Transition metal carbene chemistry. 4. Nucleophilic attachment of DABCO to fischer carbene complexes in MeCN

Rate constants for the attachment of DABCO (1,4-diazabicyclo[2.2.2]octane) to Fischer carbene complexes of the type (CO)(5)Cr=C(XR)C(6)H(4)Z (X = O and S) in dry MeCN at 25 degrees C are reported. Hammett rho values are 2.18 +/- 0.13 and 0.89 +/- 0.07 for DABCO reactions with (CO)(5)Cr=C(OMe)C(6)H(4)Z (10-Cr-Z) and (CO)(5)Cr=C(SMe)C(6)H(4)Z (11-Cr-Z), respectively. The rho values for the reaction of 10-Cr-Z and 11-Cr-Z with CH(CN)(2)(-) in 50% MeCN-50% H(2)O (v/v) are comparable to the present reactions. The reaction of DABCO with 10-Cr-Z is more closely related to the reaction of (n-Bu)(3)P with (CO)(5)W=C(OMe)C(6)H(5)-Z (23) which also provided a rho value 2.22. The much higher rho values and hence much higher reactivity of methoxy carbene complexes over the corresponding thiomethyl derivatives fit a pattern observed previously for alkoxide ion, OH(-), amine, and thiolate ion nucleophiles, and a rational explanation comes from the consideration of the substituent effects not only on the transition state but also on the reactant. A major difference between 10-Cr-Z and 11-Cr-Z is that the pi-donor effect of the methoxy group is much stronger than that of thiomethyl group. This leads to a substantial contribution of the zwitterionic form to the structure of 10-Cr-Z with much more localized positive charge on the methoxy group than the negative charge on the (CO)(5)Cr moiety. This leads to overall destabilization by an electron-withdrawing phenyl substituents resulting an increase in reactivity. The ethoxycarbene complexes are somewhat less reactive than their methoxy counterparts due to the somewhat more ground state stabilization through its stronger pi donor effect and partly due to steric crowding exerted by the slightly larger ethoxy group in the transition state. Higher k(1)(W)/k(1)(Cr) ratios for (thiomethyl)carbene complexes than methoxy or ethoxycarbene complexes are related to the intrinsic rate constant which is higher for ((thiomethyl)carbene)tungsten complexes than the corresponding Cr ones resulting in an enhanced ratio.     

Stereoselective syntheses of unnatural steroidal C(20R) aldehydes by ionic hydrogenation of C-20 tertiary alcohols

Syntheses of three unnatural steroidal C(20R) aldehydes have been realised from 16-dehydropregnenolone acetate. The salient feature of the synthesis is the ionic hydrogenation of C-20 tertiary alcohols leading to the formation of the C(20R) unnatural isomer with complete stereoselectivity. Oxidative hydrolysis of the dithiane moiety furnished the C(20R) aldehydes.     

Efficient White‐Light Generation from Ionically Self‐Assembled Triply‐Fluorescent Organic Nanoparticles

Low cost, simple, and environmentally friendly strategies for white‐light generation which do not require rare‐earth phosphors or other toxic or elementally scare species remain an essentially unmet challenge. Progress in the area of all‐organic approaches is highly sought, single molecular systems remaining a particular challenge. Taking inspiration from the designer nature of ionic‐liquid chemistry, we now introduce a new strategy toward white‐light emission based on the facile generation of nanoparticles comprising three different fluorophores assembled in a well‐defined stoichiometry purely through electrostatic interactions. The building blocks consist of the fluorophores aminopyrene, fluorescein, and rhodamine 6G which represent blue, green, and red‐emitting species, respectively. Spherical nanoparticles 16(±5) nm in size were prepared which display bright white‐light emission with high fluorescence quantum efficiency (26 %) and color coordinate at (0.29, 0.38) which lie in close proximity to pure white light (0.33, 0.33). It is noteworthy that this same fluorophore mixture in free solution yields only blue emission. Density functional theory calculations reveal H‐bond and ground‐state proton transfer mediated absolute non‐parallel orientation of the constituent units which result in frustrated energy transfer, giving rise to emission from the individual centers and concomitant white‐light emission.     

