RESUMEN
In this work, five cerium(IV) complexes were synthesized, three of which were structural isomorphs from the same pyrasal ligand with the solid-state result identified by structural analysis dependent on the initial pH of the reaction solution and the temperature at which the reaction is performed. The ligands explored here are pyrasal ligands, which are Schiff-base ligands formed by the condensation of 2,3-diaminopyrazine and a salicylaldehyde derivative. Pyrasal ligands have weaker binding than other salophen-type ligands due to the electron-withdrawing effect of the nitrogen atoms contained within the pyrazine ring. The weaker binding leaves the ligand more susceptible to the changes in pH and temperature that alternate the chelating environment from 8- to 10-coordinate. This electron-withdrawing effect of the pyrazine backbone also deactivates the second amine after the first condensation addition of salicylaldehyde. Without a metal to template the complex formation reaction, even with extended reaction times and the addition of a large excess of ligand, the result is the addition of only one salicylaldehyde.
RESUMEN
The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis.
RESUMEN
A new naphthylsalophen and its 3 : 2 ligand-to-lanthanide sandwich-type complexes were isolated. When excited at 380 nm, the complexes display the characteristic metal-centred emission for NdIII, ErIII and YbIII. Upon 980 nm excitation, in mixed lanthanide and the Er complexes, Er-centred upconversion emission at 543 and 656 nm is observed, with power densities as low as 2.18 W cm-2.
RESUMEN
Derivatives of a novel pyrrole-containing Schiff base ligand system (called "pyrrophen") are presented which feature substituted phenylene linkers (R1 = R2 = H (H2L1); R1 = R2 = CH3 (H2L2)) and a binding pocket modeled after macrocyclic species. These ligands bind neutral CH3OH in the solid state through pyrrolic hydrogen-bonding. The interaction of the uranyl cation (UO22+) and H2L1-2 yields planar hexagonal bipyramdial uranyl complexes, while the Cu2+ and Zn2+ complexes were found to self-assemble as dinuclear helicate complexes (M2L2) with H2L1 under identical conditions. The favorable binding of UO22+ over Zn2+ provides insight into the molecular recognition of uranyl over other metal species. Structural features of these complexes are examined with special attention to features of the UO22+ coordination environment which distinguish them from other related salophen and porphyrinoid complexes.
RESUMEN
The total synthesis of the lignan-based cyclobutane di-O-methylendiandrin A has been achieved using diastereoselective, vicinal alkylation and transannular McMurry reactions of a macrocyclic 1,4-diketone as key transformations for establishing relative stereochemistry and furnishing the strained 4-membered ring of the natural product.
RESUMEN
Methanesulfonyl iodide is produced in aqueous solutions from the reaction of triiodide with methanesulfinate. Dichroic crystals of (CH3SO2I)4·KI3·2I2 are formed from KI/I2 solutions with high concentrations of CH3SO2-, while dichroic crystals of (CH3SO2I)2·RbI3 are formed from RbI/I2 solutions. X-ray crystallography of these two compounds shows that the CH3SO2I molecules coordinate through their oxygen atoms to the metal cations and that the S-I bond length is 2.44 Å. At low concentrations of CH3SO2-, the solutions remain homogeneous and the sulfonyl iodide is formed in a rapid equilibrium: CH3SO2- + I3- â CH3SO2I + 2I-, KMSI = 1.07 ± 0.01 M at 25 °C (µ = 0.1 M, NaClO4). The sulfonyl iodide solutions display an absorbance maximum at 309 nm with a molar absorptivity of 667 M-1 cm-1. Stopped-flow studies reveal that the equilibrium is established within the dead time of the instrument (â¼2 ms). Solutions of CH3SO2I decompose slowly to form the sulfonate: CH3SO2I + H2O â CH3SO3- + I- + 2H+, khyd. In dilute phosphate buffer, this decomposition occurs with khyd = 2.0 × 10-4 s-1; the decomposition rate shows an inverse-squared dependence on [I-] because of the KMSI equilibrium.
