed using diisopropylfluorophosphate with a stoichiometry of 1 mole of diisopropylphosphate incorporated per mole of enzyme. This suggests the involvement of a serine residue in the active site of the enzyme, possibly for the formation of an acyl-intermediate. A new quicker assay method for lecithin:cholesterol acyltransferase was developed. This assay involved coupling reaction with acyl CoA synthetase, ΡΡᵢ-dependent phosphofructokinase, aldolase, triosephosphate isomerase and α-glycerol-3-phosphate dehydrogenase monitoring a change in the absorbance or fluorescence intensity due to the oxidation of NADH. The activity of each coupling enzyme was accurately determined to establish the optimum assay condition for lecithin:cholesterol acyltransferase. The coupled enzyme assay for lecithin:cholesterol acyltransferase by spectrofluorometry showed a significant change in relative fluorescence intensity whereas a UV absorption spectroscopy method showed no significant absorbance change for the initial rate of lecithin:cholesterol acyltransferase reaction.
Date: December 1984
Creator: Hara, Shinichi

The Study of Low Temperature Silene Generation

Description: The reactions of tert-butyl-, sec-butyl-, and n-butyllithium with dimethylfluorovinylsilane include addition to the double bond to give both silene and silenoid intermediates, fluorine substitution, and a novel vinyl substitution. For the tert-butyllithium reaction, product stereochemistry and trapping experiments using both cyclopentadiene and methoxytrimethylsilane show that silenes are not formed in THF. In hexane about 67% of the 1,3-disilacyclobutanes obtained arise from silene dimerization while 33% are formed by silenoid coupling. In hexane the order of reactivity for addition, t-Bu > sec-Bu > n-Bu, is opposite that for fluorine substitution. The vinyl substitution is most significant with secondary alkyllithium reagents including the tert-butyllithium adduct to dimethylfluorovinylsilane and with sec-butyllithium itself. Evidence for the formation of vinyllithium or ethylene in the process could not be obtained.
Date: August 1985
Creator: Cheng, Albert Home-Been

A Study of the Processing Properties of Hard-Particle Reinforced Composite Solders

Description: The microstructural, mechanical and thermal properties of various composite solder formulations were investigated. Special interest was given in observing the processing properties, microstructural characteristics, fatigue behavior, tensile strength, and the effect of environmental ageing on the composite solder formulations. The solderability parameters wetting and speed of soldering, reflow temperature, and the thermal stability of the resulting composite solder were also examined.
Date: May 1994
Creator: Calderon, Jose Guadalupe