Preparation allylzinc reagents

In particular 1- or 3-hetero-substituted allylzinc reagents are often prepared in situ by adding anhydrous zinc dichloride or dibromide to the solution of the allyllithium compound. 

Theoretical and experimental studies on the reaction of allylzinc compounds with vinylmagnesium (or other organometallic compounds with vinyl-metal bond) revealed details of the reaction involving transmetalation. The reagents prepared from a combination of allylzinc and vinylmagnesium bromide are doubly trapped... 

Here an alkynyl sulfoxide 55 is first carbocuprated with an organocopper reagent 56 to provide a vinylcopper intermediate 57, which is then zinc homologated with the primary zinc sp3-carbenoid 58 to yield the allylzinc intermediate 59. This, in a spontaneous syw-/)-climination, gives the corresponding allene 60. This protocol could also be adopted to the preparation of chiral allenes. 

As described in Section II, the preparation of 1,1-dizincioalkane can be achieved not only by reduction of 1,1-dihaloalkane, but also by allylzincation of alkenyl metal compounds. The reaction of allylzinc with alkenyl Grignard reagent, which is followed by the addition of aldehydes, gives dienes as shown in Scheme 1511,41. 

The first example of enantioselective allylzincation of an alkene was also reported for the cyclopropenone ketal 78 as substrate. The chiral allylzinc complex 135 was prepared from the corresponding bis-oxazoline derived from (,V)-valine by deprotonation with n-BuLi and transmetallation with allylzinc bromide. This reagent reacted with 78 and afforded the allylated product 136 with high optical purity ( = 99%) (equation 66)101. 

Another option for the preparation of allenyllithium reagents involves the carbolithia-tion of activated conjugated enynes15. Thus, addition of BuLi to the enyne 281 led to an allenyllithium compound 282 (in metallotropic equilibrium with its propargylic counterpart). Subsequent addition of allylzinc bromide generated the allylic 1,1-dimetallic species... 

Allylzinc and allylchromium reagents coordinated by chiral ligands have variable degrees of success in the preparation of optically active homoallylic alcohols. 

The enantioselective asymmetric allylation of imines has been a synthetic challenge, the initial solutions of which required stoichiometric amounts of chiral allylbor on [87], allylsilane [88], allylzinc [89], or allylindium reagents [90]. Itsuno showed that a chiral B allyloxazaborolidine derived from norephedrine could add to the N trimethylsilyl imine prepared from benzaldehyde in high yield and enantiomeric excess (Scheme 1.22) [91]. Brown later reported that B allyldiisopinocamphenylbor ane is also very effective for the allylation of the same electrophiles, but the addition of a molar amount of water is necessary to obtain high yields [92]. The diastereo and... 

Organozinc chlorides. To avoid generation of dibenzyls during preparation of benzylic zinc reagents from ArCH2Cl in THF, a protocol exploits the beneficial effect of LiCl. Allylzinc chlorides are similarly available, and they add to carbonyl compounds with excellent diastereoselectivity. ... 

The functionalized alkylzinc species 19S is smoothly converted to the corresponding nitrile 196 in 67% yield. Interestingly, whereas the reaction of tosyl cyanide with benzylic bromide produces selectively 1-methylbenzonitrile in 76% yield, the cyanation of the corresponding copper reagent furnishes benzyl cyanide as sole product in 80% yield (Scheme 7.59) [142]. The reaction of 1,1-mixed bimetallics of magnesium and zinc, such as 197 [143] prepared by the addition of allylzinc bromide to (Z)-octenylmagnesium bromide with tosyl cyanide produces the nitrile 198 in 93% yield (Scheme 7.59) [142]. 

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