Copper oxide, stereochemistry

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

Table 28.2 Oxidation states and stereochemistries of copper, silver and gold...

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

Bis-oxazoline ligands can also be produced by oxidative coupling of the copper derivative of diastereoisomerically pure 306 (Scheme 145) . Further lithiations of the product 317, which was produced as single diastereoisomer, occur (as in Scheme 143) at the second site adjacent to the oxazoline, giving, for example, 318, despite the (presumably) less favourable stereochemistry of the lithiation step. Bisoxazolines 318 direct the asymmetric copper-catalysed cyclopropanation of styrene using diazoacetate. 

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

Very simple experimental conditions (palladium(II) chloride (catalyst, 0.1 equivalent) copper(II) chloride (oxidant, 3 equivalents) and sodium acetate (buffer, 3 equivalents) in acetic acid under carbon monoxide at normal pressure and temperature) are necessary for this asymmetric route to saturated fused heterocycles with defined stereochemistry. 

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