Zinc homoenolates acylation

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

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)-... 

A very practical route to zinc homoenolate involves reduction of 3-iodoesters with zinc/copper couple in the presence of a polar solvent, e.g. DMF, DMA [49] Eq. (51). The nature of the species obtained in this approach is not well-defined, but appears to be essentially the same as the one obtained along the siloxycyclo-propane route. Acylation, arylation, and vinylation reactions have been reported. 

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

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. ... 

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. 

Commercially available methyl p-hydroxyisobutyrate (16) has been converted to the corresponding halide (17), then either via the silyloxycyclopropane route (A) or via the reductive route (B) to the chiral zinc homoenolate (18 Scheme 28). It is optically stable in ether and has been used for several standard carbon-carbon bond-forming reactions, e.g. carbonyl addition, arylation and acylation. ... 

One particular aspect of this chemistry is in the preparation of zinc homoenolates prepared from P-iodoesters (Scheme 2.33). This chemistry becomes particularly significant with p-iodoesters 2.95 derived from the amino acid serine 2.94. These are useful for the synthesis of novel amino acids by Negishi coupling with aryl and acyl halides (Scheme 2.34). Coupling of a complex peptidyl aryl iodide 2.98 with a zinc reagent 2.99 of this type was a late step in a synthesis of a cyclic tripeptide and aminopeptidase inhibitor OF494-III 2.100 (Scheme 2.35). ... 

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). 

Acylation and Carbonylation. The zinc homoenolate reacts more rapidly with acyl halides than with a-/3-enones to give 4-keto esters in good yield (X = Cl) (eq The Ni-catalyzed... 

---