Palladium Homoenolate

The arylation reaction is not applicable to aryl bromides and iodides. Another line of experimental evidence also indicates that an arylpalladium halide complex, instead of an arylpalladium triflate complex, is not electrophilic (Lewis acidic) enough to cleave the cyclopropane ring. 

TaUe 13. Arylated esters and ketones by direct arylation of siloxycyclopropaDes (Ref. (57]) 

Reaction of siloxycyclopropane 1 with acid chlorides in the presence of a palladium catalyst also proceeds cleanly to give 4-ketoesters in high yields (Eq. 59, Table 14) [57]. Chloroform is a suitable solvent. Kinetic studies have revealed that the interaction between 1 and an acylpalladium chloride complex is the rate limiting step. 

TaUe 14. Keto esters by direct palladium catalyzed acylaticm of oxycydopropanes (Ref. [56]) 

If the cyclopropane is heated in CHQj in the presence of a catalytic amount of palladium-phosphine complex under CO atmosphere, a symmetrical coupling product incorporating one molecule of CO forms in high yield, Eq, (60) [57]. 

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]. 

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The chiral siloxycyclopropane 106 undergoes carbonylative homocoupling to form the 4-ketopimelate derivative 108 via the palladium homoenolate 107 without racemization. The reaction is catalytic in CHCI3, but stoichiometric in benzene[93]. 

Palladium homoenolates readily undergo p-elimination to give a,P-unsaturated carbonyl compounds. Treatment of a mercurio ketone with a catalytic amount of palladium(II) in the presence of CuCh results in the formation of an enone via a 3-palladio ketone (Scheme 3). Treatment of a silyloxycyclopropane (8) with PdCh also generates in situ a palladium homoenolate which then undergoes -elimination (Scheme 3). Heating a mixture of a 3-trichlorostannyl ketone or aldehyde with DMSO results in the formation of an enone or an enal in excellent yield (Scheme 4). ... 

Palladium Homoenolates Generated In Situ 1.14.7J Silver Homoenolates... 

Next to the cyclopropane formation, elimination represents the simplest type of a carbon-carbon bond formation in the homoenolates. Transition metal homoenolates readily eliminate a metal hydride unit to give a,p-unsaturated carbonyl compounds. Treatment of a mercurio ketone with palladium (II) chloride results in the formation of the enone presumably via a 3-palladio ketone (Eq. (24), Table 3) [8], The reaction can be carried out with catalytic amounts of palladium (II) by using CuCl2 as an oxidant. Isomerization of the initial exomethylene derivative to the more stable endo-olefin can efficiently be retarded by addition of triethylamine to the reaction mixture. 

Table 10. Arylated and vinylated propionates by palladium-catalyzed reaction of zinc homoenolate (Ref. [29,40])...

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). 

Selective transfer of the homoenolate moiety characterizes the transmetalation from tin to titanium described in the previous section. A very facile transmetalation of the same kind occurs for 3-stannylketoxime, Eq. (50) [48]. Reaction of the ( )- 3-(tributyltin)ketoxime with dichlorobis(benzonitrile)palladium in CH2C12 at 0 °C for 30 min yields 78% of the transmetalation product. The... 

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]. 

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. 

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