Allyls palladium

Hard carbon nucleophiles of organometallic compounds react with 7r-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal 7T-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

Allenes also react with aryl and alkenyl halides, or triflates, and the 7r-allyl-palladium intermediates are trapped with carbon nucleophiles. The formation of 283 with malonate is an example[186]. The steroid skeleton 287 has been constructed by two-step reactions of allene with the enol trillate 284, followed by trapping with 2-methyl-l,3-cyclopentanedione (285) to give 286[187]. The inter- and intramolecular reactions of dimethyl 2,3-butenylmalonate (288) with iodobenzene afford the 3-cyclopentenedicarboxylate 289 as a main product) 188]. 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Early studies into the decomposition of ethyl diazoacetate by a ic-allyl palladium chloride complex in the presence of acetonitrile led to the isolation of 2-methyl-5-ethoxyoxazole in... 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

In 1996, Ogawa et al. reported the hydrothiolahon of an allene catalyzed by Pd(OAc)2 to provide 32, whose formahon can be explained as follows (1) insertion of the allene into the Pd-S bond of Pd(SPh)2 to give a tt-allyl palladium thiolate 33, and (2) protolysis by PhSH to form 32 and Pd(SPh)2 (Eq. 7.24) [35], The authors proposed the direct reachon of a o-aHyl palladium with PhSH before the formation of 33. 

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

Kurosawa et al. have reported that the relative stability of the ti-allyl palladium thi-olate 39 and the allyl sulfide/Pd(0) was highly ligand dependent. In the presence of PPhs or P(OMe)3 the stability was in favor of reductive elimination (Eq. 7.28), while in the presence of olefin or in the absence of any additional ligand the stability was in favor of oxidative addition (Eq. 7.29) [38]. This can explain the reactivity of the n-allyl palladium thiolate 33 and 38 proposed in Eq. (7.24) and path (c) of Scheme 7-10. The complex 33 should react with PhSH, but C-S bond-forming reductive elimination has to be suppressed in order to obtain the desired product 32. On the other hand, the complex 38 requires the phosphine ligand to promote the C-S bond-forming reductive elimination. 

Allyl carbamates also can serve as amino-protecting groups. The allyloxy group is removed by Pd-catalyzed reduction or nucleophilic substitution. These reactions involve formation of the carbamic acid by oxidative addition to the palladium. The allyl-palladium species is reductively cleaved by stannanes,221 phenylsilane,222 formic acid,223 and NaBH4,224 which convert the allyl group to propene. Reagents... 

Nucleophilic Substitution of xi-Allyl Palladium Complexes. TT-Allyl palladium species are subject to a number of useful reactions that result in allylation of nucleophiles.114 The reaction can be applied to carbon-carbon bond formation using relatively stable carbanions, such as those derived from malonate esters and (3-sulfonyl esters.115 The TT-allyl complexes are usually generated in situ by reaction of an allylic acetate with a catalytic amount of fefrafcz s-(triphenylphosphine)palladium... 

Me3SiPdSnBu3 is formed primarily from 6/1-237, which then adds to the allene moiety in 6/1-236 to give a a- or Jt-allyl palladium complex. This undergoes an intramolecular carbonyl allyl addition to afford the cis-cycloalkariols 6/1-238 (Scheme 6/1.61). 

However, some binuclear -jr-allyl palladium complexes have been obtained by facile decarboxylations from palladium chloride and some... 

Similar studies on the reactions of butadiene and bis(ir-allyl)palladium were carried out by Wilke and co-workers (4). Unlike the reactions with nickel complexes, no cyclization took place, and 1,6,10-dodecatriene... 

Next, the effect of ligand of palladium complex on the reaction of butadiene was studied. Only vinylcyclohexene (4) was obtained by the reaction of butadiene at 90°C using bis(7r-allyl)palladium (13) and tri-chlorophosphine (1 1). 

Octatriene was converted into -l,5,7,10,15-hexadecapentaene (30) with 70% selectivity by catalytic action of bis(7r-allyl)palladium (13). On the other hand, 4-(3-butenyl)-l,6,8,l 1-dodecatetraene (31) was formed as a main product when PPh3 was added in 1 1 ratio (39). 

Octatriene reacts further with butadiene in acetic acid by using 7r-allylic palladium complex as catalyst to give a mixture of acyioxydo-decatrienes (54). 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

As mentioned in the introduction, 7T-complexes of conjugated dienes with palladium(II) are not stable enough to be isolated. However, reaction of a conjugated diene with PdCl2 in alcoholic solvents or acetic acid gives a (TT-allyl)palladium complex 27 in which the... 

In 1971, Brown and Davidson reported that 1,3-cyclohexadiene undergoes a palladium-catalyzed 1,4-diacetoxylation of unspecified stereochemistry28. The oxidant employed was p-benzoquinone. They were uncertain about the mechanism at the time but later work has shown that the reaction proceeds via a (jr-allyl)palladium intermediate and subsequent nucleophilic attack by acetate6,7. 

In a stoichiometric reaction the 6jr-allyl)palladium complex 66 was isolated and characterized5815. In a subsequent reaction the jr-allyl complex was reacted with benzoquinone in acetic acid to give an allylic acetate, which was hydrolyzed and oxidized to theaspirone. Interestingly, a quite high diastereoselectivity for the turns methyl isomer was obtained in the palladium-mediated spirocyclization (equation 28). 

Related experimental and theoretical studies have been performed by the group of Szab6.458 458a 458-i They revealed that palladium pincer complexes such as 205 show high catalytic activities (Equation (100)).4S8d,g,i Mono-Z71 -allyl palladium pincer complexes are proposed to be active intermediates. 

Recently, particular ligands have been reported to enable the amination of allyl alcohols without the aid of activators (Scheme 10).13c,13d Ozawa s group found that (7r-allyl)palladium complex 58 bearing. s/>2,2a,2b-hybridized... 

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