Comparing Classes of Finite Structures

We compare classes of structures using the notion of a computable embedding, which is a partial order on the classes of structures. Our attention is mainly, but not exclusively, focused on classes of finite structures. Also, a number of problems are formulated.     

Cleavage of MEM ethers by tetrahalozincate reagents

A modification of the zinc halide-mediated removal of the MEM group is described. By the expedient of adding two molar equivalents of etheral hydrogen chloride or of lithium halide, the method is extended to substrates which otherwise chelate the zinc reagent without undergoing deprotection. The compatibility of the resulting reagent systems with other functional groups is demonstrated, and examples are presented where deprotection of mono-MEM-protecteed 1,2- and 1,3-diols can be carried out, avoiding the cyclisation which occurs normally.     

Dehalogenation of alkyl halides catalyzed by single-enantiomer complexes of titanium

Racemic alkyl bromides were dehalogenated with a stoichiometric quantity of a main group alkyl metal reductant in the presence of a chiral, single-enantiomer titanium catalyst. The reaction was monitored by chiral gas chromatography of samples from the reaction mixture. The chiral titanium compounds examined proved to be effective catalysts. However, there was no detectable difference in the rate of reduction between the two enantiomers of the alkyl halide. An enantiomerically pure secondary bromide was reduced under the same conditions without racemization during the course of the reaction. This indicates that the secondary alkyl bromide is stereochemically stable to the reaction conditions. Radical probe reactions suggest a radical mechanism.     

Comparison of the X-ray crystal structures of four closely related Mo(CO)5(R2PXR′) (R2 = OCH2CMe2CH2O, XR′ = S-2-Pr, NHC6H4-4-Me; R2 = Ph2, XR′ = NHC6H4-2-Me, OC6H4-4-SMe) complexes

The X-ray crystal structures of four closely related Mo(CO)₅(R₂PXR) (R₂ = OCH₂CMe₂CH₂O, XR = S-2-Pr, NHC₆H₄-4-Me; R₂ = Ph₂, XR = NHC₆H₄-2-Me, OC₆H₄-4-SMe) complexes have been determined. The R₂PXR ligands are oriented so that the P—X bond and one of the Mo—C bonds are nearly eclipsed. This results in the distortion of the octahedral coordination geometry via tilting of the Mo(CO)₅ group away from the XR group. As observed in related complexes, the Mo—C bond of the carbonyl trans to the phosphorus-donor group is shorter than are the Mo—C bonds of the carbonyls trans to carbonyls. In contrast, no significant differences were observed between the Mo—C bonds of carbonyls trans to phosphites and the Mo—C bonds of carbonyls trans to phosphinites. The conformations of the 1,3,2-dioxaphosphorinanes were distorted chairs with the P end of the chair significantly flattened relative to the seat of the chair. This conformation is similar to that which has been observed for 1,3,2-dioxaphosphorinanes in other transition metal complexes.     

Beamline 12.2.2: An Extreme Conditions Beamline at the Advanced Light Source

The Advanced Light Source (ALS) is a 1.9-GeV, third-generation synchrotron optimized for the production of VUV and soft X-rays from undulators. There is also a hard X-ray program at the ALS, which is based around three 6-T superconducting bending magnets [1 Robin, D. Proceedings of the 2002 European Particle Accelerator Conference. Paris, France. pp.215Geneva: EPAC..  [Google Scholar]] that shift the critical energy from 3 keV to 12 keV. The extreme conditions beamline at the ALS is situated on Beamline 12.2.2, which benefits from radiation produced by one of these superbend sources. The beamline is designed for X-ray diffraction, X-ray spectroscopy, and X-ray imaging of samples held in diamond-anvil high-pressure cells (DACs). In a DAC, samples are on the order of 10 to 50 μm in diameter and 10 to 30 μm thick and are contained in a metal gasket of typical inner diameters of 100 to 150 μm. For high-quality diffraction patterns with little or no contamination from diffraction from the gasket, the X-ray beam size needs to be on the order of 10 μm × 10 μm.