RESUMEN
Diastereoselective synthesis of γ-boryl substituted syn-ß-alkoxy- or syn-ß-amino-homoallylic alcohols is developed. Pt-catalyzed regioselective diboration of alkoxyallene or aminoallene with B2pin2 occurred at the terminal alkene unit of the allene to give ( Z)-γ-alkoxy- or ( Z)-γ-amino-ß-boryl substituted allylboronates with high selectivities. Addition of the allylboronates to aldehydes followed by protection of the resulting secondary hydroxyl group gave TES-protected syn-1,2-diols and syn-1,2-amino alcohols with high diastereoselectivities. The vinyl-Bpin group in the products is a useful handle for further transformations.
RESUMEN
Reactive oxygen species are integral to many physiological processes. Although their roles are still being elucidated, they seem to be linked to a variety of disorders and may represent promising drug targets. Mimics of superoxide dismutases, which catalyse the decomposition of O2â¢- to H2O2 and O2, have traditionally used redox-active metals, which are toxic outside of a tightly coordinating ligand. Purely organic antioxidants have also been investigated but generally require stoichiometric, rather than catalytic, doses. Here, we show that a complex of the redox-inactive metal zinc(II) with a hexadentate ligand containing a redox-active quinol can catalytically degrade superoxide, as demonstrated by both reactivity assays and stopped-flow kinetics studies of direct reactions with O2â¢- and the zinc(II) complex. The observed superoxide dismutase catalysis has an important advantage over previously reported work in that it is hastened, rather than impeded, by the presence of phosphate, the concentration of which is high under physiological conditions.
RESUMEN
The synthesis of 1,2,4,5- and 1,2,9,10-tetrasubstituted and 1,2,4,5,8-pentasubsutituted pyrenes has been achieved by initially functionalizing the K-region of pyrene. Bromination, acylation, and formylation reactions afford high to moderate levels of regioselectivity, which facilitate the controlled introduction of other functional groups about 4,5-dimethoxypyrene. Access to 4,5-dimethoxypyren-1-ol and 9,10-dimethoxypyren-1-ol enabled a rare, C-2 primary alkyl substitution of pyrene.
RESUMEN
The overproduction of reactive oxygen species has been linked to a wide array of health disorders. The ability to noninvasively monitor oxidative stress in vivo could provide substantial insight into the progression of these conditions and, in turn, could facilitate the development of better diagnosis and treatment options. A mononuclear Mn(II) complex with the redox-active ligand N,N'-bis(2,5-dihydroxybenzyl)-N,N'-bis(2-pyridinylmethyl)-1,2-ethanediamine (H4qtp2) was made and characterized. A previously prepared Mn(II) complex with a ligand containing a single quinol subunit was found to display a modest T1-derived relaxivity response to H2O2. The introduction of a second redox-active quinol both substantially improves the relaxivity response of the complex to H2O2 and reduces the cytotoxicity of the sensor but renders the complex more susceptible to transmetalation. The addition of H2O2 partially oxidizes the quinol subunits to para-quinones, concomitantly increasing the r1 from 5.46 mM-1 s-1 to 7.17 mM-1 s-1. The oxidation of the ligand enables more water molecules to coordinate to the metal ion, providing an explanation for the enhanced relaxivity. That the diquinol complex is only partially oxidized by H2O2 is attributed to its activity as an antioxidant; the complex can both catalytically degrade superoxide and serve as a hydrogen atom donor.
