Martha D. Morton

Synthesis, structure, and stability in acid of copper(II) and zinc(II) complexes of cross-bridged tetraazamacrocycles

New copper(II) and zinc(II) complexes of cross-bridged tetraazamacrocyclic ligands derived from 1,4,8,11-tetraazacyclotetradecane (cyclam), 1,4,7,10-tetraazacyclotridecane ([13]aneN4), and 1,4,7,10-tetraazacyclododecane (cyclen) have been synthesized. The X-ray crystal structures of the two cyclen-derived complexes have been determined and show both metal ions to be in distorted octahedral environments with the ligand in a cis -folded conformation. The bridged-ligand complexes are remarkably stable kinetically under harsh acidic conditions, as seen from their dissociation reactions. The electrochemical and spectroscopic properties of the Cu 2 complexes have also been investigated. # 2003 Elsevier Science B.V. All rights reserved.

meso-Tetrakis(pentafluorophenyl)porphyrin-Derived Chromene-Annulated Chlorins

The synthesis of mono- and bis-chromene-annulated meso-(pentafluorophenyl)chlorins from meso-tetrakis(pentafluorophenyl)porphyrins by an OsO(4)-mediated dihydroxylation reaction, followed by an intramolecular nucleophilic aromatic substitution reaction, is described. The reaction sequence is applicable to the free base systems as well as their Zn(II), Ni(II), Pd(II), and Pt(II) complexes. The optical properties (UV-vis and fluorescence spectra) of the (metallo)chlorin-like chromophores that possess slightly red-shifted optical spectra compared to the corresponding 2,3-dihydroxychlorins are reported. Molecular modeling and (1)H-(19)F-HOESY NMR spectroscopy provide indications for the conformation of the chromene-annulated chromophores. Using (1)H-(1)H COSY and (19)F-(19)F QF-COSY NMR spectra, we interpret the (1)H and (19)F NMR spectra of the porphyrins and chlorins, thus providing a refined reference point for the use of (19)F NMR spectroscopy as a diagnostic tool in the analysis of meso-pentafluorophenyl-substituted porphyrinoids.

The role of two-component catalysts containing chelating bisaryloxide ligands in controlling the stereochemistry of the metathesis polymerization of norbornene

Abstract The ring-opening metathesis polymerization of norbornene and 5,5-dimethylnorbornene has been studied with a range of tungsten(VI) ring-opening metathesis polymerization procatalysts containing chelating diolate ligands, including catechols, 1,8-dihydroxynaphthalene, 1,1′-bi-2-naphthol, biphenanthrol and bis(2-hydroxyphenyl) methane. One class of these catalysts, W(X)(OArO)Cl 2 (THF) (X = O or NArMe 2 ) was produced via alcoholysis reactions between W(X)Cl 4 and the respective diol. Polymers of norbornene produced with these catalysts showed a reasonable correlation between ring size and cis -selectivity. This effect is most likely steric in origin. Analyses of the tacticity of poly-5,5-dimethylnorbornene showed a correlation betwen high cis -olefin content and syndiotacticity, even when produced with catalysts containing asymmetric chelating diolate ligands.

Thiophene-Based Double Helices: Syntheses, X-ray Structures, and Chiroptical Properties

We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (β,β-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C-C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG(‡) > 48 kcal mol(-1), which may be compared to DFT-computed barrier of 51 kcal mol(-1). The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon-sulfur [7]helicene, as determined by the anisotropy factor g = Δε/ε = -0.039, based on Δεmax = -11 and ε = 2.8 × 10(2) L mol(-1) cm(-1) in cyclohexane at 327 nm.

A 19F NMR study of lomefloxacin in human erythrocytes and its interaction with hemoglobin

19F NMR spectroscopy of a model fluoroquinolone, lomefloxacin, in an erythrocyte suspension showed separate resonances for the intra- and extra-cellular compartments. The intra-cellular peak revealed significant line broadening of the fluorine signals of lomefloxacin. Line broadening also occurred in the presence of oxyhemoglobin (HbO2), hematin, globin and iron. This evidence indicated that lomefloxacin interacted with these compounds; however, ultrafiltration experiments indicated that there was only weak binding (5%) of lomefloxacin to HbO2. 19F and 31P NMR spectroscopy revealed that lomefloxacin may compete with 2,3-diphosphoglycerate for its binding site on HbO2. An apparent partition coefficient of 1.90 +/- 0.15 was observed for lomefloxacin in human erythrocytes, utilizing LC analysis.

