Dimolybdenum-containing molecular triangles and squares with diamidate linkers: structural diversity and complexity

By employing cis-Mo2(DAniF)2(2+) (DAniF = N,N'-di(p-anisyl)formamidinate) as the vertex building block and terephthaloyldiamidate as the linker, four dimolybdenum-containing cyclic oligomers have been synthesized and structurally characterized. In these compounds, described by the general formula [cis-Mo2(DAniF)2((ArNOC)2C6H4)2]n, n = 3 and 4, the geometry and composition of the products are affected by the identity of the aromatic groups of the linker. When Ar = phenyl, n = 3 (1a and 1b); however, n = 4 for Ar = p-trifluoromethylphenyl (2) and when Ar = m-trifluoromethylphenyl (3). All these compounds have a central cavity, shaped by the diamidate linker, that is capable of serving as host to guest molecules in a selective manner. For compounds 2 and 3, self-assembly that takes place in the crystalline state entails intermolecular C-H...F-C interactions. Such interactions generate a one-dimensional network with a tunnel cross section of 10 x 10 A(2) in 2, whereas in 3, they result in a cage in which two THF molecules are encapsulated. The F...H distances vary in a broad range from 2.38 to 2.70 A.     

Construction of electrocatalytic electrodes bearing the triphenylamine nucleus covalently bound to carbon. A halogen dance in protonated aminotriphenylamines

[reaction: see text]. The triarylamine nucleus has been attached to a carbon fiber electrode by diazotization of an aminotriphenylamine followed by electrochemical reduction. The resulting electrodes can electrocatalyze the oxidation of organic substrates. In acid, 4-amino-4',4' '-dibromotriphenylamine undergoes dismutation into a mixture of amines containing from 0 to 3 bromine atoms.     

Asymmetric homogeneous hydrogenation of 2,5-disubstituted furans

A homogeneous catalyst system for the asymmetric cis-hydrogenation of 2,5-disubstituted furans leading to 2',3'-dideoxynucleoside analogues is described. Best enantioselectivities (ee values of up to 72%) were obtained with cationic rhodium complexes ligated by diphospholanes of the butiphane family. The selectivity of the hydrogenation was reversed by the addition of a base or a polar protic solvent in certain cases. Ferrocene- and proline-based systems gave significant, but lower, ee values.     

Synthesis and conformational analysis of new cyclobutane-fused nucleosides

[structure: see text]. A stereselective synthesis of 3-oxabicyclo[3.2.0]heptane nucleoside analogues, which were designed as conformational mimics of the anti-HIV agents 2',3'-didehydro-2',3'-dideoxythimidine (stavudine, d4T) and 2',3'-didehydro-2',3'-dideoxyadenosine (d4A), is described. The target compounds were prepared by condensation of a common intermediate bicyclic acetate, derived from a homochiral 2(5H)-furanone, with pyrimidine and purine bases under modified Vorbrüggen conditions. The conformational behavior of the synthesized nucleoside analogues was studied by NMR spectroscopy and X-ray crystallography.     

Field-free three-dimensional alignment of polyatomic molecules

We experimentally demonstrate field-free, three-dimensional alignment (FF3DA) of polyatomic asymmetric top molecules. We achieve FF3DA in sulfur dioxide gas using two time-delayed, orthogonally polarized, nonresonant, femtosecond laser pulses. Our method avoids the use of rotational revivals and is therefore more robust to temperature. The alignment is probed using time-delayed coincidence Coulomb explosion imaging. FF3DA will be important for all molecular imaging, dynamics, or spectroscopy experiments for which random alignment leads to a loss of information.     

Quantitative real-space analysis of self-assembled structures of magnetic dipolar colloids

We present the first real-space analysis on a single-particle level of the dipolar chains and branched clusters self-assembling in magnetic fluids in zero field. Spatial correlations and chain-length distributions directly obtained from tracked particle positions in vitrified films of synthetic magnetic (Fe3O4) dispersions provide a quantitative test for simulations and theory of dipolar fluids. A pertinent example is the cluster-size distribution that can be analyzed with a one-dimensional aggregation model to yield a dipolar attraction energy that agrees well with the dipole moment found from independent magnetization measurements.     

