N-arylation of aziridines

A range of N-arylaziridines were prepared by the palladium or copper catalyzed amination reaction between N-H aziridines and aryl bromides or arylboronic acids. These results showcase the synthetic utility of metal-bound aziridine species in nitrogen transfer processes.     

OPTICAL ABSORPTION AND ELECTRON SPIN RESONANCE SPECTRA OF CATION RADICALS OF DIMERIC CHLOROPHYLL a IN LOW-TEMPERATURE MATRICES

Abstract— A chlorophyll (Chl) a solution in 3-methylpentane at 77 K exhibits an absorption spectrum with a distinct peak at 706 nm in the red-band region. The formation of the 706 nm absorbing species (S706) was reversible with respect to temperature change; no chemical change was observed. γ-Irradiation of the rigid 3-methylpentane solution at 77 K yields an absorption spectrum which can be ascribed to S706⁺ and S706⁻. When carbon tetrachloride, an electron scavenger, was added to the solution, the absorption of S706⁺ survived, which has peaks at 850 and 956 nm. It is assumed that the S706 is hydrogen-bonded dimeric Chi a , which may be regarded as a model of P700 in photosynthesis. Cation radicals of monomeric Chi a were formed in a γ-irradiated sec -butyl chloride solution at 77 K, and an absorption spectrum with peaks at 730 and 813 nm was recorded. ESR spectra of the cation radicals of S706 and monomeric Chi a are of a similar shape but their linewidths are 7.5 and 11.0 Gauss, respectively. The linewidth narrowing observed for S706⁺ is clear evidence for the assumption that S706 is dimeric Chi a. Comparison was made of the absorption spectrum of S706⁺ with the light-induced spectrum of P700 reported earlier.     

Direct catalytic asymmetric and anti-selective vinylogous addition of butenolides to chromones

An anti-selective catalytic asymmetric Michael-type vinylogous addition of β,γ-butenolides to chromones was developed. The catalyst system developed herein is characterized by tuning of the steric and electronic effects using a proper Biphep-type chiral ligand to invert the diastereoselection, and improvement of the catalyst turnover by a coordinative phenolic additive. The catalytic protocol renders potentially biologically active natural product analogs accessible in good yield with moderate diastereoselectivity and high enantiomeric purity, mostly greater than 99% ee.

An anti-selective catalytic asymmetric Michael-type vinylogous addition of β,γ-butenolides to chromones was developed.     

Cyclic tautomers of tryptophans and tryptamines—4: Synthesis of cyclic tautomers of tryptophans and tryptamines

N_b-Methoxycarbonyltryptophan methyl ester (dl- and l-13) was cyclized to the corresponding rans cyclic tautomer (14) in excellent yield in various acids such as 85% phosphoric acid or trifluoroacetic acid. The cis cyclic tautomer (15) was formed as the less stable and kinetically controlled product and converted to the more stable trans isomer (14) under the reaction condition. The trans isomer (14) was reverted to 13 on treatment with 10% sulfuric acid in methanol. Other tryptophan and tryptamine derivatives (6 and 19a) also cyclized to the corresponding cyclic tautomers in similar acidic media.     

α-methoxyallyl sulfides as a novel homoenolate dianion equivalent

α-Methoxyallyl sulfides 1 proved to serve a novel homoenolate dianion equivalent. Introduction of two electrophiles into 1 was realized by successive regiospecific reaction of an allylic moiety with electrophiles intervened by an acid-catalyzed thioallylic rearrangement. Facile desulfurization of the products under mild conditions afforded a variety of (α,β- unsaturated carbonyl compounds and 2,3,4-trisubstituted furans.     

