Palladium complexes alkyl, 3-hydrogen elimination

Kurosawa has shown that the related palladium(II) olefin complex [( ti -C5H5)Pd(Ph3P) (CHj=CHj)] undergoes clean trans attack by both methoxide and the anion of acety-lacetone. These examples of additions to iron- and palladium-olefin complexes occur by attack only at the ligand and lead to stable o-alkyl products because the metal center is coordinatively saturated. This coordinative saturation disfavors attack at the metal that would lead to either products from syn addition or displacement of the olefin. The cr-alkyl complexes that are products of these examples of nucleophilic attack are stable because the most common mode for decomposition of o-alkyl complexes, 3-hydrogen elimination, requires the presence of a vacant coordination site cis to the alkyl group, as noted in Chapters 8 and 9. Such a site is not present in these cr-alkyl products. 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

As with Pd bonds, Pd H will add to unsaturated functional groups see Hydride Complexes of the Transition Metals). In the case of alkynes, the Pd H addition is cis, to give vinyl complexes. The addition of a Pd H bond to an alkene will normally generate a Pd alkyl with jS-hydrogens, which can undergo further jS-elimination. This process can lead to alkene isomerization. However, palladium complexes have not proved as useM as those of other transition metals for alkene isomerization. While 1,5-cyclooctadiene can be isomerized to 1,3-cyclooctadiene with PdCl2(PhCN)2, palladium on charcoal is a more convenient and active catalyst for this isomerization. 

The oxidative addition to palladium reagents proceeds readily with 1-haloalkenes at room temperature. However, the oxidative addition reactions of halides other than vinyl or aryl usually are very sluggish. Moreover, alkyl-Pd(II)-X complexes in which the alkyl moiety contains an sp -bonded hydrogen at the [3-position may undergo rapid dehydropalladation by. syn- 3-hydrogen elimination, generating the hydridopalladium complex and a double bond. Thus, the substrates used for the oxidative addition reaction are usually restricted to vinyl and aryl halides and triflates. 

Alkylation of olefins. 1-Alkenes (and 1,2-disubstituted alkenes) can be alkylated primarily at the 2-position by stabilized carbanions in the presence of this Pd(II) complex and triethylamine. The reaction involves a palladium complex of the alkenc followed by 8-elimination of Pd(0) to give the unsaturated product, which can be hydrogenated if desired before work up. ... 

For example, the acyl-aryloxo bond cleavage (type b) is shown by the reaction of Ni(cod)2 with phenyl propionate in the presence of PPhs (Scheme 3.34) or 2,2 -bipyridine [65]. The reaction products are ethylene, phenol, and (car-bonyl)nickel complex. Formation of these products is conveniently understood by initial oxidative addition of EtC(0)-0Ph followed by decarbonylation, )S-hydrogen elimination and reductive elimination, though (acyl)(aryloxo)nickel(II) intermediate is not isolated. However, such an intermediate is isolated by the selective insertion of CO into the (alkyl)(aryloxo)nickel (or palladium) complexes, which smoothly affords esters by reductive elimination promoted by electron deficient olehns. The results suggest that the oxidative addition involving C-0 bond cleavage is essentially reversible. 

Stoichiometric and catalytic reactions that result from attack of nucleophiles onto square-planar palladium(II) and platinum(II) olefin complexes are legion. The o-alkyl products are stable in some cases, but p-hydrogen elimination to generate products from functionalization of the C-H bond of an olefin occurs in other cases. Nucleophihc attack on palladium(II) and platinum(II) complexes of diolefins often generate stable a-alkyl products because the resulting alkyl group is stabilized by chelation. 

The reactions of benzylic halides with carbon monoxide and alcohols form esters in good yields. However, the reactions of alkyl halides are more limited for two reasons. First, the oxidative addition of alkyl halides occurs less readily to palladium complexes than the oxidative addition of aryl halides. This difference was noted in Chapter 7. Second, the intermediate alkylpalladium halide can undergo P-hydrogen elimination. As noted in Chapters 9 and 10, these hurdles have been overcome in some cases, and cross-coupling... 

The cyclopalladation of allylic or homoallylic amines and sulfides proceeds due to the chelating effect of N and S atoms, and has been used for functionalization of alkenes. For example, i-propyl 3-butenyl sulfide is carbopalladated with methyl cy-clopentanecarboxylate and Li2PdCl4. Reduction of the chelated complex with sodium cyanoborohydride affords the alkylated keto ester in 96% yield (eq 24). Functionalization of 3-N,N-dimethylaminocyclopentene for the synthesis of a prostaglandin skeleton has been carried out via a IV-chelated palladium complex as an intermediate. In the first step, malonate was introduced regio- and stereoselectively by carbopalladation (eq 25). Elimination of a /3-hydrogen generated a new cyclopentene, and its oxypalladation with 2-chloroethanol, followed by insertion of 1-octen-3-one and /3-elimination, afforded the final product. 

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

P-hydride elimination, which is the Ayn-elimination of a hydrogen atom and Pd(II) from a palladium alkyl complex with no change in oxidation state ... 

This well known reaction can deceive us into thinking that o-complexes of Pd are stable. They are not. All the steps between 86 and 89 are reversible and in the absence of hydrogen, the reaction runs backwards. Palladium readily does the oxidative insertion into an alkyl halide to give the o-complex 92, a carbanion complex of Pd(II), but this immediately loses hydrogen by p-elimination and the alkene 86 is formed with the loss of a PdHBr complex. Notice that this Pd(II) complex immediately reverts 93 to Pd(0). 

Palladium alkyl complexes are inherently unstable because of p-elimination. We saw this in action in the discussion on hydrogenation in chapter 8. The palladium atom leaves with a hydrogen... 

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