Synthesis and structural characterization of a terminal hydroxide containing alumoxane via hydrolysis of aluminum hydrides

A novel terminal hydroxide containing dinuclear alumoxane LAl(OH)OAlL(OCH=N-tBu) (3; L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3) was prepared by treatment of aluminum dihydride LAlH2 (1) and tert-butyl isocyanate in the presence of trace amounts of water and alternatively from 1 and LAlH(OCH=N-tBu) (2) with water. Compound 2 was obtained from the reaction of 1 and tert-butyl isocyanate.     

Non-Newtonian behavior in simple fluids

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheology of a microscopic sample of simple fluid. The calculations were performed using a configurational thermostat which unlike previous nonequilibrium molecular dynamics or nonequilibrium Brownian dynamics methods does not exert any additional constraint on the flow profile. Our findings are in agreement with experimental results on concentrated "hard sphere"-like colloidal suspensions. We observe: (i) a shear thickening regime under steady shear; (ii) a strain thickening regime under oscillatory shear at low frequencies; and (iii) shear-induced ordering under oscillatory shear at higher frequencies. These results significantly differ from previous simulation results which showed systematically a strong ordering for all frequencies. They also indicate that shear thickening can occur even in the absence of a solvent.     

Enantiopure fluorous bis(oxazolines): synthesis and applications in catalytic asymmetric reactions

Various enantiopure fluorous bis(oxazolines) with fluorine content between 52.7 and 58.7% have been synthesized by a simple reaction sequence that involved the introduction of two fluorinated ponytails by alkylation of the corresponding nonfluorous bis(oxazolines). These new ligands have been used in palladium-catalyzed alkylation of rac-(E)-1,3-diphenylpropenyl acetate with carbon nucleophiles and in copper-catalyzed oxidation of cycloalkenes; these ligands exhibited enantioselectivities up to 98 and 77%, respectively, quite close to the values obtained using the analogous nonfluorous bis(oxazolines). These ligands could be easily recovered by liquid-liquid extraction or solid-liquid separation and reused with the same enantioselectivities.     

Synthesis of carbaalane halogen derivatives

The carbaalane halogen derivatives [(AlX)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] (X = F (9), Cl (7), Br (10), I (11)) were prepared in toluene from [(AlH)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] (6) and BF(3).OEt(2), BX(3) (X = Br, I), Me(3)SnF, and Me(3)SiX (X = Cl, Br, I), respectively. A partially halogenated product [(AlH)(2)(AlX)(4)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] (12) (X = Cl (approximately 40%), Br (approximately 60%)) was obtained from 5 and impure BBr(3). [(AlH)(6)(AlNMe(3))(2)(CCH(2)Ph)(6)] (5) was converted to [(AlX)(6)(AlNMe(3))(2)(CCH(2)Ph)(6)] (X = F (13), Cl (14), Br (15), I (16)) using BF(3).OEt(2) and Me(3)SiX (X = Cl, Br, I), respectively. The X-ray single-crystal structures of 11.C(6)H(6), 12.3C(7)H(8), 13.6C(7)H(8), and 15.4C(7)H(8) were determined. Compounds 7 and 9-11 are soluble in benzene/toluene and could be well characterized by NMR spectroscopy and MS (EI) spectrometry. The results demonstrate the facile substitution of the hydridic hydrogen atoms in 5 and 6 by the halides with different reagents.     

Two types of intramolecular addition of an Al-N multiple-bonded monomer LAlNAr' arising from the reaction of LAl with N3Ar' (L = HC[(CMe)(NAr)]2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3)

The reaction of beta-diketiminated aluminum(I) monomer LAl with a large bulky azide N3Ar' (L = HC(CMeNAr)2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3) in the temperature range from -78 degrees C to room temperature affords two different isomers 2 and 3, which have been characterized by spectroscopic and X-ray structural analyses, as well as elemental analysis. The variable-temperature 1H NMR kinetic studies of this reaction indicate the existence of the monomer LAlNAr' (1) at low temperature and the thermal stability of the compounds increases in the order of 1 < 2 < 3.     

