Chemistry Commons^™

Synthesis Of Cyclopropanes Via Organoiron Methodology: Stereoselective Preparation Of Cis-2-(2’-Carboxycyclopropyl)Glycine, Nathaniel J Wallock, William A. Donaldson

Chemistry Faculty Research and Publications

A stereoselective route to cis-2-(2′-carboxycyclopropyl)glycine has been developed. exo-Nucleophilic addition to the (bicyclo[5.1.0]octadienyl)iron(1+) cation establishes the relative stereochemistry at the cyclopropane ring and the α-stereocenter. Subsequent removal of the metal and cleavage of the cyclic diene gave the protected target 10, which upon hydrolysis gave 1.

A stereoselective route to cis-2-(2′-carboxycyclopropyl)glycine has been developed. exo-Nucleophilic addition to the (bicyclo[5.1.0]octadienyl)iron(1+) cation establishes the relative stereochemistry at the cyclopropane ring and the α-stereocenter.

Electrochemistry And Spectroscopy Of Sulfate And Thiosulfate Complexes Of Iron Porphyrins, Philip Walter Crawford, Michael D. Ryan

Chemistry Faculty Research and Publications

The electrochemical and spectroscopic properties of the complex formed by the addition of thiosulfate to ferric porphyrins were examined. The NMR spectrum of the thiosulfate–ferric porphyrin complex was consistent with a high-spin ferric complex, while the EPR spectrum at liquid nitrogen temperatures indicated that the complex under these conditions was low-spin. Such behavior has been previously observed for other ferric porphyrin complexes. The visible spectra were characterized by a shift in the Soret band to higher energies, with smaller changes in the longer wavelength region. The complex was reasonably stable in DMF, but slowly reduced over several hours to Fe …

Molecular Structures Of The Metastable Charge-Transfer Complexes Of Benzene (And Toluene) With Bromine As The Pre-Reactive Intermediates In Electrophilic Aromatic Bromination, Alexandr V. Vasilyev, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

Successful crystallization and X-ray crystallographic analyses of the highly metastable (1∶1) complexes of bromine with benzene and toluene establish the unique (localized) structure B that differs in notable ways from the long-accepted (delocalized) structure A. Furthermore, we demonstrate the (highly structured) charge-transfer complexes [C₆H₆,Br₂] and [CH₃C₆H₅,Br₂] to be the pre-reactive intermediates that are converted (via an overall Br⁺ transfer) to the Wheland intermediates in electrophilic aromatic bromination. The role of the dative ion pairs [C₆H₆˙⁺ Br₂˙ …

Overexpression And Divalent Metal Binding Properties Of The Methionyl Aminopeptidase From Pyrococcus Furiosus, Lu Meng, Shane Ruebush, Ventris M. D'Souza, Alicja J. Copik, Susumu Tsunasawa, Richard C. Holz

Chemistry Faculty Research and Publications

The gene encoding for the methionyl aminopeptidase from the hyperthermophilic archaeon Pyrococcus furiosus (PfMetAP-II; EC 3.4.11.18) has been inserted into a pET 27b(+) vector and overexpressed in Escherichia coli. The new expression system resulted in a 5-fold increase in purified enzyme obtained from a 5 L fermentor growth. The as-purified PfMetAP-II enzyme, to which no exogenous metal ions or EDTA was added, was found to have 1.2 equiv of zinc and 0.1 equiv of iron present by ICP-AES analysis. This enzyme had a specific activity of 5 units/mg, a 60-fold decrease from the fully loaded Fe(II) …

Silicon-Methoxide-Modified Clays And Their Polystyrene Nanocomposites, Jin Zhu, Paul Start, Kenneth A. Mauritz, Charles A. Wilkie

Chemistry Faculty Research and Publications

Silicon-methoxide-containing modified clays were obtained through cationic exchange between the sodium clay and the ammoniumcation of [3-(trimethoxysilyl) propyl]octadecyldimethylammoniumchloride (Si18). The nanocomposites were prepared through bulk polymerization of styrene in which the Si18 clay was dispersed. The silicon-methoxide offers the possibility of reaction between the methoxide and a clay hydroxyl group to link together the cation and the clay. The nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. Their thermal stability and flame retardancy were measured by thermogravimetric analysis (TGA) and cone calorimetry. Linkage between the silicon and the clay apparently occurs in the clay but …

