Palladium® complexes acetal ligands

The cationic palladium(II) complex [Pd(24a)3Cl]+ of the para-isomer of 24a (M = Na) catalyzes the carbonylation of benzyl chloride in basic medium to give phenyl-acetic acid in high yields. The Pd(0) complex [Pd(24a)3], formed by reduction of [Pd(24a)3Cl]+ with CO, is asumed to be the catalytic species [93] (see Scheme 1). Palladium complexes of ligands related to 24a (M = Na) have also been employed in aqueous ethylene glycol phases as catalysts for Suzuki-type C—C cross-coupling reactions for the syntheses of substituted biphenyls (cf. Section 6.6) [97]. 

Helquist et al. [129] have reported molecular mechanics calculations to predict the suitability of a number of chiral-substituted phenanthrolines and their corresponding palladium-complexes for use in asymmetric nucleophilic substitutions of allylic acetates. Good correlation was obtained with experimental results, the highest levels of asymmetric induction being predicted and obtained with a readily available 2-(2-bornyl)-phenanthroline ligand (90 in Scheme 50). Kocovsky et al. [130] prepared a series of chiral bipyridines, also derived from monoterpene (namely pinocarvone or myrtenal). They synthesized and characterized corresponding Mo complexes, which were found to be moderately enantioselective in allylic substitution (up to 22%). 

A palladium catalyst with a less electron-rich ligand, 2,2-dipyridyl-methylamine-based palladium complexes (4.2), is effective for coupling of aryl iodides or bromides with terminal alkynes in the presence of pyrrolidine and tetrabutylammonium acetate (TBAB) at 100°C in water.37 However, the reactions were shown to be faster in NMP solvent than in water under the reaction conditions. Palladium-phosphinous acid (POPd) was also reported as an effective catalyst for the Sonogashira cross-coupling reaction of aryl alkynes with aryl iodides, bromides, or chlorides in water (Eq. 4.18).38... 

The catalyst reported by Drent [48] was generated in situ by mixing a palladium source with the ligand. A palladium source is broadly defined as a complex or any form of palladium metal whereby upon mixing with the ligand an active catalyst is formed. Many palladium sources are possible, but the sources exemplified by Drent aretris(dibenzylideneacetone)dipalladium(0)(Pd2(dba)3),bis(dibenzylideneacetone) palladium(O) (Pd(dba)2), or palladium(II) acetate. 

The main path of the palladium-catalyzed reaction of butadiene is the dimerization. However, the trimerization to form /j-1, 3,6,10-dodeca-tetraene takes place with certain palladium complexes in the absence of a phosphine ligand. Medema and van Helden observed, while studying the insertion reaction of butadiene to 7r-allylpalladium chloride and acetate (32, 37), that the reaction of butadiene in benzene solution at 50°C using 7r-allylpalladium acetate as a catalyst yielded w-1,3,6,10-dodecatetraene (27) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 hours. 

No reaction of soft carbon nucleophiles takes place with propargylic acet-ates[37], but soft carbon nucleophiles, such as /7-keto esters and malonates, react with propargylic carbonates under neutral conditions using dppe as a ligand. The carbon nucleophile attacks the central carbon of the ir-alleny[palladium complex 81 to form the 7r-allylpalladium complex 82, which reacts further with the carbon nucleophile to give the alkene 83. Thus two molecules of the a-monosubstituted /7-keto ester 84, which has one active proton, are... 

When the Pd bears chiral ligands, these reactions can be enantioselective.1448 ir-Allylmo-lybdenum compounds behave similarly.1449 Because palladium compounds are expensive, a catalytic synthesis, which uses much smaller amounts of the complex, was developed. That is, a substrate such as an allylic acetate, alcohol, amine, or nitro compound1450 is treated with the nucleophile, and a catalytic amount of a palladium salt is added. The rr-allylpal-ladium complex is generated in situ. Alkene-palladium complexes (introducing the nucleophile at a vinylic rather than an allylic carbon) can also be used.1451... 

Although complexes with these ligands are common in palladium(II) chemistry, their occurrence is more scarce in platinum(II) compounds. Nevertheless these complexes can be prepared, examples being platinum(II) complexes of the optically active quadridentate Schiff base of salicylaldehyde and (R)-l, 2-diamines.1212 An alternative synthesis involves formation of the Schiff base by reaction of a complexed amino ligand on platinum(II) with amide acetates (equation 372).1213... 

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