Catalytic reactions palladium-catalyzed benzylation

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

The previous extension of solvent mixtures involved solvent interfaces. This organic-water interfacial technique has been successfully extended to the synthesis of phenylacetic and phenylenediacetic acids based on the use of surface-active palla-dium-(4-dimethylaminophenyl)diphenylphosphine complex in conjunction with dode-cyl sodium sulfate to effect the carbonylation of benzyl chloride and dichloro-p-xylene in a toluene-aqueous sodium hydroxide mixture. The product yields at 60°C and 1 atm are essentially quantitative based on the substrate conversions, although carbon monoxide also undergoes a slow hydrolysis reaction along with the carbonylation reactions. The side reaction produces formic acid and is catalyzed by aqueous base but not by palladium. The phosphine ligand is stable to the carbonylation reactions and the palladium can be recovered quantitatively as a compact emulsion between the organic and aqueous phases after the reaction, but the catalytic activity of the recovered palladium is about a third of its initial activity due to product inhibition (Zhong et al., 1996). 

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]. 

The addition of an oxidizing reagent makes reactions catalytic with respect to palladium(II). Palladium compounds catalyze many other reactions invol-ving C-H bond activation. For example, benzaldehyde and benzoic acid can be produced by partial oxidation of toluene applying the fuel cell reaction in the gas phase using palladium black as the anode. The authors proposed a n-aliyl-benzyl-Pd"" complex as the reactive intermediate [16]. 

Mechanistic studies performed with Freeh s pincer catalyst in the Heck reaction excluded catalytic cycles with the involvement of homogeneous palladium(O) species, as indicated by the results obtained from the (recently developed) dibenzyl-test, which is directly applicable under the reactions conditions applied [24aj. Dibenzyl formation was - in contrast to Heck reactions catalyzed by palladium(O) complexes of type [Pd(PR3)2, where Pd /Pd" cycles are operative - not detectable by gas chromatography-mass spectrometry (GC/MS) when reaction mixtures of aryl bromide, olefin, benzyl chloride ( 10 mol% relative to aryl bromide), catalyst, and base were thermally treated. On the other hand, experimental observations, such as quantitative poisoning experiments with metallic mercury and CS2, which were shown to eflfidently inhibit catalysis, as well as analysis of the reaction profiles showed sigmoidal-shaped kinetics with induction periods and hence indicated that palladium nanoparticles are the catalytically active form... 

This catalytic cycle is shown in Scheme 16.17 for the reactions of cyclohexadiene catalyzed by paUadium-phosphine complexes. By this mechanism, the combination of a diene and either a Ni(0) or Pd(0) complex and acid or a cationic nickel(ll) or palladium(ll) hydride generates a cationic ir-allyl complex. Similarly, the combination of a vinylarene and one of these combinations of species generates a cationic phenethylpalladium complex. These allyl and phenethyl complexes have been isolated using this procedure. - The amine then attacks the ir-allyl or ir-phenethyl complex to form tfie organic product and regenerate Ni(0) or Pd(0). This nucleophilic attack on ir-allyl and ir-benzyl complexes was discussed in detail in Chapter 11. 

One of the most interesting and useful applications of phase transfer catalysis in organometallic chemistry is the catalytic carbonylation reaction of alkyl halides [50]. Palladium tetrakis triphenylphosphine catalyzes the carbonylation of benzyl chloride in a two phase system containing xylene and aqueous sodium hydroxide according to equation 9.21. Phenylacetic acid (4000 moles/mole Pd) is produced in... 

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