Electrophilic reactions zinc compounds

Whereas uncatalyzed substitution reactions of organozinc compounds are limited to very reactive electrophiles, metal-transmetallated organozinc compounds are able to perform substitution reactions on various electrophiles. In the case of conjugated electrophiles, these zinc copper reagents can follow a Sn2 or Sn2 mechanism. 

Organozinc addition reactions to unsaturated systems lead to an intermediate zinc compound which is in most cases further hydrolyzed to give the corresponding protonated organic compound. This intermediate can also be trapped by reaction with electrophiles (see Electrophile) (E+) such as aldehydes, acyl chlorides, organic halides, silyl halides, and so on. Depending on the nature of the electrophile (see... 

Copper derivatives, in the form of cuprates (RCu(X)Li), are usually prepared in situ from lithium, magnesium or zinc compounds, by reaction with a Cu(I) sonrce snch as CnCN. They are often used to improve conversions using, for example, an acid chloride or an allylic halide as electrophile - a number of examples appear in other sections of this book. They are also nsefnl as the organometallic partners in some crosscoupling reactions (using cobalt catalysts). ... 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

The massive zinc (rod or plate) reacts spontaneously with activated bromides provided the preliminary electroreduction of a catalytic amount of zinc salt (ZnBr2 or ZnCl2) occurs. Reactions are carried out in nitrile solvents (CH3CN, PhCN,. ..) or their mixture with dichloromethane. An undivided cell fitted with a zinc anode and an indifferent cathode (gold, nickel, carbon, zinc,. ..) is used. As observed with benzylic bromides, the activation leads to an organozinc compound able to react with either the nitrile solvent or an electrophile reagent. The process is depicted in equation 12. 

Compound 32 may be removed, after the Ugi reaction, under particularly mild conditions, thanks to sulfur activation by soft electrophiles, such as mercury salts. The yields obtained in zinc-mediated Ugi reactions are excellent and the diastereo-meric ratios are in line with those obtained with 27. Cleavage of the chiral auxiliary can be performed, after methylamine-promoted deacylation of the sugar hydroxy groups, by a diluted solution of CF3CO2H in the presence of Hg(OAc)2. Under these conditions the acyl group on nitrogen is retained. However, the enantiomer of 32 is not easily accessible. 

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