Palladium Chloride Addition with Hydride Elimination

III. Palladium Chloride Addition with Hydride Elimination... 

An important modification of the initial protocol has been introduced by Larock ef al. to afford indanones. The saturated ring results from the termination of the process by protonolysis of palladium enolate, which is likely to be generated from reversible palladium hydride elimination-addition. Proton source required for protonolysis hypothetically comes from adventitious water coming with hygroscopic chloride or solvent (Scheme 13). ... 

In the absence of base, vinylmercury reagents and lithium tetrachloropalladate(II) react with alkenes to form ir-allylpalladium complexes arising from addition of the vinylpalladium chloride to the alkene followed by palladium hydride elimination, a reverse readdition and rr-allyl formation (equation 10).31... 

This elimination is reminiscent of the last step in the aqueous palladium chloride oxidation mentioned above and this reaction also may involve multiple hydride addition-elimination steps. Minor amounts of the normal products and Markovnikov products are also generally found in these reactions. Cupric chloride can be used as a reoxidant although the yields are generally lower than with an all acetate, non-catalytic reaction. 

Remarkably, cross-couplings of alkyl boranes with alkyl bromides or even chlorides are possible using the catalyst [Pd2(dba)3] and the ligand tricyclohexylphos-phine, PCys (Cy = CeHn). For example, the alkyl chloride 203 was coupled to the alkyl borane 204 (prepared by chemoselective hydroboration with 9-BBN see Section 5.1) to give the product 205 (1.206). The [Pd2(dba)3]/PCy3 catalyst system overcomes the normally slow oxidative addition of the alkyl halide to the palladium and promotes cross-coupling to alkyl boranes in preference to p-hydride elimination. Such B-alkyl Suzuki reactions are likely to be used as key carbon-carbon bond-forming reactions in future synthetic sequences. 

Compared with palladium-catalysed Heck reactions, the studies of nickel-catalysed Heck reactions are far fewer. Theoretical studies show that oxidative addition and olefin insertion occur easier in the nickel system than in the palladium system. On the other hand, p-hydride elimination and catalyst regeneration through HX removal are more difficult in a nickel-based catalytic system." The former characters may make the nickel-catalysed Heck reaction of alkenyl or aryl chloride proceed relatively easily and the latter... 

Subsequently, the ruthenium-catalysed alkenylation of various acrylates was accomplished with alkenyl halides [62]. Most effective catalysis was achieved with [Ru(COD)(COT)] (98) as catalyst and NEts as base in the absence of additional solvent. Interestingly, both alkenyl bromides and chlorides could be employed as electrophiles (Scheme 10.35). When using an alkenyl chloride, the catalytic activity could be improved through the addition of P(p-C6H4p)3 (99) as ligand. The efficiency of this ruthenium catalyst in the alkenylation of /3-chlorostyrene (21) compared favourably with that observed for either Pd(OAc)2 or Pd(OAc)2/P(o-Tol)3 as catalysts. With respect to the working mode of the catalyst, a radical mechanism was shown to be less likely. Instead, Mitsudo et al. [62] proposed an initial oxidative addition of the alkenyl halide to a ruthenium(O) species followed by insertion of the alkene and )8-hydride elimination, all in analogy to palladium-catalysed processes. 

Palladium chloride at 100 °C in anhydrous dimethyl formamide reacts in a stoichiometric reaction with ethylene producing vinyl chloride in 80% yield. Even though palladium chloride elimination is favored over hydride elimination, the addition recurs until the palladium is eliminated irreversibly as the hydride (Scheme 30). 

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. 

Benzyl chloride reacts easily with methyl acrylate in the presence of tri-n-butylamine and palladium acetate (1 mol %) as catalyst.51 The product is a mixture of (E)-methyl 4-phenyl-3-butenoate (67%) and (E)-methyl 4-phenyl-2-butenoate (9%), arising from elimination-addition reactions of the palladium hydride group which largely isomerize the initial elimination product. 

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