Fire Retardant Halogen-Antimony-Clay Synergism In Polypropylene Layered Silicate Nanocomposites, Marco Zanetti, Giovanni Camino, Domenico Canavese, Alexander B. Morgan, Frank K. Lamelas, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polypropylene-graft-maleic anhydride with organically modified clays have been prepared and characterized by X-ray diffraction and transmission electron microscopy. Their combustion behavior has been evaluated using oxygen consumption cone calorimetry. Synergy is observed between the nanocomposite formed and conventional vapor phase fire retardants, such as the combination of decabromodiphenyloxide and antimony oxide. The presence of bromine and antimony does not affect the heat release rate curves of the virgin polymer.

A Comparison Of Various Methods For The Preparation Of Polystyrene And Poly(Methyl Methacrylate) Nanocomposites, Dongyan Wang, Jin Zhu, Qiang Yao, Charles A. Wilkie

Chemistry Faculty Research and Publications

Polymer−clay nanocomposites of styrene and methyl methacrylate have been prepared by bulk, solution, suspension, and emulsion polymerization as well as by melt blending. Two different organic modifications of montmorillonite have been used:  one contains a styryl monomer on the ammonium ion while the other has no double bond. The organic modification as well as the mode of preparation determines if the material will be exfoliated or intercalated. Exfoliation is more likely to occur if the ammonium ion contains a double bond which can participate in the polymerization reaction, but the mere presence of this double bond is not sufficient to …

Preparation Of Pvc- Clay Nanocomposites By Solution Blending, Dongyan Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

PVC-clay nanocomposites have been prepared by solution blending of the polymer with both sodium montmorillonite and an organically modified clay. The nanocomposites have been characterized by X-ray diffraction and transmission electron microscopy; in all cases the materials are best described as mixed immiscible—intercalated nanocomposites, Thermogravimetric analysis shows that, in some cases, the onset temperature of the degradation is enhanced, and the fraction of nonvolatile residue is always decreased. This decrease in the amount of char indicates a significant change in the degradation pathway of the polymer, caused by the presence of the clay. For some preparations, the tensile strength and …

Three-Dimensional Structure Of The Complexin/Snare Complex, Xiaocheng Chen, Diana R. Tomchick, Evgenii L. Kovrigin, Demet Araç, Mischa Machius, Thomas C. Südhof, Josep Rizo

Chemistry Faculty Research and Publications

During neurotransmitter release, the neuronal SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 form a four-helix bundle, the SNARE complex, that pulls the synaptic vesicle and plasma membranes together possibly causing membrane fusion. Complexin binds tightly to the SNARE complex and is essential for efficient Ca²⁺-evoked neurotransmitter release. A combined X-ray and TROSY-based NMR study now reveals the atomic structure of the complexin/SNARE complex. Complexin binds in an antiparallel α-helical conformation to the groove between the synaptobrevin and syntaxin helices. This interaction stabilizes the interface between these two helices, which bears the repulsive forces between the apposed membranes. These results …

Hydrolysis Of Thionopeptides By The Aminopeptidase From Aeromonas Proteolytica:  Insight Into Substrate Binding, David L. Bienvenue, Danuta M. Gilner, Richard C. Holz

Chemistry Faculty Research and Publications

A series of l-leucine aniline analogues were synthesized that contained either a carbonyl or thiocarbonyl as a part of the amide bond. Additionally, the para-position on the phenyl ring of several substrates was altered with various electron-withdrawing or donating groups. The kinetic constants K_m and k_cat were determined for the hydrolysis of each of these compounds in the presence of the aminopeptidase from Aeromonas proteolytica (AAP) containing either Zn(II) or Cd(II). The dizinc(II) form of AAP ([ZnZn(AAP)]) was able to cleave both carbonyl and thiocarbonyl containing peptide substrates with similar efficiency. However, the dicadmium(II) form of AAP …

Steric Hindrance As A Mechanistic Probe For Olefin Reactivity:  Variability Of The Hydrogenic Canopy Over The Isomeric Adamantylideneadamantane/Sesquihomoadamantene Pair (A Combined Experimental And Theoretical Study), Rajendra Rathore, Sergey V. Lindeman, C.-J. Zhu, T. Mori, R. Schleyer, Jay K. Kochi

