Aqueous conditions palladium catalysis

The combination of water and an organic nitrile as the solvent system (aqueous bipha-sic system, ABS) permits one to separate the catalyst in the water solution, coordinated to a water-soluble phosphine, TPPTS, the trisodium salt of trisulfonated triphenylphos-phine.f" The groups of Sinou and Genet have studied this strategyy. " ° " ° Allylic carbonates are quite stable to the potentially hydrolytic conditions since the reactions occur in neutral medium and only traces of base are generated in the catalytic cycle. Organic-aqueous phase palladium catalysis has been reviewed extensively. 

The iodyl group (I02) is a good nucleofugue. Under mild conditions, the conversion of several iodylarenes into benzoic acids was realized in good yields, via carbonylation. Carbon monoxide was used at atmospheric pressure and 40-50°C, in aqueous sodium carbonate, with palladium catalysis [9] ... 

The transfer of the phosphine-assisted catalytic processes to aqueous media prompts the development of specific hydrophilic ligands. The most important rationale for the application of such ligands is the development of phase-separation techniques. In the biphasic liquid-liquid technique, the hydrophilic phosphine works as an effective extractor of palladium to the aqueous phase. However, numerous recent works coming primarily from Genet s group (vide infra) show that many important Pd-catalyzed reactions can be made to run under very mild conditions in homogeneous aqueous media if carried out in the presence of hydrophilic phosphines—essentially aqueous phosphine-assisted catalysis. 

It should be noted that cross-coupling in the presence of phosphine complexes of palladium usually requires high amounts of catalyst, with initial loadings of 25-30 mol% not being uncommon. An entirely new approach to the Suzuki reaction is phosphine-free palladium catalysis. The use of palladium catalysts without the addition of phosphine ligands for cross-coupling with organoboron compounds in aqueous media opened a new chapter in the story of this powerful synthetic method. This approach allows catalyst efficiency to be dramatically increased, and the reaction to be performed under milder conditions. 

An additional advantage of Freeh s palladium pincer cross-couphng catalyst includes its well-defined fate after catalysis. Treatment of 10 with aqueous hydrochloric acid and thus under work-up conditions led to a rapid and complete catalyst degradation, accompanied by the formation of phosphonate, piperidinium salts, 1,3-diaminobenzene, and other insoluble paUadium-containing products, which were easily separable from the coupling products-an important issue to be considered in particular for pharmaceutical applications. All these advantages make, [ C H3-2,6-(NHP(piperidinyl)2)2 Pd(Cl)] (10) attractive for industrial applications. Indeed, 10 was successfully applied in industry, and nowadays is commercially available. 

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