Immobilization of oxomolybdenum species in a layered double hydroxide pillared by 2,2'-bipyridine-5,5'-dicarboxylate anions

A Zn,Al layered double hydroxide (LDH) with Zn/Al = 1.45 and containing nitrate anions was prepared by coprecipitation and characterized by powder X-ray diffraction, Zn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS), thermogravimetric analysis, FTIR and FT Raman spectroscopy, and (27)Al MAS NMR spectroscopy. Three Zn...O and four Zn...M (M = Zn, Al) shells could be fitted to the low-temperature (40 K) EXAFS spectrum, in accordance with a model for an ordered cationic sheet. The nitrate anions were easily exchanged by 2,2'-bipyridine-5,5'-dicarboxylate anions, resulting in an increase in the basal spacing from 9 to 18 A. The basal spacing of the pillared derivative indicates that the anions are arranged with their longest dimension nearly perpendicular to the host layers. This material exhibits a high encapsulating ability, as evidenced by its interaction with a dichloromethane solution of the dioxomolybdenum(VI) complex MoO(2)Cl(2)(THF)(2). A material with a metal loading of 11.2 wt % was obtained. Molybdenum K-edge EXAFS analysis could not substantiate the formation of a supported complex of the type MoO(2)Cl(2)(N-N) but instead indicated the formation of unidentate-bridged entities of the type [O(2)Mo-O-MoO(2)] with a metal-metal separation of 3.29 A. The molybdenum-containing LDH was active as a catalyst for the liquid-phase epoxidation of cis-cyclooctene, 1-octene, and trans-2-octene using tert-butyl hydroperoxide as the oxygen source, yielding the corresponding epoxides as the only products. For reactions carried out with no additional solvent (other than n-decane) or in the presence of 1,2-dichloroethane, the solid catalyst could be recycled with no major loss of activity. Other tests confirmed that the systems functioned as true heterogeneous catalysts.     

Domains of Holomorphy of Generating Functions of Pólya Frequency Sequences of Finite Order*

A domain is the domain of holomorphy of the generating function of a Pólya frequency sequence of order r if and only if it satisfies the following conditions: (A) G contains the point z = 0, (B) G is symmetric with respect to the real axis, (C) T = dist(0,G)G.     

In vivo human retinal imaging by ultrahigh-speed spectral domain optical coherence tomography

An ultrahigh-speed spectral domain optical coherence tomography (SD-OCT) system is presented that achieves acquisition rates of 29,300 depth profiles/s. The sensitivity of SD-OCT and time domain OCT (TD-OCT) are experimentally compared, demonstrating a 21.7-dB improvement of SD-OCT over TD-OCT. In vivo images of the human retina are presented, demonstrating the ability to acquire high-quality structural images with an axial resolution of 6 microm at ultrahigh speed and with an ocular exposure level of less than 600 microW.     

Mass spectrometric study of aliphatic alpha-carbonyl azides

A series of derivatives of 2-azidoacetic acid and 2-azidoacetone were synthesized and their behaviour under electron ionization conditions was investigated. This paper reports the electron ionization fragmentation mechanisms for five aliphatic alpha-carbonyl azides, which were clarified by accurate mass measurements and B/E linked scans. The substituent influences the abundance and the nature of the ions resulting from the molecular ion fragmentation.     

Formal asymmetric 1,3-dipolar cycloaddition of azomethine ylides to (S)-2-p-tolylsulfinyl-2-cyclopentenone

Azomethine ylides, derived from iminoesters 1 and DBU in the presence of silver salts, react with (S)-2-p-tolylsulfinyl-2-cyclopentenone 2 in a completely regio- and endoselective manner but with a low facial selectivity, affording a mixture of two cycloadducts 3 and 4. When the ylides were prepared with LHMDS, only one diastereoisomer 3 was obtained in an almost quantitative yield. A nucleophilic addition/ring closure process easily accounts for the stereochemical results. Compounds 3 were transformed into optically pure 4-oxocyclopenta[c]dihydropyrroles and tetrahydropyrroles by elimination of the sulfinyl group.     

ITQ-12: a new microporous silica polymorph potentially useful for light hydrocarbon separations

The new synthetic form of microporous crystalline silica, denoted as ITQ-12, shows a high potential for the separation of propane and propene from its mixtures.     

A supramolecular cup-and-ball C60-porphyrin conjugate system

In addition to the ammonium-crown ether recognition, pi-stacking interactions between the C60 sphere and the porphyrin moiety have been evidenced in a supramolecular complex obtained from a porphyrin-crown ether conjugate and a fullerene derivative bearing an ammonium unit.     

Conformational preferences driven by the C-methyl substituent in chelated o-diphenylphosphino-alpha-methyl-N,N-dimethylbenzylamine rhodium complexes

The stereochemistry of the chelate rings of a number of rhodium aminophosphine complexes is studied by NMR spectroscopy. The similarity in the variable-temperature behavior for the different compounds is consistent with them having in common highly preferred chelate ring conformations. The six-membered metallacycle of coordinated (R)-PN (PN = o-diphenylphosphino-alpha-methyl-N,N-dimethylbenzylamine) adopts a delta conformation in the solid state. NMR experiments indicate that this conformation is strongly favored in solution as well. The preferred sense of helicity is imposed by the absolute configuration of the stereogenic carbon atom on the ligand, which exerts an important steric control. The complex [Rh(TFB)((C(6)H(4)CHMeNMe(2))(2)P(C(6)H(4)CHMeNHMe(2)))](BF(4))(2).H(2)O.Me(2)CO crystallizes in the monoclinic space group P2(1) with a = 12.0548(11) A, b = 16.139(2) A, c = 12.1804(10) A, beta = 100.742(9) degrees, Z = 4.     

Aziridines as a protecting and directing group. Stereoselective synthesis of (+)-bromoxone

[reaction: see text] The directing ability of an aziridine group for the epoxidation of adjacent double bonds is demonstrated. The aziridine group is also used to effectively protect a double bond in a cycloenone system for a short synthesis of the title compound.