Chiral water soluble ligands

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. 

Dehydropeptides were reduced (Scheme 3.9) on a preparative scale in two-phase systems with catalysts prepared in situ from [ RhCl(COD) 2] and chiral water-soluble ligands 35, 36, and 37 (Ch.2). The highest (87%) diastereoselectivity was obtained with [ RhCl(COD) 2] + tetrasuhbnated 2,4-bis(diphenylphosphino)pentane, BDPPTS, 36 [121]. 

When chiral water soluble ligands are used the technique can be applied to asymmetric hydrogenations [26]. Some examples are shown in Fig. 7.9. 

Recycling of the catalyst has been investigated by using Ru complexes with a chiral water-soluble ligand [286, 287], a dendritic ligand [288], or a TsDPEN immobilized on a polystyrene resin [289, 290],... 

Di)phosphines containing chiral backbones equipped with diphenylphosphino-substituents are the most successful and best-investigated chiral ligands in asymmetric homogeneous catalysis. Thus, a variety of chiral water-soluble ligands were prepared by direct sulfonation of these phosphorus ligands under conditions similar to those for the synthesis of achiral sulfonated phosphines. 

Among the other accesses to chiral water-soluble ligands, the introduction of a quaternary ammonium group is one of the most studied. Nagel et al. reported on the preparation of diphosphine 13 by quaternization of (3i ,4R)-3,4-bis(diphenyl-phosphino-l-methylpyrrolidine) with Me3OBF4 after protecting the phosphorus through complexation to rhodium [19]. 

The enantioselective hydrogenation of some a-amino add precursors 1 [Eq. (1)) in water or in an aqueous/organic two-phase system has been thoroughly investigated using rhodium or mthenium complexes assodated with chiral water-soluble ligands 3-13. Some of the most interesting results are summarized in Table 1. 

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... 

Reaction of the anion 21 with Cp or Cp metal fragments provides further metallocene-type complexes with a pendant phosphaferrocene side-chain. For example, the reaction of the thallium derivative T1 21 with [Cp RhCl2]2 yields the cationic pentamethylrhodocenium 24 as its chloride (Scheme 1.5.10). This is an interesting species because it is a chiral water-soluble P ligand. The chloride anion can be exchanged by PF,s to make the compound more soluble in organic solvents. 

The commercial success of rhodium-trisulfonated triphenylphosphine (TPPTS) catalysts1 has prompted considerable interest in TPPTS and other water-soluble ligands.2 The potential for new applications for the synthesis of both bulk and fine chemicals in water has led to methods for the preparation of a wide variety of sulfonated phosphines including chiral phosphines3 and... 

Transfer hydrogenation of ketones using metal complexes with a chiral water-soluble [97,98] and a dendritic ligand [99] was investigated for use in recycling catalysts. The reaction with immobilized catalysts has also been reported [100]. 

Allylic oxidation (acyloxylation) can also be achieved with copper catalysts and stoichiometric amounts of peresters or an alkylhydroperoxide in a carboxylic acid as solvent [108], via a free radical mechanism (Fig. 4.40). The use of water-soluble ligands [109] or fluorous solvents [110] allows recycling of the copper catalyst. In view of the oxidants required, this reaction is economically viable only when valuable (chiral) products are obtained using asymmetric copper catalysts [111-113]. The scope of the reaction is rather limited however. 

Interesting chiral water-soluble aminosulfonamide ligands containing a phenyl-sulfonic acid substituent have been synthesized and engaged directly with a ruthenium precursor to reduce enantioselectively aromatic ketones to the corresponding alcohols [84]. As concluded by the authors, these ligands should be evaluated in bi-phasic catalysis. 

Stelzer and co-workers reported a number of chiral water-soluble secondary phosphines [14], prepared by nucleophilic phosphination of primary phosphines with fluorinated aryl sulfonates in the superbasic medium DMSO/KOH. Further reaction with alkyl halides gives bidentate tertiary phosphines with P-chirality, but only racemic versions have been reported so far. Hanson et al. introduced so-called surface-active phosphines into asymmetric aqueous-phase catalysis. One of the main problems inherent to two-phase catalysis is the often very low miscibility of the substrates in the aqueous phase. Insertion of long alkyl chains between phosphorus atoms and phenyl groups in sulfonated phosphine ligands has been proven to increase reaction rates in the Rh-catalyzed hydroformylation of 1-octene [15], This concept was extended to a number of chiral ligands, i.e., the monoden-... 

Figure 6.6 Chiral water-soluble, polyhydroxyphosphine ligands.

A remarkable rate enhancement of palladium-catalyzed allylic alkylation in water using non-water-soluble ligands was observed in the presence of surfactants, and when BINAP was used as the chiral ligand, enantioselectivities up to 94% ee were obtained in the allylic alkylation of l,3-diphenyl-3-acetoxyprop-l-ene with carbon nucleophiles (Scheme... 

Gomez Arrayas R, Adrio J, Carretero JC (2006) Recent applications of chiral ferrocene ligands in asymmetric catalysis. Angew Chem Int Ed 45 7674—7715 Dai LX, Hou XL (2010) Chiral ferrocenes in asymmetric catalysis. Wiley-VCH, Weinheim Rigaut S, Delville MH, Losada J, Astrac D (2002) Water-soluble mono- and star-shaped hexanuclear functional organoiron catalysts for nitrate and nitrite reduction in water syntheses and electroanalytical study. Inorg Chim Acta 334 225-242... 

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