Imidazol(in)ium-2-Carboxylates as N-Heterocyclic Carbene Ligand Precursors for Ruthenium Metathesis Initiators

Summary: Imidazol(in)ium-2-carboxylates were used as N-heterocyclic carbene (NHC) ligand precursors to convert the [RuCl₂(p-cymene)]₂ dimer into three ruthenium-arene complexes of the [RuCl₂(p-cymene)(NHC)] type. The decarboxylation of NHC · CO₂ betaines also provided a convenient synthetic path to prepare five well-known ruthenium-NHC catalysts for olefin metathesis and related reactions, including the second generation Grubbs and Hoveyda–Grubbs catalysts, via ligand exchange with phosphine-containing, first generation ruthenium-benzylidene or indenylidene complexes. Both procedures are particularly attractive from a practical point of view, because NHC · CO₂ adducts are stable zwitterionic compounds that can be stored and handled with no particular precautions.     

Oxadisilole fused triptycene and extended triptycene: precursors of triptycyne and extended triptycyne

With benzobisoxadisilole 1 as a 1,4-benzdiyne equivalent, oxadisilole fused triptycene 5 and extended triptycene 6 were synthesized. Triptycenes 5 and 6 are new precursors of triptycyne 7 and extended triptycyne 8 respectively via the phenyliodination/fluoride induced elimination protocol. Using these two arynes, a series of triptycene derivatives were synthesized.     

Surprising Inability of Singlet Oxygen-generated 6-Hydroperoxycholesterol to Induce Damaging Free Radical Lipid Peroxidation in Cell Membranes†

Singlet oxygen attack on cholesterol (Ch), a prominent monounsaturated lipid of mammalian cell plasma membranes, gives rise to three hydroperoxide (ChOOH) isomers, 5α-OOH, 6α-OOH and 6β-OOH, the latter two in lower yield than 5α-OOH, and 6α-OOH in lowest yield. A third possible positional isomer, 7α-OOH and 7β-OOH, is produced by free radical attack. In the presence of iron and ascorbate (Fe/AH), 5α-OOH or 6β-OOH in phosphatidylcholine/Ch/ChOOH (20:15:1 by mol) liposomes was reduced to its corresponding alcohol, the rate constant being approximately the same for both ChOOHs. Using [¹⁴C]Ch as an in situ probe, we found that liposomal 5α-OOH readily set off free radical-mediated (chain) peroxidation reactions when exposed to Fe/AH, whereas 6β-OOH under the same conditions did not. Moreover, liposomal 5α-OOH triggered robust chain peroxidation in [¹⁴C]Ch-labeled L1210 cells, leading to cell death, whereas 6β-OOH was essentially inert in this regard. Thus, 5α-OOH and 6β-OOH undergo iron-catalyzed reductive turnover, but only the former can provoke toxic membrane damage. These novel findings have important implications for UVA-induced photodamage in Ch-rich tissues like skin and eye, where ¹O₂ often plays a major role.     

Synthesis of substituted β-carbolines via gold(III)-catalyzed cycloisomerization of N-propargylamides

Indole-substituted N-propargylamides undergo a gold(III)-catalyzed cyclization to give oxazoles or β-carbolinones, depending on the substitution pattern at the amide nitrogen. Secondary amides furnished oxazoles via a 5-exo-dig cyclization, while tertiary indole-2-carboxamides cycloisomerized to give 2,9-dihydro-β-carbolin-1-ones upon treatment with catalytic amounts of gold(III) chloride. An optimized procedure was developed to prepare several new N-benzyl-β-carbolinones as well as the corresponding β-carbolines, which are of interest as core structures found in various natural products such as the harman, ervolanine, and lavendamycin class of alkaloids.