Asunto(s)
Antioxidantes/farmacología , Medios de Contraste/química , Peróxido de Hidrógeno/química , Hidroquinonas/química , Manganeso/farmacología , Compuestos Organometálicos/farmacología , Animales , Antioxidantes/síntesis química , Antioxidantes/química , Muerte Celular/efectos de los fármacos , Línea Celular , Proliferación Celular/efectos de los fármacos , Cristalografía por Rayos X , Imagen por Resonancia Magnética , Manganeso/química , Modelos Moleculares , Estructura Molecular , Compuestos Organometálicos/síntesis química , Compuestos Organometálicos/química , Oxidación-Reducción , RatasRESUMEN
The tetradentate ligand N,N'-dibenzyl-N,N'-bis(2-pyridylmethyl)-1,2-cyclohexanediamine (bbpc) was used to prepare cobalt(II) diacetonitrilo and cobalt(III) peroxo complexes, the latter of which was structurally characterized. The cobalt(III) peroxo compound forms from reactions between the cobalt(II) complex, hydrogen peroxide, and a base, and it stoichiometrically reacts with aldehydes to yield mixtures of alkenes and ketones. The cobalt(II) precursor is capable of catalyzing the activation of weak C-H bonds by either iodosobenzene or m-chloroperbenzoic acid. This chemistry differs from most previously characterized cobalt-mediated C-H activation in that (1) it is catalytic, rather than stoichiometric, with respect to the cobalt and (2) it does not need a second Lewis acid metal ion in order to proceed.
RESUMEN
A series of p-terphenyl-based macrocycles, containing highly distorted p-phenylene units, have been synthesized. Biaryl bonds of the nonplanar p-terphenyl nuclei were constructed in the absence of Pd-catalyzed or Ni-mediated cross-coupling reactions, using 1,4-diketones as surrogates to strained arene units. A streamlined synthetic protocol for the synthesis of 1,4-diketo macrocycles has been developed, using only 2.5 mol % of the Hoveyda-Grubbs second-generation catalyst in both metathesis and transfer hydrogenation reactions. Under protic acid-mediated dehydrative aromatization conditions, the central and most strained benzene ring of the p-terphenyl systems was susceptible to rearrangement reactions. To overcome this, a dehydrative aromatization protocol using the Burgess reagent was developed. Under these conditions, no strain-induced rearrangement reactions occur, delivering p-phenylene units with up to 28.4 kcal/mol strain energy and deformation angles that sum up to 40°.
RESUMEN
The 3-deaza analogs of the naturally occurring adenine-based carbocyclic nucleosides aristeromycin and neplanocin possess biological properties that have not been optimized. In that direction, this paper reports the strategic placement of a fluorine atom at the C-2 and C-3 positions and a methyl at the C-3 site of the 3-deazaadenine ring of the aforementioned compounds. The synthesis and S-adenosylhomocysteine hydrolase inhibitory and antiviral properties of these targets are described. Some, but not all, compounds in this series showed significant activity toward herpes, arena, bunya, flavi, and orthomyxoviruses.
Asunto(s)
Adenosina/análogos & derivados , Antivirales/farmacología , Adenosina/síntesis química , Humanos , Estructura MolecularRESUMEN
A new synthetic strategy that employs a relatively unstrained, 1,4-diketo-bridged macrocycle as a precursor to a strained, 1,4-arene-bridged (bent para-phenylene) macrocycle has been developed. The distorted p-terphenyl nucleus (CPP fragment) of the macrocycle has been characterized by X-ray crystallography, and a direct, regioselective bromination protocol of the macrocyclic system is reported.
RESUMEN
A manganese(II) complex with a ligand containing an oxidizable quinol group serves as a turn-on sensor for H2O2. Upon oxidation, the relaxivity of the complex in buffered water increases by 0.8 mM(-1) s(-1), providing a signal that can be detected and quantified by magnetic resonance imaging. The complex also serves as a potent antioxidant, suggesting that this and related complexes have the potential to concurrently visualize and alleviate oxidative stress.
