Transmetalation allylzincation

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

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. 

Metallated allylic ethers can also be used as partners in allylzincations of alkenyl-metals144. Allyl methyl ether could be metallated with s-BuLi in ether provided that TMEDA was present and, after transmetallation with ZnBr2, addition to the alkenyl-lithium derived from 205 proceeded slowly at room temperature. Nevertheless, after hydrolysis the corresponding allylic ether 227 was obtained with high diastereoselectivity (equation 110)146. 

When 303 was directly treated with Me2Cu(CN)Li2, the transmetallation failed to discriminate between the two carbon-metal bonds. By contrast, the allylzincation of the alkynyllithium derived from the propargylic alcohol 309 produced the alkenyl 1,1-dimetallic species 310, in which the two carbon-metal bonds exhibit different reactivities due to the presence of a metal-alkoxide. Indeed, transmetallation with Me2Cu(CN)Li2 led to the alkenyl copper-zinc species 311, which was relatively poorly reactive towards electrophiles but underwent successful 1,4-addition to ethyl propiolate leading to 312 in satisfactory overall yield (equation 145)180. 

With A-benzyl or A-allyl amides, the a-lithiated products are more stable. N,N-dibenzylbenzamide 48, for example, is a-lithiated with LDA to give 49 and hence 50,38 while two equivalents of BuLi or LDA are sufficient toa-lithiate the secondary amides 51 and 54 to give 52 or 55 which react with electrophiles to produce 53 and 56.39 The lithiated A-Boc allyl amides 57 react with selectivity which is highly electrophile dependent, but transmetallation of 58 to the allylzinc reagent leads to good a-selectivity.40... 

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... 

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