Cuprates effect

A priori, an allenyl cuprate intermediate might account for the observed products. However, the reaction was carried out in two stages with only 5mol% of CuBr. Thus, if a cuprate intermediate was the reactive species, a theoretical yield of 5% would have been expected. However, the reaction did not take place in the absence of the copper catalyst. Accordingly, a catalytic cycle was proposed in which the initial dibutyl cuprate effects a well precedented SN2 displacement on the vinyloxirane,... 

Acylcuprate reagents.1 The reaction of cuprates of the type R2(CN)CuLi2 with carbon monoxide at -110° results in carbonylated reagents, possibly with the composition (RCO)R(CN)CuLi2. In any case, these cuprates effect 1,4-acylation of a,p-enones and -enals to provide 1,4-dicarbonyl compounds. 

The basicity of Gilman cuprates is so low that they do not undergo acid/base reactions with acetylene or higher terminal alkynes. Instead, Gilman cuprates effect the car-bocupration of the C=C triple bond (Figures 13.12 and 13.13). This reaction formally resembles the hydroboration of a C=C triple bond ( see example in Figure 13.10). The regioselectivity also is the same hence, the metal is connected to the Cl center of a terminal alkyne. Finally, the reaction shows the same stereoselectivity as in the case of the hydroboration of a C=C triple bond carbocupration occurs as a cis addition. 

Keywords High-Tc cuprates, effective band-structure, slave boson method, La2 Sr Cu04... 

Doping of a cuprate effectively means that some Cu " ions are oxidized to Cu +. Also, a reduction to Cu" leads to superconductivity. The Cu " sites are very likely localized. However, the Cu " /Cu + exchange may still have a quite low time constant (1 ps to 1 ns). 

In addition to the boron trifluoride-diethyl ether complex, chlorotrimcthylsilanc also shows a rate accelerating effect on cuprate addition reactions this effect emerges only if tetrahydrofuran is used as the reaction solvent. No significant difference in rate and diastereoselectivity is observed in diethyl ether as reaction solvent when addition of the cuprate, prepared from butyllithium and copper(I) bromide-dimethylsulfide complex, is performed in the presence or absence of chlorotrimethylsilane17. If, however, the reaction is performed in tetrahydrofuran, the reaction rate is accelerated in the presence of chlorotrimethylsilane and the diastereofacial selectivity increases to a ratio of 88 12 17. In contrast to the reaction in diethyl ether, the O-silylated product is predominantly formed in tetrahydrofuran. The alcohol product is only formed to a low extent and showed a diastereomeric ratio of 55 45, which is similar to the result obtained in the absence of chlorotrimethylsilane. This discrepancy indicates that the selective pathway leading to the O-silylated product is totally different and several times faster than the unselective pathway" which leads to the unsilylated alcohol adduct. A slight further increase in the Cram selectivity was achieved when 18-crown-6 was used in order to increase the steric bulk of the reagent. 

Addition of 15-crown-5 to the higher-order cuprate led to a reagent that is totally unrcac-tive towards 2-phenylpropanal even at room temperature18. If, however, boron trifluoride-diethyl ether complex was added as additional ingredient, the reactivity was restored and, furthermore, the Cram selectivity increased to 90 10 (Table 4). Analogous results could be obtained by placing the crown-ether effect within the cuprate itself, as in reagent 10. 

The /J-amino aldehyde 1. readily available from aspartic acid, is configurationally very stable. No epimerization of the stereocenter is observed upon addition of Grignard Or cuprate reagents and the transformations display relatively high levels of 1,3-asymmetric induction73. This C-3-directing effect is superimposed upon the 1,2-directing effects of the C-2-substituents (Rl). 

There has been a great deal of study concerning the effect of solvents and other reaction conditions on the stability and reactivity of organocuprate species.8 These studies have found, for example, that (CH3)2S-CuBr, a readily prepared and purified complex of CuBr, is an especially reliable source of Cu(I) for cuprate preparation.9 Copper(I) cyanide and iodide are also generally effective and, in some cases, preferable.10... 

Some reactions of this type are accelerated further by use of naphthoic acid as an additive. This effect is believed to result from formation of a mixed anionic cuprate... 

Karpfen A (2007) Theoretical Characterization of the Trends in Halogen Bonding. 126 1-15 Keller H (2005) Unconventional Isotope Effects in Cuprate Superconductors 114 143-169 Keller H, see Bussmann-Holder A (2005) 114 367-386 Khan AI, see Williams GR (2005) 119 161-192... 

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