Stille copper effect

The synthesis of pyrone derivatives attracted attention due to their synthetic potential. In an illustrative example Cho and co-workers studied the Suzuki-coupling of 3,5-dibromo-2-pyrone with arylboronic acids (8.9.). Under regular conditions the aryl group is introduced selectively into the more electron deficient 3-position, while in the presence of an equimolar amount of copper(I) iodide the coupling is diverted selectively into the 5-position (N.B. drop of the reaction temperature from 50 °C to ambient temperature negated the effect of copper and led to 3-arylation). The way copper effects the coupling is still unclear, but it was successfully used in the preparation of a range of 5-aryl-2-pyrons.17... 

The rate-enhancing influence of Cu(l) salts (the so-called "Copper Effect") in normally nonproductive and sluggish Stille couplings was first pointed out by Liebeskind et al.6 in 1990. A greater insight into this phenomenon was obtained later by Farina and co-workers.7 A number of modifications of the Stille reaction have since... 

The use of tributyltin and copper biocides may not be a matter of concern for ocean going vessels where the dilution is infinite, but the use of these biocides in conjunction with say cooling water systems, is not recommended because of the toxic effect and environmental risks. In order to utilise the technique for heat exchangers therefore, it would be necessary to develop suitable biocides that may be incorporated in the resin matrix and still be effective in controlling biofilm formation and growth, without detrimental effects. 

It is reported in literature that additives like LiCl or Cu(I) salts can have a dramatic influence on the coupling. The copper effect in Stille coupling reactions was investigated by Farina and Liebeskind and coworkers. For instance in the reaction of iodobenzene and vinyltributyltin in dioxane at 50 °C catalyzed by Pd2(dba)3 in presence of a strong ligand like PPhs, it was found that the addition of 2 molar equivalents of Cul per mol of catalyst led to a > 100 fold increase in reaction rate. 

Farina V, Kapadia S, Krishnan B, Wang C, Liebeskind LS. On the nature of the copper effect in the Stille cross-coupling. J. Org. Chem. 1994 59(20) 5905-5911. 

The dried malted barley is ground and mashed in a tub, after which the Hquid portion is drained off, cooled, and placed in the fermentor. After fermentation, a batch distillation system is usually used to separate the whisky from the fermented wort. The stiU consists of a copper ketde with a spiral tube or "worm" leading from the top. The dimensions and shape of the stills have a critical effect on the character of the whisky. The product taken off in the first part of the distillation is called foreshots (heads). The middle portion is the high wines and the last portion is the feints (tails). The middle portion is redistilled at the 140—160° proof (70—80%) range and matured in used oak cooperage. 

Manganese, copper, iron, cobalt and nickel ions can all initiate oxidation. Untinned copper wire can have a catastrophic effect on natural rubber compounds with which it comes into contact. Inert fillers for use in rubbers are usually tested for traces of such metal ions, particularly copper and manganese. The problem is perhaps less serious in saturated hydrocarbon polymers but still exists. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

Hie products of this catalytic enanlioselective 1,4-addition still contain an enone moiety, prone lo subsequenl 1,4-addition [73]. An inlriguing queslion regarding stereoconlrol was posed would the stereoselectivity in the second addition step be governed by the catalyst or would there be a major effect fToni the stereocenlers already present Sequential 1,4-addition lo dimetlioxy-substiltiled cyclobexadienone 66 (Scheme 7.18) using the copper catalyst based on IS, R, i j-ligand 18 both in the... 

Use of diis tedinique results in an equivalent of litiiium balide being present in die reaction mixture, unlike when die isolated copper arenediiolates ate employed. Litiiium salts can have very profound effects on coppet-niediated reactions, but in tliis case a sinidat ee f4096) and complete y selectivity were still obtained fbt die reaction between 21 and n-BuMgl when die catalyst was prepared from Cut. Nei-... 

On the other hand, this MO description matches a crystal field description of a Cu(II) ion insofar as the five highest occupied MO s are concerned four of these are doubly occupied and have a predominant copper 3 d character (65 % at least), whereas the fifth orbital contains the unpaired electron and still has 50% copper 3 d character. The very small MuUiken charge on the copper atom, which does not fit in the description of a Cu(II) ion, is caused by the partial occupation of the 4 p orbitals, which effect is neglected in the crystal field theory. 

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