Jt-allyl palladium complex

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). 

Fttrstner has employed the Trost pyrrole synthesis in the first total synthesis of roseophilin, wherein this A-benzylpyrrole-ring forming step occurred in 70% yield [23]. Backvall has found that primary amines react with dienes under the guidance of Pd(II) to form pyrroles 170 in variable yields [121]. The intermediate Jt-allyl-palladium complexes are quite stable. 

Scheme 17.3 Possible stereoisomers of the Jt-allyl (palladium) complex and the equilibrium between them via a it-a-it rearrangement.

The palladium(II)-catalyzed oxidation of allenes with chloride was studied by Hege-dus et al. [3], In this reaction the dimeric products 4 and 6 as shown in Scheme 17.4 were obtained. The (allene)palladium(II) complex formed can react with chloride ions in two different ways (Scheme 17.4) [4]. Attack at the terminal carbon gives a vinylpalladium intermediate 2 whereas attack at the middle carbon produces a 2-chloro(jt-allyl)palladium complex 3. The former complex is the kinetic intermediate (k2 > kj) and is in equilibrium with the (allene)palladium complex. The 2-chloro(jt-allyl)palladium complex is formed more slowly but is more stable and has been isolated [2]. The vinyl complex can undergo further reaction with excess allene to give a new (jt-allyl)palladium complex, which undergoes attack with chloride to give the observed dimer 6 [3]. The dichloride from attack on the 2-chloro-(jT-allyl)palladium complex 3 was not observed. 

Several catalytic systems have been investigated for hydroamination of unsaturated bonds [16]. Takahashi et al. reported the telomerization of 1,3-dienes in the presence of an amine leading to octadienylamine or allylic amines when palladium catalysts are used in association with monodentate or bidentate phosphine ligands, respectively [17]. Dieck et al. demonstrated the beneficial effect of addition of an amine hydroiodic salt in the hydroamination reaction of 1,3-dienes in which the allylic amines are produced via an intermediate Jt-allyl palladium complex [18]. Coulson reported the Pd-catalyzed addition of amines to allenes where dimerization is incorporated [4]. This reaction presumably proceeds via a cyclic palladium intermediate in which the Pd activates the olefinic bond for nucleophilic attack the reactions are therefore different from pronucleophilic additions. 

Palladium salts react with allylic hydrocarbons and with functionalized allylic derivatives to produce Jt-allyl complexes. These complexes can react with nucleophilic species, including enolate anions, to produce new carbon-carbon bonds.206 jn these reactions, Jt-allyl palladium complexes function as equivalents thus they are included in this chapter. 

D.1. Reactions with Nucleophiles. Previously, a jr-allylic palladium complex was generated by reaction of palladium reagents with allylic hydrocarbons prior to reaction with nucleophiles. In the catalytic version of this reaction, an allylic halide or an allylic acetate is used with a palladium(O) reagent. Why use a palladium complex when enolate alkylation is a well-known process (sec. 9.3.A) A typical enolate coupling reaction is the conversion of 2-methylcyclopentane-l,3-dione (373) to the enolate anion by reaction with NaOH, allowing reaction with allyl bromide. Under these conditions only 34% of 374 was obtained. When allyl acetate was used in place of allyl bromide in this reaction and tetra w(triphenylphosphino)palladium was used as a catalyst, a 94% yield of 374 was obtained.224 in this reaction, formation of the Jt-allyl palladium complex facilitated coupling with the nucleophilic enolate derived from 373, which exhibited poor reactivity in the normal enolate alkylation sequence. 

Trost proposed the following mechanism to account for these catalytic transformations. Reaction of the palladium catalyst with 377 generates jt-alkene palladium complex 378. Palladium removes the allylic hydrogen, with expulsion of the acetate moiety to generate the Jt-allyl palladium complex (379). Attack of a nucleophile at Ca leads to 380, with expulsion of the PdL2 species, whereas attack at Cb leads to 381. Palladium coordinates on the face of the alkene distal to the acetate (distant from the acetate Ca rather than Cb). Palladium displaces acetate with inversion (378 - 379). When the nucleophile displaces the palladium, a second inversion occurs at Ca or Cb, whichever is less sterically hindered, to give a net retention of configuration for the conversion 377 - 380 and/or 381. 

The first step of the reaction generated a cationic Jt-allyl palladium complex 185/186 which was easily detectable by ESI-MS. Then, the nucleophile attack afforded the... 

The whole transformation is an oxidative intramolecular 1,4-addition. Stereochemistry of the 1,4-addition is explained by an intramolecular anti attack of the allylsilane moiety to a Pd(II)-coordinated diene functionality in 26 to generate (jt-allyl)palladium complex 27 followed by an intermolecular anti attack of a chloride ion. The substrate allylsilane is easily prepared by the reaction of the corresponding allylic acetate with PhMCjSiLi. 

---