Palladium® complexes diene conjugation, allylic intermediates

Intramolecular reactions of allylic acetates with conjugated dienes catalyzed by Pd(0) lead to a 1,4-addition of a carbon and an oxygen nucleophile to the diene. The reaction, which is formally an isomerization, involves tw different yr-allyl complexes (Scheme 8-4) [44]. Reaction of 22 in the presence of the Pd(0) catalyst Pd2(dba)3-CHCl3 (dba = dibenzyl-ideneacetone) and LiOAc/HOAc in acetonitrile at reflux produces the cyclized isomer 25 in 62% yield. The double bond was exclusively of E stereochemistry, while the ring stereochemistry was a mixture of cis and tram isomers. Oxidative addition of the Pd(0) to the allylic acetate gives the intermediate jr-allyl complex 23. Subsequent insertion of a diene double bond into the allyl-palladium bond produces another jr-allyl intermediate (24), which is subsequendy attacked by acetate to give the product 25. 

Palladium(II)-catalyzed 1,4-additions to conjugated dienes, in which at least one alkoxide function is added, require the presence of an alcohol function. In all cases known so far, this involves an alkoxypalladation of tlie conjugated diene to give an intermediate 4-alkoxy-l,2,3-(jr-allyl)palladium complex. Subsequent nucleophilic attack on the jr-allyl... 

If the unsaturated hydrocarbon is a diene, both double bonds may coordinate to palladium ). (Diene)palladium(II) complexes have been isolated and characterized. For example, 2 and 3 are stable complexes in which both double bonds are coordinated to the metal10. Conjugated dienes constitute a special case and although /j4-diene complexes, e.g. 4, are postulated as intermediates, they have not yet been isolated. The butadiene complex 4 is in equilibrium with the zr-allyl complex 5 in solution, and attempts to isolate the diene complex from this mixture lead to formation of a yellow crystalline complex 511. 

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and (E,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and mesa (>95% me so) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a t vans-accto xy pal I adation of the conjugated diene to give an intermediate (rr-allyljpalladium complex, followed by either a cis or trans attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

Dicarboration of conjugated dienes can occur in either a 1,2- or a 1,4-addition mode. Several examples of stepwise stereoselective c -l,4-dicarborations of open-chain and cyclic dienes, via palladium-allyl complexes, which use stoichiometric amounts of palladium salts and soft, delocalized carbanions have been reported65 67. Catalytic dicarboration of 1,3-butadiene, stereoselectively leading to ( )-alkenes with up to 70% yield, is achieved using aryl iodides and delocalized carbanions of the malonate type in the presence of palladium(II) chloride and l,2-bis(diphenylphosphino)ethane68. A Tt-allyl-palladium intermediate is thought to determine the E geometry of the product. 

Insertions of vinylic palladium halides with olefins not conjugated with carbonyl groups is more complex. Rearrangement of the initial insertion product to a i-allyl-palladium derivative usually occurs as in Eq. (p). Since the a -allylic complexes are relatively stable, catalytic reactions to form dienes with tertiary amines as bases are slow and inefficient . A useful catalytic reaction occurs, however, if a nucleophilic secondary amine is used as base instead of the tertiary amine. The a-allylic palladium intermediates are attacked by the amine to form tertiary allylic amines and regenerate the catalyst ... 

This technique to generate a vinylpalladium intermediate was later applied to the palladium-catalyzed 1,4-oxidation of conjugated dienes [132]. Thus, the use of substrate 110 inapalladium(II)-catalyzed oxidation in the presence of Li Cl afforded 113 in 65% yield (Eq. (47)). The reaction is an overall 1,4-trans-carbochlorination, and proceeds via chloropalladation of the acetylene to give the vinylpalladium intermediate 111, which in turn reacts in a migratory insertion reaction to produce the 7t-allyl complex 112. Subsequent chloride attack on 112 anti to palladium accounts for the product. The chlorodimer of the proposed 7t-allylpalladium intermediate was isolated and fuUy characterized. The chloromethylene function in 113 occurred asamixtureof( )-and(Z)-isomers (Z/ = 1.5 1), indicating that chloropalladation of the acetylene is a nonstereoselective process [130,133). 

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