Tr-Allylpalladium complexes

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

Preparation of Tr-Allylpalladium Complexes from Alkenes and Their Reactions with Carhon Nucleophiles... 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

The regioselective and stereospecific construction of C-20 stereochemistry is explained by the following mechanism. The Pd(0) species attacks the ( )-/3-carbonate 616 from the a-side by inversion to form the Tr-allylpalladium species 620, which has a stable syn structure[392]. Then concerted decarboxylation-hydride transfer as in 621 takes place from the a-side to give the unnatural configuration in 617. On the other hand, the Tr-allylpalladium complex 622... 

It is widely recognized that an allyl group is a useful protecting group for acids, amines, and alcohols. Facile formation of a Tr-allylpalladium complex from... 

Scheme 16.22 A proposed reaction mechanism via an nnti-Tr-allylpalladium complex.

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

When butadiene is treated with PdCl2 the l-chloromethyl-Tr-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed[296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-l,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the 7r-allyl complex 339 after the cyclization[288]. 

Another reaction occurs by the attack of a soft nucleophile at the central carbon to form the 7r-allylpalladium complex 7, which undergoes further reaction with the nucleophile typical of Tr-allylpalladium complexes to form the alkene 8,... 

Oxygen nucleophiles can be added successfully to Tr-allylpalladium complexes by various methodologies. Those additions that are fostered by the use of classical oxidizing agents will be treated separately in Section 3.3.2.3.2. 

The most intense interest in the addition of oxygen nucleophiles to Tr-allylpalladium complexes has centered on the delivery of OAc. For example, in allylpalladium-OAc complexes, acetate can be induced to migrate to the allyl ligand by the addition of CO (equation 46).162-164 Rearrangements and isomerizations of allyl acetates can also be readily accomplished via Pd catalysis (equations 47 and... 

Oxa- Tr-allylpalladium complexes (10), which can also be envisioned as palladium enolates (11), are susceptible to (3-hydride elimination, and as such have been principally used in methodologies for the preparation of (3,(3-unsaturated carbonyl compounds. 

What may be characterized as an aza-Tr-allylpalladium complex has been proposed as an intermediate in the preparation of a, -unsaturated nitriles by decarboxylation-dehydration of allyl a-cyanocarboxy-lates (equation 144).369 These reactions have been summarized in the context of several reviews.89,117 If the oxa-7r-allyl species is blocked from the elimination of a (3-H, then it can undergo typical ir-allylpalla-dium chemistry, such as insertion by alkenes, dienes and CO (equation 145).370 371... 

It is clear that although substantial progress has been made in the understanding and control of regioselectivity in the addition of nucleophiles to Tr-allylpalladium complexes via attack at the metal, considerable variability still exists in these reactions that is not readily mechanistically rationalized. 

Acyliron and acylnickel anions, as well as NaCo(CO)4, acylate Tr-allylpalladium complexes selectively at the less substituted allyl terminus (equation 294).223... 

Z)-Phenol-substituted alkenes (67) can be produced by the palladium(0)-catalysed reaction of propargylic oxiranes (66) with phenols. This regio- and stereo-selective (g) addition is believed to occur via the formation of 7t-propargyl- and Tr-allylpalladium complexes. The phenoxy-substituted enones were obtained as by-products and their proportion depended on the reaction conditions.75... 

AUylic acetoxy lotion. A new method for regioselective ally lie oxidation of ger-anylacetone (1) is based on oxidation of the Tr-allylpalladium complex known to be formed selectively with the terminal allylic methyl group. The most satisfactory results are obtained... 

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