Boron in copper: A perfect misfit in the bulk and cohesion enhancer at a grain boundary

Using first principles electronic structure methods, we calculate the effects of boron impurities in bulk copper and at surfaces and grain boundaries. We find that boron segregation to the Sigma5(310)[001] grain boundary should strengthen the boundary up to 1.5 ML coverage (15.24 at./nm2). The maximal effect is observed at 0.5 ML and corresponds to boron atoms filling exclusively grain boundary interstices. In copper bulk, B causes significant distortion both in interstitial and regular lattice sites, for which boron atoms are either too big or too small. The distortion is compensated to a large extent when the interstitial and substitutional boron combine together to form a strongly bound dumbbell. Our prediction is that bound boron impurities should appear in a sizable proportion if not dominate in most experimental conditions. A large discrepancy between calculated heats of solution and experimental terminal solubility of B in Cu is found, indicating either a significant failure of the density functional approach or, more likely, strongly overestimated solubility limits in the existing B-Cu phase diagram.

Efficient heterogeneous asymmetric catalysis of the Mukaiyama aldol reaction by silica- and ionic liquid-supported Lewis acid copper(II) complexes of bis(oxazolines)

Lewis acid complexes based on copper(II) and an imidazolium-tagged bis(oxazoline) have been used to catalyse the asymmetric Mukaiyama aldol reaction between methyl pyruvate and 1-methoxy-1-tri-methylsilyloxypropene under homogeneous and heterogeneous conditions. Although the ees obtained in ionic liquid were similar to those found in dichloromethane, there was a significant rate enhancement in the ionic liquid with reactions typically reaching completion within 2 min compared with only 55% conversion after 60 min in dichloromethane. However, this rate enhancement was offset by lower chemoselectivity in ionic liquids due to the formation of 3-hydroxy-1,3-diphenylbutan-1-one as a by-product. Supporting the catalyst on silica or an imidazolium-modified silica using the ionic liquid or in an ionic liquid-diethyl ether system completely suppressed the formation of this by-product without reducing the enantioselectivity. Although the heterogeneous systems were characterised by a drop in catalytic activity the system could be recycled up to five times without any loss in conversion or ee.

Recyclable copper catalysts based on imidazolium-tagged bis(oxazolines): A marked enhancement in rate and enantioselectivity for Diels-Alder reactions in ionic liquid

Imidazolium-tagged bis(oxazolines) have been prepared and used as chiral ligands in the copper(II)-catalysed Diels-Alder reaction of N-acryloyl- and N-crotonoyloxazolidinones with cyclopentadiene and 1,3-cyclohexadiene in the ionic liquid 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [emim][NTf2]. A significant and substantial enhancement in the rate and enantioselectivity was achieved in [emim][NTf2] compared with dichloromethane. For example, complete conversion and enantioselectivities up to 95 % were obtained for the reaction between N-acryloyloxazolidinone and cyclopentadiene within 2 min in [emim][NTf2] whereas the corresponding reaction in dichloromethane required 60 min to reach completion and gave an ee of only 16 %. The enhanced rates obtained in the ionic liquid enabled a catalyst loading as low as 0.5 mol % to give complete conversion within 2 min while retaining the same level of enantioselectivity. The imidazolium-tagged catalysts can be recycled ten times without any loss in activity or enantioselectivity and showed much higher affinity for the ionic liquid phase during the recycle procedure than the analogous uncharged ligand.

Ionic Liquid Effect on the Reversal of Configuration for the Magnesium(II) and Copper(II) Bis(oxazoline)-Catalysed Enantioselective Diels-Alder Reaction

Ionic liquids have been used to support a range of magnesium-and copper-based bis(oxazoline) complexes for the enantioselective Diels-Alder reaction between N-acryloyloxazolidinone and cyclopentadiene. Compared with reaction performed in dichloromethane or diethyl ether, an enhancement in ee is observed with a large increase in reaction rate. In addition, for non-sterically hindered bis(oxazoline) ligands, that is, phenyl functionalised ligands, a reversal in configuration is found in the ionic liquid, 1-ethyl-3-methylimidazolium bis[(trifluoromethanesulfonyl)imide], compared with molecular solvents. Supported ionic liquid phase catalysts have also been developed using surface-modified silica which show good reactivity and enantioselectivity for the case of the magnesium-based bis(oxazoline) complexes. Poor ees and conversion were observed for the analogous copper-based systems. Some drop in ee was found on supporting the catalyst due a drop in the rate of reaction and, therefore, an increase in the contribution from the uncatalysed a chiral reaction.

Model calculations for copper clusters on Au(111) surfaces

Using the semi-empirical embedded-atom method, the structure of small copper clusters on Au(111) surfaces has been investigated both by static and dynamic calculations. By varying the size of roughly circular clusters, the edge energy per atom is obtained; it agrees quite well with estimates based on experimental results. Small three-dimensional clusters tend to have the shape of a pyramid, whose sides are oriented in the directions of small surface energy. The presence of a cluster is found to distort the underlying lattice of adsorbed copper atoms. (C) 2002 Published by Elsevier Science B.V.