Zinc Chloride homoenolates

Zinc chloride (cuprous bromide jhexamethylphosphor amide ZnCl2lCuBrl(Me2N)2PO Syntheses via zinc homoenolates <-... 

The zinc homoenolate prepared by the treatment of the siloxycyclopropane with zinc chloride is a versatile synthetic reagent (eq 4). Reduction of 3-iodopropionate with activated zinc also produces a zinc homoenolate species. ... 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclo-propanes under suitable conditions. Whereas acylation of zinc alkyls makes a straightforward ketone synthesis [32], that of a zinc homoenolate is more complex. Treatment of a purified zinc homoenolate in CDC13 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ester, as the single product (Eq. (22) [33]). The reaction probably proceeds by initial electrophilic attack of acyl cation on the carbonyl oxygen. A C-acylation leading to a 4-keto ester can, however, be accomplished in a polar solvent Eq. (44)-... 

Interestingly, the carbonyl addition of the zinc homoenolate, in the presence of acid chloride, affords 4-acyloxyesters Eq. (35) [27]. 

The reaction of zinc homoenolate 9 with acid chlorides in ethereal solvents containing 2 equiv of HMPA rapidly produces 4-ketoesters in high yield Eq. (44) [33]. A palladium catalyst [40] (or less effectively a copper catalist) [28] accelerates the reaction. This is in contrast to the cyclopropane formation in a nonpolar solvent see (Eq. 22 above). 

The zinc homoenolate a reacts with acid chlorides in CH2C12 to form l-(acyl-oxy)cyclopropanes. 

Alkoxy-l-siloxycyclopropane, e.g. 30, on treatment with zinc(II) chloride gives a stable zinc homoenolate, that can, for example, be reacted via an intermediate cuprate with enones to afford the Michael addition product.Addition of a palladium catalyst to the zinc species allows for arylation and for vinylation of the homoenolate in good yield. 

Treatment with zinc/copper couple activated by ultrasound in dimethylacetamide gave the zinc homoenolate 57 stabilised by chelation to either the ester or the carbamate. Acylation with acid chlorides and Pd(0) catalysis gave the enantiomerically pure y-oxo-amino acids 58 in good yield. 

Internal nucleophilic cyclization is one of the most typical reactions of reactive metal homoenolates (equation 3 and Scheme and provides a convenient route to silyloxycyclopropanes (e.g. 8) through cyclization of 3-halo esters. Zinc homoenolates (9) also cyclize to cyclopropanes under suitable conditions. Treatment of the zinc homoenolate in CHCb with an acid chloride at room temperature gives an 0-acylation product (Scheme 6), instead of a 4-keto ester (see Section 1.14.7.1). The reaction of the zinc homoenolate wifo MesSiCl in a polar solvent gives a silyloxycyclopropane (Scheme 6), providing a very mild route to silyloxycyclopropanes. ... 

Zinc homoenolates (9) react rapidly with acid chlorides in ethereal solvents containing a dipolar aprotic solvent to give 1,4-keto esters in high yield (Scheme 23). 9 palladium catalyst (or, less effective, a copper catalyst) ° accelerates the reaction, in contrast to cyclopropane formation in halometh-ane solvents (see Section 1.14.3.1). 

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). 

---