The mixed alkali effect in lithium-sodium borate glasses

The electrical conductivity of a series of 0.35 (Li, Na)₂O·B₂O₃ glasses shows a minimum at the composition Na/(Na+Li)～0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.     

Tracer diffusion and electrical conductivity in sodium-cesium silicate glasses

The tracer diffusion coefficients of ²²Na and ¹³⁷Cs, and the electrical conductivity have been measured in the (Na, Cs)₂O:3SiO₂ glasses as a function of temperature and Cs/Na ratio. Complex impedance analysis was used for the conductivity measurements. The Haven ratio at 396.5°C increases from 0.3–0.4 in single-alkali glasses to 0.8 for the mixed-alkali compositions. The results are explained in terms of a single-jump mechanism; interactions between alkali ions and non-bridging oxygen ions, and between different alkali ions, produce the observed correlation effects.     

Mixed isotope electrical conductivity in lithium borate glasses

Electrical conductivity of (Li⁶/Li⁷)₂O·3B₂O₃ glasses with varying Li⁶/Li⁷ ratios has been measured as a function of temperature. The conductivity obtained after complex impedance analysis decreases continously and the activation enthalpy increases slightly as Li⁷ is substituted for Li⁶. This suggest that the difference in mass of the alkali ions is not the cause of the mixed alkali effect.     

Oxidation of p-aminophenols and formal radical cyclization onto benzene rings: formation of benzo-fused nitrogen heterocycles

p-Iodophenol and its O-MOM-protected ether can be converted into iodoamines 2. These give cross-conjugated ketones 3 on oxidation with hypervalent iodides in the presence of methanol, and the ketones undergo radical cyclization. Exposure of the products to acid or sequential treatment with a Grignard reagent and acid effects rearomatization to produce benzo-fused nitrogen heterocycles 4.     

Palladium-mediated fragmentation of meta photocycloadducts using carbon based electrophiles. Part 1

Substituted benzene derived meta photocycloadducts have been shown to undergo a fragmentation/arylation reaction under Heck reaction conditions to give bridged bicyclo[3.2.1] compounds in a highly atom-efficient manner. When an anisole derived meta photocycloadduct is used, a bridgehead ketone is generated. However, if an alkylbenzene derived meta photocycloadduct is used, a bridgehead alkene is formed. This strategy has been used to create novel enol ether and transient allyl silane compounds.     

Towards conformationally-locked difluorosugar analogues: an unexpected sense of dihydroxylation

Difluorinated cyclooctenones, synthesised using RCM, can be used as templates for stereoselective oxidative transformations to products that undergo transannular reactions to afford conformationally-locked analogues of 2-deoxy-2,2-difluorosugars with different stereochemical relationships between the C-2 and C-3 hydroxyl groups.     

A New General Approach to Enantiomerically Pure Cyclic and Open-Chain (R)- and (S)-α,α-Disubstituted α-Amino Acids

A wide range of cyclic and open-chain α,α-disubstituted α-amino acids 1a-p were prepared. The racemic N-acylated α,α-disubstituted amino acids were resolved by coupling to chiral amines 15-18 derived from (S)-phenylalanine to form diastereoisomers 19/20 or 21/22 that could be separated by crystallization and/or flash chromatography on silica gel (Scheme 3). Selective cleavage via the 1,3-oxazol-5(4H)-ones 10a-p gave the corresponding optically pure α,α-disubstituted amino-acid derivatives 11 or 12 in high yield (Scheme 3). The absolute configurations of the α,α-disubstituted amino acids were determined from X-ray structures of the diastereoisomers 20, 21g′, 22d .     

Preparation of polycyclic systems by sequential 5-exo-digonal radical cyclization, 1,5-hydrogen transfer from silicon, and 5-endo-trigonal cyclization

Radicals of type 1 undergo 5-exo diagonal cyclization, and the resulting vinyl radical abstracts hydrogen from silicon to afford a silicon-centered radical. This radical closes in a 5-endo trigonal manner to generate radicals of type 4, which are reduced (4 --> 5) by stannane, except when the starting acetylene carries a terminal trimethylstannyl group. In this case, radicals 4 expel trimethylstannyl radical to afford vinyl silanes 6. The stereochemical outcome of the radical cascade 1 --> 5 is controlled by the stereochemistry of the oxygen-bearing carbon in 1 (see starred atom). The sequence can be initiated by carbon-, alpha-substituted carbon-, oxyacyl-, and carbamoyl radicals and generates a silicon-containing ring fused onto a carbocycle or heterocycle. Numerous examples are described, as well as a number of transformations of the final cyclization products, especially their response to n-Bu(4)NF and to BF(3).OEt(2), reagents that cleave the newly formed carbon-silicon bond.     

Synthesis and structural characterization of cis-Pt(SH)2(PPh3)2 and related alkyl and aryl thiolato complexes

The synthesis and spectroscopic characterization of the complexes cis-Pt(SR)₂(PPh₃)₂ (R = H. Me, n-Bu and Ph) are reported, together with the single crystal X-ray crystallographic characterization of cis-Pt(SH)₂(PPh₃)₂. The ability of these complexes to act as bidentate ligands towards other metal centres is also briefly described.