Water-soluble ligands

A variety of new water-soluble ligands are being developed and the role of additives, particularly for enhancing solubility of sparingly soluble solutes in water, has been assessed even the use of (3-cyclodextrin to increase solubility has been studied. 

Buhling et al. (1995) have used amphiphilic ligands in oxo-reactions, where with a pH swing the catalyst is made organic or water soluble (ligand Ph2Ar(P) with Ar = 3-hydroxy phenyl or 4- caroxy phenyl). 

Gulyas etal. (1997) have suggested new amphiphillic or water-soluble ligands. 

Miscellaneous Reactions of Phosphines.- The role of chiral phosphines as ligands in the catalysis of reactions leading to the formation of chiral products has been reviewed.1111 A procedure for the determination of the enantiomeric excess in chiral phosphines has been developed, based on 13C n.m.r. studies of the diastereoisomeric complexes formed by phosphines with the chiral pinenyl nickel bromide complex. 111 Studies of the sulphonation of triphenylphosphine and of chiral arylphosphines have been reported in attempts to prepare water soluble ligands which aid... 

Biphasic systems were found to have a unique effect on the selectivity of the addition of arylboronic acids to alkynes. It was found that the use of [Rh(COD)OH]2 associated with the water-soluble ligand, m-TPPTC, was highly effective for such a reaction in the biphasic water/toluene system (Eq. 4.51).91 The reaction was completely stereo-and regioselective. In addition, the catalyst did not lose any activity... 

Diethyl arylphosphonates were synthesized by reactions of diethyl phosphonate with aryl iodides or bromides containing electron-donor or electron-acceptor substituents in the aromatic ring in aqueous MeCN or neat H2O in the presence of Pd complexes with water-soluble ligands.34 For example, MeCN/H20 (1 mL), PhBr (8.2 mmol), and Ph2P(C6H4S03Na-m) (approximately 0.4 mmol) were successively... 

Collum129 reported that while the Stille coupling can proceed without using a phosphine ligand, the addition of a water-soluble ligand improved the yield of the reaction. Water-soluble aryl and vinyl halides were coupled with alkyl-, aryl-, and vinyltrichlorostannane derivatives in this way (Eq. 6.39). 

By using the more water-soluble ligand, TPPTS, Grosselin et al. converted several unsaturated aldehydes into the corresponding unsaturated... 

Another approach for ion-sensing (here copper and zinc) is based on the water-soluble ligand zincon and the ion pair with quaternary ammonium halides shown in Figure 17 which can be homogeneously dissolved in polymers such as plasticized poly(vinyl acetate), ethyl cellulose, and polyurethane. 

In a recent report by Wang, microwave-mediated Heck couplings were rapidly performed in water under phase-transfer conditions (Eq. 11.8) [22]. Villemin has also reported the use of water and acetonitrile in the context of using water-soluble ligands in microwave-assisted Heck couplings [23],... 

This Chapter will concentrate on the hydroformylation of propene by means of rhodium catalysts, modified by water-soluble ligands such as TPPTS (triphenylphosphine m-trisulfonate). 

The RCH/RP process converts propylene to n- and Lo-butyraldehydes (or butenes to valeraldehydes) in the presence [HRh(CO)(TPPTS)] (with TPPTS = tris[sodium-m-sulfonatophcnvl phosphine as water soluble ligand) according to Equation 5.1. 

The use of amphiphilic water-soluble ligands which influence the alkene solubility or increase the catalyst concentration at the interface area of the phases [48,49] ... 

The use of water-soluble ligands was referred to previously for both ruthenium and rhodium complexes. As in the case of ruthenium complexes, the use of an aqueous biphasic system leads to a clear enhancement of selectivity towards the unsaturated alcohol [34]. Among the series of systems tested, the most convenient catalysts were obtained from mixtures of OsCl3 3H20 with TPPMS (or better still TPPTS) as they are easily prepared and provide reasonable activities and modest selectivities. As with their ruthenium and rhodium analogues, the main advantage is the ease of catalyst recycling with no loss of activity or selectivity. However, the ruthenium-based catalysts are far superior. 

The water-soluble ligand (TPPTS) was discussed earlier with regard to aldehyde reduction [17]. Similarly, in ketone transfer hydrogenation, high yields are obtained for a variety of substrates with the ability for efficient catalyst recycling at no expense of activity or selectivity (Fig. 15.10). 

Several methods have been described to liberate the hydroxyl groups from 24 to produce the water-soluble, tetrahydroxyl bidentate ligand 25 [52, 53b]. Water-soluble ligands are of interest due to the prospect of recycling the catalyst into an aqueous phase, ideally without loss of performance. The enantiomeric hydrogenation of itaconic acid was performed in aqueous methanol over a range of solvent compositions (MeOH H20, 9 1 to 3 97), with consistently excellent levels of performance (100% conversion, 99% ee, SCR 100, 12 h) [52 b]. Interest-... 

Another solution to the problem of catalyst/product separation is the biphasic catalysis. The liquid biphasic catalysis became an attractive technology for potential commercial application of enantioselective homogeneous catalysis. The most important features of such systems are related to the fact that both reaction rate and e.s. may be influenced by the number of ionic groups in water-soluble ligand or by addition of surfactants. Descriptions of water-soluble ligands and the recent results in the rapidly progressing area of biphasic enantioselective catalysis are available in recent reviews [255,256],... 

Bidentate chiral water-soluble ligands such as (S,S)-2,4-bis(diphenyl-sulfonatophosphino)butane BDPPTS (Fig. 2) or (R,R) 1,2-bis(diphenylsul-fonatophosphinomethyl)cyclobutane have been prepared [25]. Their palladium complexes catalyze the synthesis of chiral acids from various viny-larenes and an ee of 43% has been reached for p-methoxystyrene with the BDPPTS ligand. Furthermore, recycling of the aqueous phase has shown that the regio- and enantioselectivity are maintained and that no palladium leaches. 

The water-soluble ligands described above, together with many others, are used to conduct a wide range of catalytic reactions in water. These reactions include hydrogenation, hydroformylation, oxidation, C-C coupling and polymerization reactions [30], Many of these reactions are discussed in detail in Chapters 7-11. 

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