Palladium water-soluble

The reaction is performed in a biphasic aqueous-organic medium using a water-soluble palladium catalyst (see later). 

A new water soluble palladium(O) complex was also prepared. This compound and a previously reported related species were examined as catalysts in the hydrogenation of unsaturated nitriles in water. It was found that the pH of the solvent plays a crucial role in determining both yield and reaction course.869... 

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. 

Similarly, a water-soluble palladium complex of a sulfonated phenanthroline ligand catalyzed the highly selective aerobic oxidation of primary and secondary alcohols in an aqueous biphasic system in the absence of any organic solvent (Figure 1.8) [40]. The liquid product could be recovered by simple phase separation, and the aqueous phase, containing the catalyst, used with a fresh batch of alcohol substrate, affording a truly green method for the oxidation of alcohols. 

Two-phase systems utilizing water-soluble palladium phosphine catalysts, formate and allyl chlorides or acetates have also been developed.303... 

Another recent development is the use of water soluble palladium complexes as recyclable catalysts for the aerobic oxidation of alcohols in aqueous/organic biphasic media (Fig. 1.22) [73]. 

Fig. 4.36 Water-soluble palladium complexes for Wacker oxidation.

A much more active catalyst is constituted by a water-soluble palladium(II) complex of sulfonated bathophenanthroline as a stable, recyclable catalyst for the aerobic oxidation of alcohols in a two-phase aqueous-organic medium, e.g. in Fig. 4.64 [16, 172, 173]. Reactions were generally complete in 5 h at 100°C/ 30 bar air with as little as 0.25 mol% catalyst. No organic solvent is required (unless the substrate is a solid) and the product ketone is easily recovered by phase separation. The catalyst is stable and remains in the aqueous phase which can be recycled to the next batch. 

The importance of water as a solvent has led to the development of water-soluble phosphines. This has been achieved by the incorporation of hydrophilic substituents into the phosphine stmcture. The choice of substituents, whether anionic, cationic, or neutral, depends on the exigencies of the end use. The anionic species [Ph2P(3-C6H4S03)l Na+ has been used to afford a water-soluble palladium catalyst (37) by Du Pont workers. ... 

Water-soluble palladium(O) complexes have also been used as homogeneous catalysts in aqueous-solution alkylation reactions. The particular complex that has been used is Pd(TPPMS>3. Aryl or heteroaromatic halides can be coupled with aryl or vinyl boronic acids, alkynes, alkenes, or dialkyl phosphites with this palladium(0) complex. This complex in aqueous solution can also be used for the coupling of alkynes with unprotected iodonucleotides, iodonucleosides, and iodoamino acids (133). 

The cross-coupling reactions of various aryl halides and triflates with vinyl- or arylboronic acids and esters (Suzuki cross-coupling reaction) was also carried out in water in the presence of tetrabutylammonium bromide and a base such as Na2C03, using a phosphine-free palladium catalyst to give biaryl derivatives [Eq. 18)1 [108,109]. More recently, Casalnuovo [101] and Gen t [102,110] have performed this reaction using water-soluble palladium catalysts PdCla (tppms)2 and Pd(OAc)2/tppts in water/acetonitrile. 

As revealed in our previous study, palladium complexes with heteropolytungstate anion PWnOag do catalyze oxidation of benzene with O2/H2 mixture. Water soluble palladium complexes operate in a two-phase liquid medium. This paper describes preparation, characterization and catalytic performance of Si02 supported palladium complexes with heteropolytungstates. 

Beletskaya and co-workers have shown that the reaction is possible in neat water as solvent. Thus, aryl iodides have been carbonylated with various palladium salts lacking phosphine ligands as depicted in Eq. (4) [7]. Although this reaction is not a truely biphasic process the results are remarkable regarding catalyst efficiency. Thus, a maximum turnover number (TON) of 100000 was described (R = p-COOH, quantitative yield after 6 days). Quite different is the performance of a water-soluble palladium phosphine catalyst described by Kalck et al. [8]. The hy-drocarboxylation of the less activated bromobenzene with either Pd(TPPTS)3 or a mixture of Pd(OAc)2 and TPPTS proceeds only sluggishly (turnover frequency TOF < 10 h 1). In order to prevent decomposition of palladium an excess of phosphine has to be used. At least 15 equiv. of ligand is necessary to prevent formation of metallic palladium. Because of rapid oxidation of the ligand the re-use of the water phase is not possible. 

Based on the palladium-catalyzed carbonylation of benzylic halides Sheldon and co-workers investigated the functionalization of 5-hydroxymethylfurfural (HMF) to 5-formylfuran-2-acetic acid (FFA) in aqueous medium in the presence of a water-soluble palladium/TPPTS catalyst (Scheme 3) [5], Here, the hydroxy group displays similar reactivity under acidic conditions compared with benzylic halides. 

In general for carbonylations, palladium as catalyst metal is preferable to nickel with respect of catalyst efficiency. Thus, Okano, Kiji, and co-workers described some other efficient palladium-catalyzed carbonylations of allyl chloride and substituted allyl halides (Eqs. 5-10). In greater detail, the water-soluble palladium complex PdCl2[Ph2P(w-C6H4S03Na)]2 has been used in a two-phase system (e.g., aqueous NaOH/benzene medium) at atmospheric carbon monoxide pressure, giving 3-butenoic acids [20], In the carbonylation of allyl chloride a mixture of 2-bute-noic acid, which was formed by base-catalyzed isomerization, and 3-butenoic acid was obtained in up to 90% yield (TON = 135), albeit at moderate selectivity (24 76). Clearly, the isomerization depends on the concentration of the base and was therefore suppressed by a method of continuous addition to the aqueous medium. 

The isolation of water-soluble palladium(O) complexes was achieved by Herrmann and co-workers by gel-permeation chromatography for Pd(TPPTS)3 [5], An X-ray determination was reported by Casalnuovo and Calabrese for Pd(TPPMS), [6] this is the first published structure of a transition metal complex containing a sulfonated phosphine. 

The water-soluble palladium(O) complex was generated in situ from Pd(OAc)2 or Pd2(dba)3 (dba = dibenzylideneacetone) in association with TPPTS in water/nitrile mixture [7]. The use of a nitrile as the co-solvent being beneficial very important for the recycling of the catalyst, probably because of stabilization of the palladium ) catalyst. The formation of a Pd(TPPTS)3 species was demonstrated through a series of 31P-NMR experiments and by cyclic voltametry [8, 9]. 

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