Chemistry Faculty Research and Publications

Access to each CC face of adamantylideneadamantane (AA) and sesquihomoadamantene (SA) is hindered by the hydrogenic canopy consisting of four β-hydrogens; otherwise, these olefins have quite normal environments. X-ray crystallography and density functional (DFT) calculations show a 0.5 Å larger annular opening in the protective cover of AA than that in SA. This contributes to the remarkable differences in reactivity toward various reagents, not only by limiting access to the olefin site in SA but also by inhibiting reactions which force these hydrogens closer together. Thus, AA is subject to typical olefin-addition reactions with bromine, …

Novel Bis-Arene (Sandwich) Complexes With No+ Acceptor. Isolation, X-Ray Crystallography And Electronic Structure, Sergiy V. Rosokha, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

The unusual charge-transfer complexes of various arene donors (ArH) with the nitrosonium cation (NO⁺) resulting from bimolecular [1 ∶ 1] associations can be extended at suitably high ArH concentrations to termolecular processes leading to the analogous [2 ∶ 1] complexes. Spectral analyses of the intense color changes accompanying the arene interaction with NO⁺ provide optimum conditions for the isolation of pure crystalline ternary complexes. Single crystal X-ray crystallographic determinations establish the unique sandwich structure consisting of the NO moiety interposed (parallel) between a pair of cofacial arene donors—reminiscent of the well-known transition metal sandwich complexes with aromatic …

Polystyrene/Graphite Nanocomposites: Effect On Thermal Stability, Fawn Marie Uhl, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites consisting of polymer and clay have been shown to exhibit a significant reduction in flammability and an increase in mechanical properties. This work examines the effect of thermal stability and mechanical properties of nanocomposites prepared from potassium graphite and styrene. Synthesis of nanocomposites was accomplished by using potassium graphite (KC₈) as the initiator in the polymerization of styrene. A slight increase in thermal stability is observed but mechanical properties are decreased.

An Xps Investigation Of Thermal Degradation And Charring Of Pmma Clay Nanocomposites, Jianxin Du, Jin Zhu, Charles A. Wilkie, Jiangqi Wang

Chemistry Faculty Research and Publications

Poly(methyl methacrylate)–clay nanocomposites have been studied using X-ray photoelectron spectroscopy. It is clear that as the polymer undergoes thermal degradation, the clay accumulates at the surface and the barrier properties which result from this clay accumulation have been described as the reason for the decreased heat release rate for nanocomposites. The surface composition of the clay changes as the nanocomposite is heated and the changes are affected by the organic-modification that were applied to the clay in order to prepare the nanocomposite.

An Xps Study Of The Thermal Degradation Of Polystyrene-Clay Nanocomposites, Jianqi Wang, Jianxin Du, Jin Zhu, Charles A. Wilkie

Chemistry Faculty Research and Publications

X-ray photoelectron spectroscopy, XPS, has been used to examine several polystyrene-clay nanocomposites. The accumulation of oxygen, from the almuniosilicate, on the surface of the polymer was observed, along with the loss of carbon. This confirms that the barrier properties of the clay provide a mechanism by which nanocomposite formation can enhance the fire retardancy of the polymers. No difference is detected depending upon the extent of exfoliation or intercalation of the nanocomposite. #2002 Elsevier Science Ltd. All rights reserved.

Thermal Stability And Flame Retardancy Of Poly(Methyl Methacrylate)-Clay Nanocomposites, Jin Zhu, Paul Start, Kenneth A. Mauritz, Charles A. Wilkie

Chemistry Faculty Research and Publications

Three ammonium salts, hexadecylallyldimethyl ammonium chloride (Allyl16), hexadecylvinylbenzyldimethyl ammonium chloride (VB16) and hexadecylvinylbenzyldimethyl ammonium chloride (Bz16) were synthesized and ion exchanged onto montmorillonite. Poly(methyl methacrylate)–clay nanocomposites of all three clays were prepared by bulk polymerization and the resulting nanocomposites were characterized by X-ray diffraction and transmission electron microscopy. The clays which contain a pendant double bond are more likely to give an exfoliated material while that which contains no double bond is intercalated. The thermal stability and flame retardancy were measured by thermogravimetric analysis (TGA) and Cone Calorimetry respectively.

