Homoenolate complexes

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... 

Allyltitanium complexes derived from a chiral acetal have been reacted with carbonyl compounds and imines [63], While the chiral induction proved to be low with carbonyl compounds, high induction was observed with imines. This complex represents the first chiral homoenolate equivalent that reacts efficiently with imines. Finally, the reactions with electrophiles other than carbonyl compounds and imines, namely a proton source, NCS, and I2, furnished the corresponding alkene, chloro, and iodo derivatives in good yields [64]. 

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

The chemical reactivities of such titanium homoenolates are similar to those of ordinary titanium alkyls (Scheme 2). Oxidation of the metal-carbon bond with bromine or oxygen occurs readily. Transmetalations with other metal halides such as SnCl4, SbClj, TeCl4, and NbCls proceed cleanly. Reaction with benzaldehyde gives a 4-chloroester as the result of carbon-carbon bond formation followed by chlorination [9]. Acetone forms an addition complex. No reaction takes place with acid chloride and tm-alkyl chlorides. 

The reaction of the cyclopropane 1 with ZrCU gave a complex mixture [11]. 3.1.2 Group 12 Metal Homoenolates... 

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)-... 

Polladium(II) chloride or its phosphine complex smoothly reacts with siloxy-cyclopropane 1 to produce acrylic ester and a palladium mirror. This reaction probably involves the formation of a chloropalladium homoenolate followed by elimination of palladium hydridochloride (Eq. (56) [56]. 

A further example of the effectiveness of these reagents is demonstrated in the synthesis of the complex spiroventivane phytoalexin-lubiminol 3 (Scheme 1.2).4 Generation in situ of a zinc homoenolate, Et02C(CH2)2ZnCl,3 allows formation of the functionalized cyclopentenone intermediate, essential for the synthesis. 

The complexes generated by oxidative addition of haloarenes and haloalkenes to palladium(O) are electrophilic at the metal-substituted center, and can therefore react with carbon nucleophiles other than alkenes, especially with enolate and homoenolate ions to form new C—C bonds [176, 177]. 

The synthesis and characterization of the neutral homoenolate mono-Gp CpTiC CF CF COOEt) and cationic bis-Cp titanium [Cp2TiCFl2CF[2COOEt]+Znl3 derivatives have been reported. The molecular structure of the cationic complex shows ester coordination to give a five-membered metallacycle (Scheme 499).53... 

On the other hand, transition metal mediated conversions of heterofunctionalized cyclopropane derivatives can be used in the generation of three-carbon building blocks. Thus, starting from hydroxy- or siloxycyclopropanes and similar compounds, metal homoenolates can be generated and transformed in the presence of transition-metal complexes. ... 

Zirconium ester homoenolates 43 or 44 can be prepared from the triethyl orthoacrylate 42 with the zirconocene complex of but-l-ene 41 (Cp means cyclopentadiene). These resemble the zinc and titanium species we have been discussing but are not derived from cyclopropanes.11... 

Hoppe has extended this work to d3 reagents 159 (homoenolates - see chapter 13 for achiral versions) by the addition of a double bond.29 Lithiation occurs at C-l by removal of one of the enantiotopic protons at C-3. Aldol reaction with acetone occurs at C-3 of the complex 160 as expected for a homoenolate (chapter 13) giving a single enantiomer of the homoaldol product 161. All these reactions use an excess of sparteine. 

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