Allylic zinc derivative

Several methods have been described for preparing allylic zinc derivatives. In contrast to alkylzincs, allylic zinc reagents are much more reactive due to the more ionic nature of the carbon-zinc bond in these organometallics. The chemistry displayed by these reagents is not representative of the usually moderate reactivity of organozinc derivatives. Tamaru and coworkers have converted various allylic benzoates to the corresponding organozinc intermediates in the presence of palladium(O) as catalyst. 

In the case of allylic zinc derivatives, which isomerize much more readily in a similar fashion, the rearrangement takes place by reversal of the addition, and readdition of the allylic organometallic to give the thermodynamically favored, less hindered product (51). More recently, Benkeser has demonstrated the reversibility of the addition of the allyl Grignard itself (6). 

A regiospecific transposition of the allyl fragment involving exclusive y-reaction is observed e.g. 27 -> 28) in both studies. A related reaction between (presumably) allylic zinc derivatives and aldehydes or ketones occurs when a mixture of an allylic bromide and the carbonyl compound is passed along a column of zinc granules heated at the reflux temperature of THF. Hydrolysis of the resultant zinc alkoxide provides another route to homoallylic alcohols (Scheme 12), with no problem from Wurtz coupling of the bromide. The column may be used repeatedly without contamination, providing the zinc is replenished as it is used up. ... 

SCHEME 91. Reaction of allenic, propargylic and allylic zinc reagents with carbonyl derivatives... 

BINOL-derived phosphoramidites are versatile ligands in palladium-catalysed umpol-ung allylation of aryl aldehydes mediated by diethylzinc.172 The possible roles of allyl-zinc and -palladium species in the mechanism are discussed in detail. 

Vinyl isoxazolines 48 can be prepared by cycloaddition of nitrile oxide to 1,3-butadiene. An alternative synthetic approach has been recently described. The method is based on domino nucleophilic addition-anionic C-O-heterocyclization of nitrile oxides with allyl organometallics derived from trans-1,4-dihalobutene and a metal such as zinc, magnesium or indium. In the case of indium an aqueous media could be used <07SL1449>. 

Displacement of a cyano group in a-cyanoketones is possible. Treatment of the a-cyanoketone with Sml2 followed by addition of an excess of allyl bromide gave the a-allyl ketone derivative. a-Cyano amines react with allyl bromide and then zinc metal to give homoallylic amines after treatment with dilute acetic acid in... 

This last example succeeds partly because nickel catalyses the union of the three reagents in the second step. Similar things happen in the combination of nickel allyls with zinc borates 76. The allyls are derived from unsymmetrical allylic acetates 75 and react at the end remote from the phenyl group (R1 is an alkyl group and R2 is an aryl or alkyl group).19... 

The problem with forming allyl metal derivatives by exchange of zerovalent metal with halide ions is that we often cannot be sure where the halide ion is and never sure where the metal is because of rapid [1,3] shifts, cf. compounds 21 and 23. If the allylic system has a substituent that chelates with the metal then only one derivative is likely to be formed. We need a halide at one end of the allyl group and a chelating substituent at the other. Just such compounds can be made by acylation of acrolein 72 in the presence of zinc chloride. No [1,3] shift of Br will occur as the alkene much prefers to be disubstituted and conjugated with oxygen 151 rather than unconjugated and terminal 152. 

Allylic boron and zinc derivatives in synthesis of nitrogen heterocycles 00PAC1641. 

Negishi and his co-workers have developed several approaches to allylated arenes involving cross-coupling reactions. Preliminary studies established that aryl-aluminium, zirconium, or zinc compounds readily undergo palladium-catalysed cross-coupling with allylic halides or acetates (Scheme 19). Later in the year it was shown that allylic alcohol derivatives containing OAlR2,... 

---