An enantioselective access to 1-alkyl-1,2,3,4-tetrahydroisoquinolines. Application to a new synthesis of (-)-argemonine

Potassium ferricyanide oxidation of salt 1 gave isoquinolinone 7 whose treatment with Grignard reagents resulted in a high-yield formation of substituted isoquinolinium salts 5. The selectivity of the reduction of these salts to give derivatives 6 has been studied. Particularly good selectivities (82-84%) were observed when R is a benzylic group. On the basis of these results, a practical and enantioselective synthesis of the natural alkaloid (-)-argemonine is presented.     

Synthesis of 4-amino-5-H-2,3-dihydroisothiazole-1,1-dioxide ring systems on sugar templates via carbanion-mediated sulfonamide intramolecular cyclization reactions (CSIC protocols) of glyco-alpha-sulfonamidonitriles

The carbanion-mediated sulfonate intramolecular cyclizations (CSIC protocols) of glyco-alpha-sulfonamidonitriles derived from readily available monosaccharides have been extensively investigated using potassium carbonate, cesium carbonate, n-BuLi, and LDA as bases. As a result, a series of enantiomerically pure spiro(4-amino-5-H-2,3-dihydroisothiazole-1,1-dioxide) derivatives have been prepared efficiently and isolated in good yield. The synthesis of these new bicyclic systems is key to accessing a novel range of aza analogues of TSAO nucleosides (ATSAOs).     

Direct detection of oxygen intermediates in the non-heme Fe enzyme taurine/alpha-ketoglutarate dioxygenase

The reaction of substrate-bound taurine/alpha-ketoglutarate dioxygenase with O2 has been studied using cryogenic continuous-flow spectroscopy. Transient absorption spectra acquired at -38 degrees C show an exponential decay of a 318-nm chromophore with an apparent rate of 1.3 s-1. The observed optical changes and their kinetics are consistent with the profile of an Fe(IV) species detected recently by M?ssbauer spectroscopy (Price et al., Biochemistry 2003, 42, 7497-7508). Resonance Raman measurement upon excitation at 363.7 nm reveal at least two oxygen isotope-sensitive vibrations at 821/787 cm-1 and 583/555 cm-1 for 16O and 18O derivatives, respectively. An additional mode is likely to be obscured by an ethylene glycol vibration at 865 cm-1 and/or 1089 cm-1. The 821 cm-1 vibration is assigned to the stretching mode of Fe(IV)=O species on the basis of its frequency and isotopic shift amplitude. The 583 cm-1 band is likely to originate from an Fe-O2 precursor of the Fe(IV)=O species, although its structural details are unclear at present.     

Extension of the tandem conjugate addition-Dieckmann condensation: the formal synthesis of tetracenomycin A2

Tandem cuprate addition-Dieckmann condensation is featured in the construction of the polyketide metabolite tetracenomycin A(2). Thus, cyclization substrate 11 was treated with Gilman cuprate Me(2)CuLi to afford anthracene 12. The phenolic acetate protecting group of 12ensured its chemoselective oxidation to reveal terminal quinone 13, which intercepts the previously reported synthesis of tetracenomycin A(2).     

Investigation of hydroxyl-mediated spectral interference from easily ionized elements in laser-enhanced ionization spectrometry in the 281–285 nm range

The spectral background from 281 to 285 nm in the laser-enhanced ionization (LEI) spectrum of aqueous samples containing easily ionized elements (EIE) at concentrations similar to those found in blood was investigated. A complex, structured spectral background was observed, which appears in the presence of Na or K, but does not match the spectral signature of either element. The same behavior was also observed for Li. It was established that this background originates from an energy transfer between laser-excited hydroxyl (OH) molecules and ground-state EIEs. The intensity of this spectral feature was found to increase with EIE concentration and applied electrode voltage. This unexpected source of spectral interference may complicate the determination of trace metals by LEI in the presence of EIEs, since it can not be prevented by simply avoiding interference from atomic lines.