The simple α,β-unsaturated ketones and 2-pyrones are readily available and synthetically important dienophiles and dienes, respectively, for Diels−Alder reactions. However, both prove to be challenging substrates for catalytic asymmetric Diels−Alder reactions. By exploring a new catalysis strategy featuring cooperative catalysis with readily available cinchona catalysts, an unprecedented asymmetric Diels−Alder reaction of simple α,β-unsaturated ketones with 2-pyrones has been successfully developed. With broad scopes for both reactants, the reaction provides a direct and versatile asymmetric access to a wide range of structurally novel bicyclic chiral building blocks amenable for further synthetic elaborations.
One-dimensional nanostructure-based chemiresistive sensors
have
received great attention because of their compact design and excellent
sensing performance, including high sensitivity, low detection limits,
low power consumption, and the ability to integrate multisensor arrays.
However, these sensors generally suffer from slow response and recovery
times when they are operated at ambient conditions because of slow
catalytic or absorption/desorption processes. The sensors overcome
this obstacle by operating at high temperature, which increases the
device complexity with a higher power consumption rate. In this work,
we demonstrated the construction of an ultrafast NO2 sensor
at ambient conditions by utilizing feather-like tellurium (Te) nanostructures
functionalized on single-walled carbon nanotube networks. By tailoring
the morphology and density of Te nanostructures, hybrid nanostructures
show excellent response and recovery times of approximately 63 s and
7 min to 100 ppbV NO2 gas at room temperature.
Metal/DNA/SWNT hybrid nanostructure-based gas sensor arrays were fabricated by means of ink jet printing of metal ion chelated DNA/SWNTs on microfabricated electrodes, followed by electroless deposition to reduce metal ions to metal. DNA served as a dispersing agent to effectively solubilize pristine SWNTs in water and as metal ion chelating centers for the formation of nanoparticles. Noble metals including palladium, platinum, and gold were used because the high binding affinity toward specific analytes enhances the selectivity and sensitivity. The sensitivity and selectivity of the gas sensors toward various gases such as H2, H2S, NH3, and NO2 were determined at room temperature. Sensing results indicated the enhancement of the sensitivity and selectivity toward certain analytes by functionalizing with different metal nanoparticles (e.g., Pd/DNA/SWNTs for H2 and H2S). The combined responses give a unique pattern or signature for each analyte by which the system can identify and quantify an individual gas.
Achiral epoxides are isomerized to optically active allylic alcohols under the influence of catalytical amounts of cob(1)alamin in protic polar solvents.Introduction. -The isomerization of epoxides to allylic alcohols is an important transformation in organic synthesis and an industrial process [l] Enantioselective isomerization of achiral epoxides to optically active allylic alcohols with chiral lithium amides (from BuLi and enantiomerically pure secondary amines) has recently been described by Whitesell and Felman [17] and Asami [18]. The yield and enantiomeric excess strongly depend on the epoxide, the base, and the solvent.Continuing our investigations on the application of vitamin B,, [19] as catalyst in organic synthesis [20], we report here on preliminary results on the B,,-catalyzed isomerization of achiral epoxides to optically active allylic alcohols. This work was stimulated to a considerable extent by earlier findings of Fischli et al. on the cob(1)alamin-catalyzed enantioselective hydrogenation of Michael-olefins [2 11 and the work of Golding and coworkers on the stereochemistry of the alkylation of cob(1)alamin by racemic epoxides [221.Results and Discussion. ~ If achiral epoxides like la, b, d are dissolved in a polar protic solvent containing a catalytic amount of vitamin B,,, [23], the corresponding optically active allylic alcohols 2a, b, dare formed on standing at room temperature (Scheme). The
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