Hydrogen bonding in tungsten(VI) salicylate free acids

Abstract Hydrogen-bonded free acid dimers of the formula [W(X)Cl3(Hsal–R)]2 (where X=O (1), NC6H3-2,6-Me2 (2), Ph2C2 and Hsal–R=substituted salicylate monoanion) are prepared through reactions between W(X)Cl4 precursors and functionalized salicylic acids (H2sal–R). Download : Download full-size image The free acids are stable at ambient temperature and exist as dimers in solution, although the structure of the dimer is not known. Spectroscopic studies show that the electronic characteristics of the π-donor ligands directly affect the electronic environments of the carboxylate functionality. This influence is observed in weaker binding constants of diethyl ether (KX) as the X ligand becomes more strongly π donating: O⩾NC6H3–2,6-Me2>Ph2C2. In other words, the oxo and aryl imido species are more acidic than the diphenylacetylene compounds in the Bronsted sense owing to the higher Lewis acidity of their tungsten centers. Salicylate adducts of the type W(X)Cl3(Hsal–R)⋯OR2′ (X=O (3), NC6H3–2,6-Me2 (4)) and W(X)Cl3(Hsal–R)⋯NEt3 (X=O (5), NC6H3–2,6-Me2 (6)) have been isolated and characterized. A comparison of two structures, W(NC6H3–2,6-Me2)Cl3(Hsal–3-Me⋯L) where L=OEt2 (4c) and NEt3 (6c), shows that 6c has more charge localization on both the carboxylate group and tungsten center than 4c. The charge separation in amine adducts (i.e. salts) contributes to association of these salts with the free acid (i.e. W(X)Cl3(Hsal–R⋯L)⋯[W(X)Cl3(Hsal–R)]). The strong hydrogen bonding exhibited by the free acids leads to the formation of supramolecular complexes organized around poly(ether) templates, including dimethoxyethane (in 3e) and 18-crown-6.

Hydrolysis of the Prodrug, 2′, 3′, 5′ -Triacetyl-6-azauridine

AbstractPurpose. The purposes were to study the kinetics of hydrolysis of 2′,3′,5′-triacetyl-6-azauridine (1) in aqueous solution (µ = 0.5) and to identify the main intermediates and products of the reaction. Methods. A stability indicating isocratic LC assay was used to study the rate of degradation of 1 A gradient LC assay was used to study the time courses of the degradants. The products of hydrolysis were isolated by preparative liquid chromatography and identified by 1H-NMR and CI-MS. The pKa value was obtained by potentiometric titration. Results. At 36.8°C, the pH-rate profile of 1 in water was adequately described by a four-term rate equation. The intermediates were identified as the primary and secondary di-acetates, and the primary and secondary mono-acetates. The final product was 6-azauridine. Conclusions. A simplified kinetic scheme could be used to describe the concentration-time profiles of 1, the intermediates and the final product.

Dimetallacyclobutadiene derivatives: Catalysts or catalyst precursors for the ROMP of norbornene?

Abstract Metallacyclobutadiene complexes with the formula (Me3SiCH2)4M2(μ-CSiMe3)2 (M = Ta or W) induce the ring-opening metathesis polymerization (ROMP) of norbornene. The tungsten compound acts as a stand-alone catalyst, polymerizing norbornene with a first-order dependence on both W2 and monomer. In contrast, the tantalum complex requires oxygen as an initiator, and generates an as yet undefined catalyst that shows a zero-order dependence on the monomer concentration.

Preparation of Some Homologous TEMPO Nitroxides and Oxoammonium Salts; Notes on the NMR Spectroscopy of Nitroxide Free Radicals; Observed Radical Nature of Oxoammonium Salt Solutions Containing Trace Amounts of Corresponding Nitroxides in an Equilibrium Relationship

Three new homologous TEMPO oxoammonium salts and three homologous nitroxide radicals have been prepared and characterized. The oxidation properties of the salts have been explored. The direct 13C NMR and EPR spectra of the nitroxide free radicals and the oxoammonium salts, along with TEMPO and its oxoammonium salt, have been successfully measured with little peak broadening of the NMR signals. In the spectra of all ten compounds (nitroxides and corresponding oxoammonium salts), the carbons in the 2,2,6,6-tetramethylpiperidine core do not appear, implying paramagnetic properties. This unpredicted overall paramagnetism in the oxoammonium salt solutions is explained by a redox equilibrium as shown between oxoammonium salts and trace amounts of corresponding nitroxide. This equilibrium is confirmed by electron interchange reactions between nitroxides with an N-acetyl substituent and oxoammonium salts with longer acyl side chains.

A Benzylation Study of 2,3-Dichloronaphthazarin

Abstract Depending on different reaction time and temperature employed, benzylation of 2,3-dichloronaphthazarin (I) using silver oxide as the catalyst can form either exclusively 5,8-dibenzyloxy-6,7-dichloronaphthalene-1,4-dione (II), or exclusively 5-benzyloxy-2,3-dichloro-8-hydroxynaphthalene-1,4-dione (IV); or a mixture of II and 5,8-dibenzyloxy-2,3-dichloronaphthalene-1,4-dione (III). Structures of these compounds were identified by nmr analysis.