Allyl electrophiles alkynylation

The reactivity of ggm-dizmcioalkenes (prepared by metal allylation of alkynyl metals (see Sections. )) with aldehydes leads to the corresponding aUene derivatives in moderate yields (Scheme 38). As already seen for gm-dizincioalkanes, an internal chelation of the 1,1-dimetalloalkene with a Lewis functional group allows differentiation of the reactivity of the two metal centers towards electrophile (see Electrophile) reagents. Scheme 39 summarizes the reactivity... 

The overall reaction makes it possible to perform intramolecular caibozincations [65-71] via a radical cyclization. This useful preparation of cyclopentylmethylzinc derivatives proceeds with excellent stereoselectivity and allows preparation of quaternary centers. After cyclization, the zinc organometallic can be transmetallated with CuCN-2LiCl and made to react with a broad range of electrophiles such as acid chlorides, allylic and alkynyl halides, ethyl propiolate, 3-iodo-2-cyclohexen-l-one, and nitroolefins such as nitrostyrene, leading to products of type 40 (see Section 9.6.9 Scheme 9-34) [65,70]. 

This reaction works with aryl, allylic and 1-alkynyl halides, but not vinylic halides. A vinylic group can be introduced into the 4 position of an isoquinoline by using PdBr2 as the electrophile and an alkene (Scheme 9).10 This process... 

The group R1 can be allyl, acyl, or alkynyl, and arynes can also act as the acceptors. The catalysts are usually Ni(cod)2, or ligated palladium. The mechanisms are not understood in detail, but a catalytic cycle involving the product of oxidative addition, Sn-M-R1, is thought to be involved. The stannylalkenes that are formed can then be subjected to reaction with electrophiles (e.g., AczO or RCH=0), or to coupling reactions in the presence of transition metals (e.g., the Stille reaction). 

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

Pentafluorophenylcopper exhibits high reactivity towards a variety of organic substrates such as aryl, vinyl, alkynyl, allyl halides etc. [226,227,229,235-238] (Scheme 77). Similar to trifluorovinylcopper, pentafluorophenylcopper readily adds to hexafluoro-2-butyne to form the syn addition product, which can be quenched with electrophiles [230] (Scheme 78). 

The presence of an alkenyl, phenyl, or alkynyl group one carbon away from a halogen, oxygen, or another electronegative atom, such as sulfur, makes allylic, benzyllic, and propargyllic electrophiles very reactive in oxidative addition... 

In general, allylic and propargyllic electrophiles are much more reactive than alkenyl and alkynyl electrophiles. Thus, a wide variety of electrophiles containing halogens, e.g. I, Br, and Cl, and oxygen groups, e.g. sulfonates, phosphates, carboxylates, carbonates, alkyl and aryl ethers, and even silyl... 

Note that the multiple bond of unsaturated o-organyls (e.g. alkenyls, alkynyls, acyls, allyls) may present an alternative site to the M-C bond for electrophilic attack. 

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