Electronic interactions and their influence on the conformational stability of trans-2-halocyclopentanol

Conformational preferences and electronic interactions of trans-2-fluorocyclopentanol (1), trans-2-chlorocyclopentanol (2), and trans-2-bromocyclopentanol (3) were analyzed using experimental and theoretical (3)J(HH) coupling constants, theoretical calculations, and natural bond orbital (NBO) analysis. The conformational equilibria of compounds 1-3 can be represented by their diaxial and diequatorial conformers as supported by theoretical calculations. From (3)J(HH) coupling constant values, it can be found that the diequatorial conformer is present in the equilibrium as 55% for compound 1 and as 60% for compounds 2 and 3. This behavior is in agreement with orbital interaction analyses obtained from NBO.     

Consecutive palladium-catalyzed Hiyama-Heck reactions in aqueous media under ligand-free conditions

Symmetric and asymmetric (E)-1,2-diarylethenes are synthesized from aryl bromides by consecutive one-pot Hiyama-Heck reactions carried out in water and under air; the only additives required are sodium hydroxide, palladium acetate and poly(ethylene glycol), and the products are isolable in many cases by simple filtration of the water solution.     

Asymmetric synthesis of rubiginones A(2) and C(2) and their 11-methoxy regioisomers

Convergent enantioselective syntheses of angucyclinone-type natural products rubiginones A(2) (2) and C(2) (1) and their 11-methoxy regioisomers 3 a and 3 b have been achieved by using two domino processes from a common enantiomerically pure 1-vinylcyclohexene 4. Key steps in the synthesis of this diene were the stereoselective conjugate addition of AlMe(3) on (SS)-[(p-tolylsulfinyl)methyl]-p-quinol (9) and the elimination of the beta-hydroxy sulfoxide fragment, after oxidation to sulfone, to recover a carbonyl group. The first domino sequence comprised Diels-Alder reaction with a sulfinyl naphthoquinone followed by sulfoxide elimination. An efficient opposite regioselection in the cycloaddition step was achieved in the convergent construction of the tetracyclic skeleton using a sulfoxide at C-2 or C-3 of the dienophiles 5 or 6, derived from 5-methoxy-1,4-naphthoquinone. The second domino process, triggered by oxygen and sunlight, allowed the transformation of the initial tetracyclic adducts into the final products after B ring aromatization, silyl deprotection and C-1 oxidation.     

Donor/conductor/acceptor triads spatially organized on the micrometer-length scale: an alternative approach to photovoltaic cells

We have used porous anodised Al(2)O(3) membranes as inert matrix for constructing and organizing spatially ternary donor/conductor/acceptor (DCA) systems exhibiting photovoltaic cell activity on the micrometric-length scale. These DCA triads were built stepwise by first growing a conducting polymer inside the membrane pores, thus forming nanorods that completely fill the internal pore space of the membrane. Then, an electron donor and an electron acceptor were adsorbed one on each side of the membrane, so that they were separated by a distance equal to the membrane thickness (ca. 60 microm), but electronically connected through the conductive polymer. When this device was placed between two electrodes and irradiated with visible light, electrons jumped from the donor molecule, crossed the membrane from side to side through the conductive polymer (a journey of about 60 microm!) until they finally reach the acceptor molecule. In so doing, an electric voltage was generated between the two electrodes, capable of maintaining an electric current flow from the membrane to an external circuit. Our DCA device constitutes the proof of a novel concept of photovoltaic cells, since it is based on the spatial organization at the micrometric scale of complementary, but not covalently linked, electron-donor and electron-acceptor organic species. Thus, our cell is based in translating photoinduced electron transfer between donors and acceptors, which is known to occur at the molecular nanometric scale, to the micrometric range in a spatially organised system. In addition our cell does not need the use of liquid electrolytes in order to operate, which is one of the main drawbacks in dye-sensitised solar cells.     

Structural transformation of MoO<Subscript>3</Subscript> nanobelts into MoS<Subscript>2</Subscript> nanotubes

The structural transformation of MoO₃ nanobelts into MoS₂ nanotubes using a simple sulfur source has been reported. This transformation has been extensively investigated using electron microscopic and spectroscopic techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), and energy-dispersive X-ray analysis (SEM-EDAX and TEM-EDX). The method described in this report will serve as a generic route for the transformation of other oxide nanostructures into the chalcogenide nanostructures.     

Chromium(III) complexes with terdentate 2,6-bis(azolylmethyl)pyridine ligands: Synthesis, structures and ethylene polymerization behavior

Reactions of 2,6-bis(bromomethyl)pyridine with 3,5-dimethylpyrazole and 1H-indazole yield the terdentate ligands 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (5) and 2,6-bis(indazol-2-ylmethyl)pyridine (6). The molecular structure of the new compound 6 was determined by single-crystal X-ray diffraction. These ligands react with the CrCl₃(THF)₃ complex in THF to form neutral complexes of general formula [CrCl₃{2,6-bis(azolylmethyl)pyridine-N,N,N}] (7, 8) which are isolated in high yields as stable green solids and characterized by means of elemental analysis, magnetic moments, IR, and mass spectroscopy. Theoretical calculations predict that the thermodynamically preferred structure of the complexes is the fac configuration. After reaction with methylaluminoxane (MAO) the chromium(III) complexes are active in the polymerization of ethylene.     

Theoretical studies of a singlet oxygen-releasing dioxapaddlane (1,4-diicosa naphthalene-1,4-endoperoxide)

Theoretical calculations have been used to examine singlet oxygen release from a naphthalene endoperoxide which bears a flexible (CH₂)₂₂ polymethylene “lid”. Monte Carlo and ONIOM calculations that incorporated semi-empirical and density functional theory predicted the conformational influence of the polymethylene chain in the cycloreversion of dioxapaddlane, 1,4-diicosa naphthalene-1,4-endoperoxide, to ¹O₂ and 1,4-diicosa naphthalene. This study attempts to build a connection between ¹O₂ generation and “jump rope” dynamics of the dioxapaddlane. The polymethylene chain appears to function as a gatekeeper for the oxygen. Instead of coming full circle, a semi-circle rotation of the polymethylene bridge protected the peroxide group, limiting the dissociation of ¹O₂ from the naphthalene site. Dedicated to István Hargittai for his first twenty years at Structural Chemistry.     

Semitoric integrable systems on symplectic 4-manifolds

Let (M,ω) be a symplectic 4-manifold. A semitoric integrable system on (M,ω) is a pair of smooth functions J,H∈C ^∞(M,ℝ) for which J generates a Hamiltonian S ¹-action and the Poisson brackets {J,H} vanish. We shall introduce new global symplectic invariants for these systems; some of these invariants encode topological or geometric aspects, while others encode analytical information about the singularities and how they stand with respect to the system. Our goal is to prove that a semitoric system is completely determined by the invariants we introduce. A. Pelayo was partially supported by an NSF Postdoctoral Fellowship.     

On products of quadratic twists and ranks of elliptic curves over large fields

In this paper, we give examples of elliptic curves E/K overa number field K satisfying the property that there exist P₁,P₂ K[t] such that the twists and are of positiverank over K(t). As a consequence of this result on twists, weshow that for those elliptic curves E/K, and for each , the rank of E over the fixed field (K^ab) under is infinite, where K^ab is the maximal abelian extension ofK.