Zinc homoenolates allylation

Among the characterized metal homoenolates, only zinc homoenolate of alkyl propionate undergoes substitution reactions with electrophiles under suitable conditions. Two types of metal catalysts, copper(I) and metals of the nickel triad (e.g. Pd), have successfully been used to effect allylation, arylation, and vinylation reactions. 

Zinc homoenolate reacts with allylic halides and diene monoepoxides under copper catalysis [29]. Treatment of the zinc nomoenolate with a catalytic amount of Cu(II) in a polar solvent (e.g. hexamethylphosphoramide, HMPA, N,N-dimethylacetamide, DMA) generates a copper species which undergoes clean Sn2 allylation reactions Eq. (40). Polar solvents not only accelerate the reaction but greatly improve the SN2 selectivity. A variety of allylating reagents can be employed in this reaction (Table 9). The SN2 /SN2 ratio is particularly high (close to 100%) when the alkylated carbon bears no substituents. The reaction of... 

The allylation and the conjugate addition (vide supra) of zinc homoenolate proceed under essentially the same conditions except that the latter requires the presence of Me,SiCl as well. Due to this subtle difference, selective allyl substitution is possible even with an enone function is present in the same molecule (Table 9). 

Table 9. Unsaturated esters by -allylation of zinc homoenolate of esters (Ref. [29])...

Kinetic resolution (chapter 28) of each diastereoisomer with a lipase gives single enantiomers of the hemiacetals 51 and hence the zinc homoenolates 52. Various electrophiles (E in 53) gave enantiomerically pure (>99% ee) adducts where E = H, allyl or imine.13... 

Copper(I) and metals of the nickel triad have been used to effect allylation, aryladon, vinylation and acylation reactions of zinc homoenolates. Transient homoenolates generated under Pd catalysis and Ag catalysis have also been used for direct arylation and acylation of silyloxycyclopropanes. 

The zinc homoenolate undergoes copper-catalyzed allylation with allylic chlorides. The reaction is not only extremely Sn2 regioselective but stereoselective for 5-chiral allylic chlorides. Arylation and vinylation of the zinc homoenolates proceed in the presence of a palladium-phosphine complex. Similarly, palladium-catalyzed acylation reaction gives y-keto esters (eq 6). 

Homoenolate Reactivity. Since the previous e-EROS report, a number of examples have been described using the cyclopropanone acetals. Thus, the zinc homoenolate, known to undergo a highly regioselective and stereoselective Sn2 allylation reaction (eq 6), is used in the synthesis of moenomycin analogues. The activated titanium homoenolate reacts with aldehydes or ketones to give y-hydroxy esters that serve as precursors to y-lactones. ... 

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