Efficient and Green Approaches for the Synthesis of 4H-Benzo[g]chromenes in Water,Under Neat Conditions,and Using Task-Specific Ionic Liquid

Facile and convenient procedures have been described for the synthesis of 4H-benzo[g]chromenes by one-pot condensation of aromatic aldehydes, malononitrile/ethyl cyanoacetate, and 2-hydroxy-1,4-naphthoquinone. The reaction has been accomplished using catalytic cetyltrimethylammonium bromide (CTAB) in water under reflux or under neat conditions at 110 °C. A task-specific ionic liquid, 1-butyl-3-methyl imidazolium hydroxide ([bmim]OH), has also been found to be an effective catalyst for this transformation. The protocols prove to be environmentally benign and offer advantages of good yields, short reaction times, and simple workup procedures. The ionic liquid could be recycled and reused for up to four runs without appreciable loss in the activity.     

Highly Efficient One-Pot Synthesis of 2-Aminobenzoxazoles Using Triflic Acid as a Cyclodesulfurizing Reagent

An efficient one-pot synthetic strategy for 2-aminobenzoxazoles was developed from isothiocyanates and 2-aminophenol using triflic acid as a cyclodesulfurizing reagent.

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40–80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.     

Synthesis and Onwards Coordination of an AsI Centered Zwitterion

The bis(phosphino)borate ligand class is used as an anionic anchor to stabilize reactive, low coordinate arsenic centers. The neutral, zwitterionic As^I species, 2 , is formed very cleanly, and isolated in good yields using cyclohexene as a halogen scavenger. The uniqueness of this heterocyclic As^I compound is on display with the coordination to Group 6 metal centers, ( 2 M(CO)₅ ; M=Cr, Mo, W). The arsenic? metal bond lengths are longer than the related AsPh₃ complexes suggesting that compound 2 is a weak sigma donor. The metal complexes reveal a trigonal pyramidal arsenic atom, which provides the first experimental evidence for the presence of two “lone pairs” of electrons on the As^I center. When more flexible and more electron‐donating isopropyl substituents were used, an intermediate (compound 5 ) in the formation of low coordinate pnictogen compounds was crystallographically characterized. This structure, formally a base‐stabilized dichloroarsenium cation, provides an alternative mechanistic proposal to the one described in the literature.     

Memory effects in combining isotope ratio monitoring with isotope dilution mass spectrometry

Combining isotope ratio monitoring with isotope dilution techniques provides very accurate results in the quantitative analysis of volatile organic chemical compounds by gas chromatography/mass spectrometry (GC/MS). However, this method requires that spikes highly enriched in (13)C be used. This may lead to memory effects which will be investigated in more detail. They occur when the component of the mixture to be investigated exhibits an isotope ratio which is different from that of the component eluted earlier from the column during the chromatographic separation process. A residue of this component, which is shown in the gas chromatogram as tailing, falsifies the result of the isotope ratio measurement. This also leads to false amount-of-substance measurement results. Memory effects can be avoided by using spikes of low (13)C content, by adjusting the composition of the reference solution to that of the sample, or by ensuring effective sample preparation, thus separating disturbing mixture components prior to the measurement. Copyright 1999 John Wiley & Sons, Ltd.     

Control of Nanospaces with Molecular Devices

Cavitands and capsules define nanoliter spaces for recognition, isolation and reactions of small molecules. These systems are usually self-assembled and factors such as solvent size, stoichiometry, and packing factors determine what goes into the spaces. Here we examine two switching devices to control what and when guests get in and out of these hosts: bipyridyl-metal chelation and azobenzene photoisomerization. The effects are reversible by treatment with conventional chelating agents and brief heating, respectively. Accordingly, it is possible to trigger reactions that take place within a cylindrical capsule by light, even though the reaction process is not photochemical by nature. Likewise the presence of metals can regulate reactions without acting as direct catalysts.     

Determination of biogenic amines in lake water by micellar electrokinetic chromatography with fluorescence detection after derivatization with fluorescamine

A simple and rapid method has been developed for the determination of biogenic amines in lake water using micellar electrokinetic chromatography with fluorescence detection. Separation of fluorescamine derivatized biogenic amines was accomplished by using borate buffer of pH 9.5 containing 40 mM of sodium dodecyl sulphate. The method has been optimized with respect to fluorescamine concentration, reaction pH, reaction time, separation voltage and injection time. Detection was performed by using UG-11 excitation filter and 495 nm emission filter. The proposed method for histamine, tyramine and dopamine allowed their separation within 2 min with detection limits in nM range. The interday and intraday reproducibility of peak areas were less than 6.5%. Recovery of spiked samples was 95.76–116.31%.     

Strategic applications of Baylis-Hillman adducts to general syntheses of 3-nitroazetidines

A novel one-pot highly diastereoselective synthesis of substituted 3-nitroazetidines via an anionic domino process is described. The synthesis involves a high yielding annulation of Baylis-Hillman alcohols and their aldehydes with either N-aryl/tosylphosphoramidates or N-aryl/tosylphosphoramidates in combination with a task-specific ionic liquid [bmim][X-Y] to afford the corresponding 1,2,3-tri- and 1,2,3,4-tetrasubstituted azetidines, respectively. Plausible mechanisms for the formation of various 3-nitroazetidines have been suggested.     

Structure and properties of an eight-coordinate Mn(II) complex that demonstrates a high water relaxivity

An air-stable eight-coordinate (8C) Mn(II)N(8) complex has been synthesized utilizing an N(4) imidazole/imine ligand. The 8C dodecahedral geometry is structurally robust as the Mn complex is stable to air, NO(g), and potential coordinating anions. The structural, spectroscopic and water relaxivity properties of this complex are reported.