Asunto(s)
Antioxidantes/química , Medios de Contraste/química , Complejos de Coordinación/química , Peróxido de Hidrógeno/análisis , Manganeso/química , Estrés Oxidativo/efectos de los fármacos , Animales , Antioxidantes/farmacología , Línea Celular , Medios de Contraste/farmacología , Complejos de Coordinación/farmacología , Imagen por Resonancia Magnética , Manganeso/farmacología , Ratones , Modelos Moleculares , Oxidación-Reducción , RatasRESUMEN
The compound N,N'-dineopentyl-N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine (dnbpn) and its ferrous complex [Fe(dnbpn)(OTf)2] were synthesized. The Fe(II) complex was used to catalyze the oxidation of hydrocarbons by H2O2 and O2. Although the catalyzed alkane oxidation by H2O2 displays a higher preference for secondary over tertiary carbons than those associated with most previously reported nonheme iron catalysts, the catalytic activity is markedly inferior. In addition to directing the catalyzed oxidation toward the less sterically congested C-H bonds of the substrates, the neopentyl groups destabilize the metal-based oxidants generated from H2O2 and the Fe(II) complex. The presence of benzylic substrates with weak C-H bonds stabilizes an intermediate which we have tentatively assigned as a high-spin ferric hydroperoxide species. The oxidant generated from O2 reacts with allylic and benzylic C-H bonds in the absence of a sacrificial reductant; less substrate dehydrogenation is observed than with related previously described systems that use O2 as a terminal oxidant.
Asunto(s)
Compuestos Ferrosos/química , Peróxido de Hidrógeno/química , Oxígeno/química , Catálisis , Etilenodiaminas/química , Ligandos , Modelos Moleculares , Oxidación-ReducciónRESUMEN
Six gallium(III) complexes with N-donor ligands were synthesized to study the mechanism of Ga(III)-catalyzed olefin epoxidation. These include 2:1 ligand/metal complexes with the bidentate ligands ethylenediamine, 5-nitro-1,10-phenanthroline, and 5-amino-1,10-phenanthroline, as well as 1:1 ligand/metal complexes with the tetradentate N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, the potentially pentadentate N,N,N'-tris(2-pyridylmethyl)-1,2-ethanediamine, and the potentially hexadentate N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethanediamine. In solution, each of the three pyridylamine ligands appears to coordinate to the Ga(III) through four donor atoms. The six complexes were tested for their ability to catalyze the epoxidation of alkenes by peracetic acid. Although the complexes with relatively electron-poor phenanthroline derivatives display faster initial reactivity, the gallium(III) complexes with the polydentate pyridylamine ligands appear to be more robust, with less noticeable decreases in their catalytic activity over time. The more highly chelating trispicen and tpen are associated with markedly decreased activity.
Asunto(s)
Alquenos/química , Compuestos Epoxi/síntesis química , Galio/química , Compuestos Organometálicos/química , Catálisis , Compuestos Epoxi/química , Ligandos , Modelos Moleculares , Estructura Molecular , Compuestos Organometálicos/síntesis químicaRESUMEN
The redox-active ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (Hptp1) was prepared and complexed to manganese(II). The isolated [Mn(Hptp1)(MeCN)](2+) serves as a magnetic resonance imaging contrast agent, with an r(1) value comparable to those of other mononuclear gadolinium(III) and manganese(II) complexes. The metal and ligand are stable in aerated aqueous solutions, but the addition of H(2)O(2) causes the complex to oxidatively couple to itself through a bimolecular reaction involving the phenol groups of two Hptp1 ligands. The binuclear product is less paramagnetic per manganese(II) than its mononuclear precursor, lowering the measured r(1) per manganese(II). The manganese(II) complex with Hptp1 can thereby serve as a sensor for oxidative stress.
Asunto(s)
Medios de Contraste/química , Peróxido de Hidrógeno/análisis , Peróxido de Hidrógeno/química , Imagen por Resonancia Magnética , Compuestos Organometálicos/química , Estrés OxidativoRESUMEN
We demonstrate that a simple gallium(III) complex, [Ga(phen)(2)Cl(2)]Cl (phen = 1,10-phenanthroline), can serve as a homogeneous catalyst for the epoxidation of alkenes. The olefin epoxidations proceed relatively quickly at mild temperatures and, under optimum conditions, are highly selective for the epoxide product.