Deposition of gold nanoparticles on silica spheres by electroless metal plating technique

A previously proposed method for metal deposition with silver [Kobayashi et al., Chem. Mater. 13 (2001) 1630] was extended to uniform deposition of gold nanoparticles on submicrometer-sized silica spheres. The present method consisted of three steps: (1) the adsorption of Sn(2+) ions took place on surface of silica particles, (2) Ag(+) ions added were reduced and simultaneously adsorbed to the surface, while Sn(2+) was oxidized to Sn(4+), and (3) Au(+) ions added were reduced and deposited on the Ag surface. TEM observation, X-ray diffractometry, and UV-vis absorption spectroscopy revealed that gold metal nanoparticles with an average particle size of 13 nm and a crystal size of 5.1 nm were formed on the silica spheres with a size of 273 nm at an Au concentration of 0.77 M.     

Electrochemical lithium intercalation into WO3 and lithium tungstates Li x WO3+ x /2 of various structures

Hexagonal and monoclinic tungsten trioxides WO₃ and hexagonal lithium tungstates Li _x WO_{3+ x /2} (x = 0.10–0.42) from a soft chemistry route were used as the active cathode material in secondary lithium batteries. The hexagonal structures, regardless of their being an oxide or a tungstate, showed higher specific capacities and better cycling behavior in Li⁺ intercalation reactions than the monoclinic form. The presence of pre-allocated lithium (as Li₂O) in hexagonal tungstates decreased the capacity for lithium intercalation. Additionally, the plot of open-circuit voltage (OCV) against the depth of intercalation (n) for anhydrous tungstates showed two straight lines with different slopes that can be related to the structural changes in lithium intercalation. The effective diffusion coefficients of lithium insertion into the host structure, D˜, were also found to be dependent on the structure and the composition of these compounds. Received: 28 November 1997 / Accepted: 6 March 1998     

Deep levels related to transition metals in Si under hydrostatic pressure

The influence of hydrostatic pressure up to 5×10⁸ Pa on deep levels related to transition metal impurities in silicon is determined by means of an isothermal capacitance method. Under pressure, donor levels of isolated Fe, V, Ti, and Mn shift towards the valence band in contrast to earlier results for deep chalcogen donors. This behavior is contrary to what is expected by considering only effects of hybridization. Quantitative differences between Fe, Ti, V, and, on the other hand, Mn suggest a different microscopic structure of these defects. The Fe-acceptor pairs FeB, FeAl, and FeGa move towards the valence band with a rate comparable to that of the ₁ conduction band. The thermal capture coefficients of isolated Fe, V, and Ti are found to be pressure independent up to 5×10⁸ Pa.On leave from Sony Corp., Research Center, Yokohama, Japan     

Substitution effect,absorption, and fluorescence behaviors of 11,12-benzo-1,7,10,13-tetraoxa-4-aza- cyclopentadec-11-ene (benzoaza-15-crown-5) derivatives upon cation complexation in solvent extraction

Substitution effect, absorption, and fluorescence behaviors of some benzoaza-15-crown-5 derivatives upon cation complexation in solvent extraction were studied. The introduction of a substituent on the nitrogen atom in benzoaza-15-crown-5 enhanced extractabilities in the solvent extraction of aqueous alkali metal picrates. The nondonating substituents raised the cation selectivity for Na(+) over K(+), but the donating substituents reduced the cation selectivity. The absorption and fluorescence spectral behavior was different with the alkali metal cations.     

Synthesis of monodisperse,magnetic latex particles with polystyrene core

A novel method for producing monodisperse, submicron-sized magnetic latex particles is described. The method provides coating of polymer particles with surface-modified magnetic particles during soap-free polymerization. Experiments were performed with styrene monomer, potassium persulfate initiator, Fe ₃O ₄ magnetic particles, and silane-coupling reagents of methacryloxypropyltrimethoxysilane (MPTMS) and methacryloxypropyldimethoxysilane (MPDMS). The morphology of the magnetic particles depended on the silane-coupling reagents. Use of the tri-functional coupling reagent MPTMS produced particles having a disk-like or concave-like shape, whereas use of the di-functional coupling reagent MPDMS produced spherical particles that had a coefficient of variation of 4.4%, which was much smaller than the standard criteria of monodispersity, 10%.