Structural Evidence That The Methionyl Aminopeptidase From Escherichia Coli Is A Mononuclear Metalloprotease, Nathaniel J. Cosper, Ventris M. D'Souza, Robert A. Scott, Richard C. Holz

Chemistry Faculty Research and Publications

The Co and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra of the methionyl aminopeptidase from Escherichia coli (EcMetAP) have been recorded in the presence of 1 and 2 equiv of either Co(II) or Fe(II) (i.e., [Co(II)_(EcMetAP)], [Co(II)Co(II)(EcMetAP)], [Fe(II)_(EcMetAP)], and [Fe(II)Fe(II)(EcMetAP)]). The Fourier transformed data of both [Co(II)_(EcMetAP)] and [Co(II)Co(II)(EcMetAP)] are dominated by a peak at ca. 2.05 Å, which can be fit assuming 5 light atom (N,O) scatterers at 2.04 Å. Attempts to include a Co−Co interaction (in the 2.4−4.0 Å range) in the …

Synthesis Of Cyclopropane Containing Natural Products, William A. Donaldson

Chemistry Faculty Research and Publications

The synthesis of select cyclopropane containing natural products is presented.

Reactivity Of Acyclic (Pentadienyl)Iron(1+) Cations With Weak Carbon Nucleophiles, M. Azad Hossain, Myung-Jong Jin, William A. Donaldson

Chemistry Faculty Research and Publications

The reaction of acyclic tricarbonyl(pentadienyl)iron(1+) cations with allyl trimethylsilane or with excess furan leads to (E,E-diene)iron complexes. Attack of these weak nucleophiles on the transoid form of the pentadienyl cation is presumably faster than attack on the more stable cisoid form.

Cross-Linking Of Polystyrene By Friedel-Crafts Chemistry: Reaction Of P-Hydroxymethylbenzyl Chloride With Polystyrene, Hongyang Yao, Michael A. Mckinney, Caroline Dick, John J. Liggat, C.E. Snape, Charles A. Wilkie

Chemistry Faculty Research and Publications

p-Hydroxymethylbenzyl chloride was found to be an effective cross-linking agent for polystyrene. The reaction was found to occur by Friedel–Crafts alkylation between the benzyl alcohol/chloride functional groups in the additive and phenyl ring in polystyrene. The reaction was studied by TGA–IR to monitor the evolution of hydrogen chloride and water, and the structure of the resultant gel was analyzed by solid state NMR and elemental analysis. The potential application in flame-retardancy was evaluated using Cone calorimetry.

An Efficient And Simple Synthesis Of Optically Pure Tricarbonyl(Methyl 6-Oxo-2,4-Hexadienoate)Iron, Kamil Godula, Heiko Bärmann, William A. Donaldson

Chemistry Faculty Research and Publications

No abstract provided.

Synthesis And Reactivity Of Tricarbonyl(1-Ethoxy-Carbonyl-2-Methylpentadienyl)Iron(1+) Cation, Young K. Yun, Heiko Bärmann, William A. Donaldson

Chemistry Faculty Research and Publications

The title cation was prepared in two steps from the known (ethyl 3-methyl-6-oxo-2,4-hexadienoate)Fe(CO)3. Reation of the cation with NaBH3CN, methyl cuprate, phthalimide, water, PPh3, or malonate anions gave predominantly the products from nucleophilic attack at the C5 pentadienyl carbon.

Intramolecular (Electron) Delocalization Between Aromatic Donors And Their Tethered Cation–Radicals. Application Of Electrochemical And Structural Probes, Duoli Sun, Sergey V. Lindeman, Rajendra Rathore, Jay K. Kochi

Chemistry Faculty Research and Publications

To study the mechanism of electronic transduction along (poly)phenylene chains, a series of aromatic donors with general formula D–B–D has been synthesized [where D = 2,5-dimethoxy-4-methylphenyl donor and B = (poly)phenylene bridge]; and the corresponding cation–radical salts D–B–D⁺˙ SbCl₆⁻ have been isolated for X-ray crystallographic analyses. The magnitude of the electronic interaction between the D and D⁺˙ moieties through the various B bridges has been measured (i) as the difference between the first and the second oxidation potentials of D–B–D donors and (ii) as the structural changes induced in neutral D by the presence …

Thermal Degradation Of Pvc In The Presence Of Polystyrene, Qiang Yao, Charles A. Wilkie

Chemistry Faculty Research and Publications

The thermal degradation of poly(vinyl chloride) (PVC) and blends of PVC with polystyrene has been studied using thermogravimetric analysis coupled to Fourier transform infrared spectroscopy. The degradation of PVC commences at weak links by an ionic pathway, but there is evidence to suggest the presence of a radical pathway at high temperatures. While there is some chemical interaction between PVC and polystyrene, the principal mode of stabilization is a physical process.

