Allylic zinc reagent

The chiral a-pyrrolidino imine that is formed in situ in the reaction of a-sulfonyl amide 141 with allylic zinc reagents undergoes addition with mod-... 

Most importantly, the 4,5-diamino-1,7-octadienes 248a, 256 and 257, available by the double addition of allylic zinc reagents to the diimine 247, are... 

D. Diverse Methods of Preparation of Allylic Zinc Reagents. 317... 

During the preparation of allylic zinc reagents, the formation of Wurtz-coupling products may be observed, especially if the intermediate allylic radical is well stabilized. However, the direct insertion of zinc foil to allyl bromide in THF at 5 C is one of the best methods for preparing an allylic anion equivalent. Allylic zinc reagents are more convenient to prepare and to handle than their magnesium and lithium counterparts... 

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. 

The addition of substituted allylic zinc reagents to aldehydes is usually unselective" . Furthermore, the direct zinc insertion to substituted allylic halides is complicated by radical homocoupling reactions. Both of these problems are solved by the fragmentation of homoallylic alcohols. Thus, the ketone 166 reacts with BuLi providing a lithium alcoholate which, after the addition of ZnCl2 and an aldehyde, provides the expected addition product... 

SCHEME 50. Preparation and uses of allylic zinc reagents by fragmentation reaction... 

SCHEME 51. Diastereoselective reactions of substituted allylic zinc reagents generated by fragmentation... 

SCHEME 56. Reactions of functionalized allylic zinc reagents... 

Allylic zinc reagents are highly reactive and add to acid chlorides and anhydrides. A double addition of the allylic moiety usually occurs, leading to tertiary alcohols . The double addition can be avoided by using a nitrile as substrate (Blaise reaction). By... 

SCHEME 90. Acylation by the addition of an allylic zinc reagent to a nitrile... 

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

Allylic zinc reagents are highly reactive reagents which are prone to undergo carbozincation of weakly activated alkenes . Thus, the addition of the mixed methallyl(butyl)zinc 393 with the vinylic boronate 394 provides the intermediate zinc reagent 395. After the addition of an extra equivalent of ZnBri, CuCN 2LiCl and allyl bromide, the reaction mixture was worked up oxidatively providing the alcohol 396 in 83% yield. ... 

Later work by Barbot and Miginiac2 and Jones and Knochel3 showed that additions of allylic zinc reagents to aldehydes are also reversible and occur by S/,-2 pathways. In the former case, the mixture of regioisomeric alcohol adducts from prenylzinc bromide and 3-pentanone changed composition in favor of the unbranched isomer with increased reaction times and temperature (equation 4)2. 

Addition of an allylic zinc reagent to cyclic aldimines has been reported129. Lithiated bisoxazoline 61 and allylzinc reagent form the reactive species, which adds to a cyclic aldimine enantioselectively with up to 97.5% (equation 29)129. 

Allylic zinc reagents undergo cross-coupling reactions with reactive halides leading to 1,5-dienes. Usually the new carbon-carbon bond is formed from the more-substituted end of the allylic system (Scheme 9-11) [30]. 

The allylic zinc reagents are generally prepared by mixing zinc metal with the requisite allyl halide using either sonication or a saturated aqueous solution of... 

Several new syntheses of 2,5-disubstituted oxazoles have been reported. The reaction of allyl zinc bromides with )V-cyanomethylimidates (136) leads to 2-alkyl- or aryl-5-allyloxazoles in 50-65% yields after hydrolysis of intermediate imine salts (137) <91SC1501>. The allyl zinc reagents add to the nitrile, in contrast to organolithium and magnesium reagents, which are too basic (Scheme 62). 

Following their earlier findings on the enantioselective allylzincation of cy-clopropenone acetals [43], Nakamura and co-workers investigated the same reaction with imines of aromatic aldehydes [44]. In an initial experiment, ( )-ben-zaldehyde N-phenyl imine (59) was treated with preformed chiral allylic zinc reagent 57a (R=z-Pr) obtained by mixing a lithiated bis(oxazoline) (55, R=z-Pr) with an allylic zinc bromide (56a) (Scheme 22). The allylation product was obtained in 95% yield, but the enantioselectivity was only 6% ee. Fortunately, the enantioselective allylzincation of Z- (i.e., cyclic) aldimines was more successful. 

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