Palladium/diphosphine catalysts

Scheme 6.11 Asymmetric hydrogenation of N tosylated imines by a palladium/diphosphine catalyst.

The addition-cyclization reaction of HP(0) compounds catalyzed by palladium-diphosphine catalyst systems [22] proceeds more selectively by the addition of Brpnsted acids. For instance, the Ph2P(0)0H-assisted reaction of 1,6-heptadiyne or analogues with HP(0)Ph2 affords the cyclized products in high yields (Scheme 38) [40]. A similar cyclization reaction with HP(0)Bu2 also proceeds in an acceptable yield (Scheme 39) [41]. 

In the 1980s Drent and colleagues at Shell found a remarkable change in catalytic behaviour when monodentate phosphines were replaced with certain bidentate diphosphines. Cationic palladium(II) catalysts generated in methanol from Pd(OAc)2, PPhs and the acid of a weakly coordinating anion (e.g. tosylate, triflate) catalyzed the methoxycarbonylation of ethylene to... 

Palladium diphosphine complexes located at the core have also been found to be efficient, recoverable catalysts in a Stille coupling involving iodobenzene ant tributylvinyltin. This palladium complex, composed of a dendrimer of generation 3 bearing only one metallic center located at the core, was prepared in situ by reacting the dendrimer (Figure 38) with Pd(OAc)2. The obtained complex (1 or 5 mol.%) appeared to be a very efficient catalyst under mild conditions. Steric hindrance around the metallic center due to the dendrimer shell allowed the system to be recycled several times without loss of activity. 

Miquel-Serrano MD, Aghmiz A, Dieguez M, Masdeu-Bulto AM, Claver C, Sinou D. Recoverable chiral palladium-sulfonated diphosphine catalysts for the as3mimetric hydrocarboxylation of vinyl arenes. Tetrahedron Asymm. 1999 10 4463 467. 

In order to keep the mild conditions, hydroxycarbonylation has been performed in biphasic media, maintaining the catalyst in the aqueous phase thanks to water-soluble mono- or diphosphine ligands. In the presence of the sodium salt of trisulfonated triphenylphosphine (TPPTS), palladium was shown to carbonylate efficiently acrylic ester [19], propene and light alkenes [20,21] in acidic media. For heavy alkenes the reduced activity due to the mass transfer problems between the aqueous and organic phases can be overcome by introducing an inverse phase transfer agent, and particularly dimeihyl-/-i-cyclodextrin [22,23]. Moreover, a dicationic palladium center coordinated by the bidentate diphosphine ligand 2,7-bis(sulfonato)xantphos (Fig. 2) catalyzes, in the presence of tolylsulfonic acid for stability reasons, the hydroxycarbonylation of ethylene, propene and styrene and provides a ca. 0.34 0.66 molar ratio for the two linear and branched acids [24],... 

The water-soluble palladium complex prepared from [Pd(MeCN)4](Bp4)2 and tetrasulfonated DPPP (34, n=3, m=0) catalyzed the copolymerization of CO and ethene in neutral aqueous solutions with much lower activity [21 g copolymer (g Pd) h ] [53] than the organosoluble analogue in methanol. Addition of strong Brpnsted acids with weakly coordinating anions substantially accelerated the reaction, and with a catalyst obtained from the same ligand and from [Pd(OTs)2(MeCN)2] but in the presence of p-toluenesulfonic acid (TsOH) 4 kg copolymer was produced per g Pd in one hour [54-56] (Scheme 7.16). Other tetrasulfonated diphosphines (34, n=2, 4 or 5, m=0) were also tried in place of the DPPP derivative, but only the sulfonated DPPB (n=4) gave a catalyst with considerably higher activity [56], Albeit with lower productivity, these Pd-complexes also catalyze the CO/ethene/propene terpolymerization. 

Better results were obtained by using in situ prepared palladium complexes of a G4 dendrimer (calculated molecular weight 20 564 Da for 100% palladium loading of the 32 diphosphines). After 100 residence times, the conversion had decreased from 100% to approximately 75% (Fig. 3). A small amount of palladium was leached from the catalyst during this experiment (0.14% per residence time), which only partly explains the decrease in conversion. The formation of inactive PdCl2 was proposed to account for the additional drop in activity. A sound conclusion about the effect of this dendritic catalyst requires more experiments. 

Maraval et al 39) synthesized core- and periphery-functionalized ruthenium and palladium dendritic diphosphines (Fig. 12) that were applied in three reactions (Stille coupling, Knoevenagel condensation, and diastereoselective Michael addition). The catalyst was recovered by using the precipitation strategy. 

Mizugaki et al. 74) have recently utilized thermomorphic properties of Pd(0)-complexed phosphinated dendrimers for dendritic catalyst recycling. Using the method developed by Reetz 16), they prepared dendritic ligands containing, respectively, 2, 8, 16, and 32 chelating diphosphines. Palladium dichloride was com-plexed to the dendrimers, and a reduction in the presence of triphenylphosphine gave the Pd(0)-complexed dendrimers (80—83). The dendritic complexes were active... 

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