Organocatalytic oxidative synthesis of C2-functionalized benzoxazoles,naphthoxazoles, benzothiazoles and benzimidazoles

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Carbon-Nanotube-Appended PAMAM Dendrimers Bearing Iron(II) α-Keto Acid Complexes: Catalytic Non-Heme Oxygenase Models

Poly(amidoamine) dendrimers grafted on carbon nanotubes have been appended with iron(II)-α-keto acid (benzoylformate) complex of polypyridyl ligand to design artificial non-heme oxygenase model. This nano-enzyme was applied for selective catalytic oxidation of organic molecules. Although the carbon nanotubes serve as a robust heterogeneous platform, the amine terminals of dendrimers provide catalysts binding sites and the amide bonds provide a necessary second coordination sphere similar to the enzymatic polypeptide chains. Such a hybrid design prevented the deactivation of the primary active sites leading to 8 times faster oxidative decarboxylation rates than those of its homogeneous analogue. An electrophilic iron(IV)-oxo intermediate has been intercepted, which catalyzes the selective oxidation of alcohols to aldehydes and incorporates single oxygen atoms into sulfides and olefins by using aerial oxygen with multiple turnover numbers. The catalyst was consecutively regenerated three times by mild chemical treatment and showed negligible loss of activity.     

HPTLC Determination of Cefpodoxime Proxetil in Formulations

An HPTLC method has been developed that coelutes both the isomers of cefpodoxime proxetil (CFP). CFP was chromatographed on a silica gel 60 F₂₅₄ TLC plate using toluene:acetonitrile (6:4) as a mobile phase and was quantified at 234 nm. The method was validated with respect to linearity, accuracy, precision and specificity. The limit of detection and limit of quantification for CFP were found to be 0.150 and 0.4 μg spot⁻¹, respectively. The proposed method was successfully used to determine the amount of CFP present in the marketed tablets and self-nanoemulsifying systems.

     

Chemical modification of metallocene‐based polyolefinic elastomers by acrylic acid and its influence on physico‐mechanical properties: Effect of reaction parameters,crystallinity and pendant chain length

In this investigation, solution grafting of acrylic acid (AA) in presence of benzoyl peroxide (BPO) was carried out onto metallocene‐based “poly(ethylene‐octene) elastomers” (POE) as well as “poly(ethylene‐butene) elastomers” (PBE), to impart polarity on the non‐polar rubbery matrix and also to study the effects of crystallinity and pendant chain length on the “grafting percentage” and “percent gel yield” at optimized conditions for all the POE and PBE systems. Reaction parameters were optimized on the basis of the relative proportions of graft and gel formations obtained through %weight gain, Fourier Transform infrared spectroscopy and elemental analysis. The effect of grafting at its maximum level on various physico‐mechanical properties was also thoroughly investigated by using X‐ray diffraction (XRD), differential scanning calorimetry (DSC), mechanical, dynamic mechanical (DMTA), and thermogravimetric analysis (TGA) and the properties were correlated with the structure of the modified polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5529–5540, 2007     

Multifunctional silk fibroin/PVA bio-nanocomposite films containing TEMPO-oxidized bacterial cellulose nanofibers and silver nanoparticles

Cellulose - Silk fibroin (SF) has been intensively studied as a biobased renewable material for biomedical and other applications. However, its relatively low mechanical properties have limited its...     

Hydrothermal synthesis of dimeric lanthanide compounds: X-ray structure,magnetic study and heterogeneous catalytic epoxidation of olefins

A series of dimeric lanthanide carboxylato complexes [La(5-Br-NIC)₃(H₂O)₂]₂·H₂O (1); [Gd(5-Br-NIC)₃(H₂O)₂]₂ (2), [5-Br-NIC = 5-bromonicotinate] and [Sm(NIC)₃(H₂O)₂]₂ (3) [NIC = nicotinate], have been hydrothermally synthesized and structurally characterized by single crystal X-ray analysis. Complexes 1, 2 and 3 are of similar structure and consist of a basic unit [La(carboxylato)₃(H₂O)₂]₂. In compound 1 lanthanide cation is surrounded by one chelating 5-bromo-nicotinato ligand, two bridging oxygen atoms from 5-bromo-nicotinato and two water molecules, in which each La(III) ion is nine coordinated in a tricapped prismatic geometry. However, in compounds 2 and 3 four carboxylate groups link a pair of lanthanide atoms in the O,O′-bridging mode to generate a paddle-wheel-like centrosymmetric dimer. All the compounds exhibit excellent catalytic performance in olefin epoxidation reaction. The variable temperature magnetic susceptibility measurements showed that the magnetic interaction in [Gd(5-Br-NIC)₃(H₂O)₂]₂ (2), is antiferromagnetic (J = −0.048 cm⁻¹), while compound [Sm(NIC)₃(H₂O)₂]₂ (3), showed a complicated low-temperature magnetic property.