Electron Redistribution Of Aromatic Ligands In (Arene)Cr(Co)3 Complexes. Structural (Bond-Length) Changes As Quantitative Measures, P. Le Maguères, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

Arene ligands experience significant ring expansion upon coordination with chromium tricarbonyl, as established by precise X-ray crystallographic analyses of various (η⁶-arene)Cr(CO)₃ complexes. Such changes in ligand structures result from the charge (electron) redistribution, Ar⁺−Cr^-, upon arene coordination, since they are closely related to those found in the intermolecular 1:1 complexes of the corresponding series of arenes with nitrosonium cation (NO⁺). The latter are prototypical examples of charge-transfer complexes as described by Mulliken. As such, they show enhanced degrees of charge (electron) transfer that approach unity, which is confirmed by quantitative comparison …

Noncovalent Binding Of The Halogens To Aromatic Donors. Discrete Structures Of Labile Br2 Complexes With Benzene And Toluene, Alexandr V. Vasilyev, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

Precise molecular structures resulting from the noncovalent interaction of Br₂ with benzene (and toluene) reveal the unusual localized bonding to specific (one or two) carbon centers in prereactive complexes leading directly to the transition states for electrophilic aromatic brominations.

Photooxidation Of Polymeric-Inorganic Nanocomposites: Chemical, Thermal Stability And Fire Retardancy Investigations, Adams Tidjani, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polypropylene-graft-maleic anhydride/clay and polypropylene/clay were prepared by melt blending using two different approaches. X-Ray diffraction results showed an intercalated structure. Samples of nanocomposites were exposed to UV light under atmospheric oxygen and their photo-oxidative stability was studied using FTIR and UV spectroscopy. The consequences of this photo-oxidation on the thermal stability and fire retardant performance of the nanocomposites were also addressed from thermogravimetry analysis and Cone calorimetry.

An Xps Investigation Of Thermal Degradation And Charring Of Cross-Linked Polyisoprene And Polychloroprene, Jianwei Hao, Charles A. Wilkie, Jiangqi Wang

Chemistry Faculty Research and Publications

Our interest in butadiene-containing polymers had led to an investigation of the thermal degradation of polyisoprene, PIP, and polychloroprene, PCP. The connection between cross-linking and thermal stability through an examination of PIP and PCP has been reported. Like the course of thermogravimetric analysis (TGA) the cross-linking and charring of polymers subjected to heat can also be experimentally observed as function of temperatures by the pseudo-in-situ XPS (X-ray Photoelectron Spectroscopy). Data acquisition of C1s spectra as function of temperature permits us to explore: (1) the extent of cross-linking and/or carbon accumulation of systems of PCP and PIP with/without initiators, BPO and …

Synthesis Of Cyclopropanes Via Organoiron Methodology: Preparation Of 2-(2′-Carboxy-3′-Ethylcyclopropyl)Glycine, Kamil Godula, William A. Donaldson

Chemistry Faculty Research and Publications

A route to 1,2,3-trisubstituted cyclopropanes has been developed. The relative stereochemistry at the three cyclopropane centers is established by nucleophilic attack on the pentadienyl ligand on the face opposite to iron and subsequent oxidatively induced reductive elimination with retention of configuration. This methodology was applied to the synthesis of 2-(2′-carboxy-3′-ethylcyclopropyl)glycines. The diastereomeric glycine dimethyl esters are separable as their diphenylmethylene imines.

The conformationally restricted glutamate analogs (2S,1′S,2′S,3′R)- and (2R,1′S,2′S,3′R)-ECCG's 5a and5b were prepared in five steps (17 and 15% yield, respectively) from (pentadienyl)Fe(CO) …

Fire Retardancy In 2001, Gordon L. Wilson, Charles A. Wilkie

Chemistry Faculty Research and Publications

Fire is a world-wide problem which claims lives and causes significant loss of property. Some of the problems are discussed and the solution delineated. This peer-reviewed volume is designed to be as the state-of-the-art. This chapter provides a perspective for current work.