Jt-allyl 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). 

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. 

A single reaction has been described in which a palladium-catalyzed reaction was employed to form an alkyne [45], Thus, attempted alkylation of carbonate 145 with dimethyl malonate in the presence of Pd(PPh3)4 gave a mixture of enyne 87 and the alkylation product 86 in a 15 1 ratio (Scheme 14.37). Methoxide caused an elimination in (jT-allyl)palladium intermediate 146, which is apparently faster under these conditions than a reaction with the nucleophile (cf. Eq. 14.9). The synthetic importance of this process seems to be limited. 

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

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. 

Allenyl alcohols 10 react with lithium bromide in the presence of a palladium(II) catalyst to afford tetrahydrofurans and tetrahydropyrans 11 in good yield (Scheme 17.6) [7]. The mechanism of the reaction is similar to that discussed in Sect 17.2.1. i.e. it proceeds via a 2-bromo(jt-allyl)palladium(II) complex. In this case, however, the second nucleophile is not bromide ion but the alcohol moiety. As stoichiometric oxidant p-benzoquinonc (BQ) or copper(II) together with oxygen can be used. 

A likely mechanism of these reactions is that they proceed via a (jt-allyl)palladium intermediate 29 or 31 as shown in Scheme 17.12. Intramolecular attack by either... 

Regioselective polycondensations with transition-metal catalysts were also reported. Nomura et al. developed palladium-catalyzed allylation polycondensation, in which nucleophile predominantly reacted with jt-allyl palladium at the terminal allylic carbon to give fi-linear products [122,123]. On the other hand, polymerization with an iridium catalyst selectively proceeded at the internal allylic carbon to yield branched polymers with pendant vinyl groups (Scheme 30). These polycondensations demonstrate that polymers having different structures can be synthesized from the same monomers by changing the catalyst [124],... 

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 deuteride 1 may act as an activator of C-H bonds at allylic positions and may form Jt-allyl palladium. Via the equilibrium between Jt-allyl palladium and alkene, H-D exchange will occur by C=C double bond migration. As shown in Scheme 5, cyclododecene was converted into the fully deuterated compound by treatment with hydrothermal deuterium oxide in the presence of Pd/C catalyst. Without Pd/C, no deuteration was observed under these reaction conditions [16]. 

Olefin isomerization catalyzed by ruthenium alkylidene complexes can be applied to the deprotection of allyl ethers, allyl amines, and synthesis of cyclic enol ethers by the sequential reaction of RCM and olefin isomerization. Treatment of 70 with allyl ether affords corresponding vinyl ether, which is subsequently converted into alcohol with an aqueous HCl solution (Eq. 12.37) [44]. In contrast, the allylic chain was substituted at the Cl position, and allyl ether 94 was converted to the corresponding homoallylic 95 (Eq. 12.38). The corresponding enamines were formed by the reaction of 70 with allylamines [44, 45]. Selective deprotection of the allylamines in the presence of allyl ethers by 69 has been observed (Eq. 12.39), which is comparable with the Jt-allyl palladium deallylation methodology. This selectivity was attributed to the ability of the lone pair of the nitrogen atom to conjugate with a new double bond of the enamine intermediate. 

Acyl Esters Aloe formation of a Jt-allyl palladium TFA, piperidine Boc, OtBu Fmoc,... 

Vinyl epoxides and allyHc carbonates are especially useful electrophiles because under the influence of palladium(O) they produce a catalytic amount of base since is an alkoxide anion. This is sufficiently basic to deprotonate most nucleophiles that participate in aUylic alkylations and thus no added base is required with these substrates. The overaU reaction proceeds under almost neutral conditions, which is ideal for complex substrates. The rehef of strain in the three-membered ring is responsible for the epoxide reacting with the paUadium(O) to produce the zwitterionic intermediate. Attack of the negatively charged nucleophile at the less hindered end of the Jt-allyl palladium intermediate preferentially leads to overall 1,4-addition of the neutral nucleophile to vinyl epoxides. 

Allylic malonate 100 completely isomerizes to the thermodynamically favored linear isomer 101 on treatment with a palladium catalyst [119]. Formation of a stabilized carbanion and Jt-(allyl)palladium species facilitates the C-C bond cleavage. Analogous isomerization is also catalyzed by a nickel complex [120]. These results demonstrate that the transition metal-catalyzed nucleophilic substitution of an allylic substrate with a carbon nucleophile is reversible, if the cleaved nucleophile is sufficiently stabilized. 

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. 

As alluded to above, this version of the Jt-allyl palladium reaction uses an allylic acetate or chloride. The use of the acetate is more common because acetate is a much weaker nucleophile than chloride. When it involves a substrate where diastereomeric products can result, the stereochemistry of the nucleophilic displacement is an important issue. Palladium assisted alkylation proceeds with net retention of configuration of the acetate or chloride, as seen in the conversion of 375 to 376.223e,f... 

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. 

Since the Jt-allyl species is bound to palladium, there is a template effect in the Jt-allyl palladium reactions that can be exploited to form large rings (sec. 6.6.B).228 Ester 388 was converted to 389 in 70% yield by this procedure, but the yields have been poor in some cases.228... 

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