Phosphines amphiphilic

Rhodium also has been reported as a catalyst for [2+2+2] alkyne cycloaddition in water. Uozumi et al. explored the use of an amphiphilic resin-supported rhodium-phosphine complex as catalyst (Eq. 4.60). The immobilized rhodium catalyst was effective for the [2+2+2] cycloaddition of internal alkynes in water,113 although the yields of products were not satisfactory. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

Amphiphilic resin supported ruthenium(II) complexes similar to those displayed in structure 1 were employed as recyclable catalysts for dimethylformamide production from supercritical C02 itself [96]. Tertiary phosphines were attached to crosslinked polystyrene-poly(ethyleneglycol) graft copolymers (PS-PEG resin) with amino groups to form an immobilized chelating phosphine. In this case recycling was not particularly effective as catalytic activity declined with each subsequent cycle, probably due to oxidation of the phosphines and metal leaching. 

The use of surfactants in hydrogenation and hydroformylation immediately followed the practical implementation of the original idea of aqueous biphasic catalysis [57, 118]. Not only the effect of well-known tenzides (SDS, CTAB, etc.) was studied, but new amphiphilic phosphine... 

Amphiphilic tertiary phosphines have their phosphorus donor atom located somewhere in the hydrophobic part of the molecule and should have at least one long alkyl or alkyl-aryl chain carrying a polar head group (Scheme 4. 10). Some of them, such as the sulfonated derivatives, are quite well soluble in water, others, such as Ph2P(CH) COOH (n = 3, 5, 7, 9, 11) are practically insoluble, however, can be easily solubilized with common surfactants (SDS, CTAB etc.). 

Concerning monodentate amphiphilic phosphines one of the latest developments is the use of Rh/phosphonate-phosphine catalysts for the hydroformylation of 1-octene and 1-dodecene [54]. The catalysts were prepared in situ from [Rh(acac)(CO)2] and from the appropriate Ph2P(CH2)ioP03M2 (M = Na, K", NKi, l aPr) phosphine. Pretreatment under 30 bar syngas significantly improved the catalytic performance. At 120 °C, 30 bar syngas, [P]/[Rh] = 5, in 4 h, 1-octene reacted with 52 % conversion and 47 % aldehyde yield. This means a 91 % selectivity to... 

Special mention has to be made of the use of surfactants. Aryl halides are insoluble in water but can be solubilized in the aqueous phase with the aid of detergents. A thorough study [24,25] established that the two-phase reaction of 4-iodoanisole with phenylboronic acid (toluene/ethanol/water 1/1/1 v/v/v), catalyzed by [PdCl2 Ph2P(CH2)4S03K 2], was substantially accelerated by various amphiphiles. Under comparable conditions the use of CTAB led to a 99 % yield of 4-methoxybiphenyl, while 92 % and 88 % yields were observed with SDS and n-Bu4NBr, respectively (for the amphiphiles see Scheme 3.11). Similar effects were observed with Pd-complexes of other water-soluble phosphines (TPPTS and TPPMS), too. 

Uozumi, Y. Suzuka, T. pi-AUyhc sulfonylation in water with amphiphilic resin-supported paUadium-phosphine complexes. Synthesis 2008,1960-1964. 

Uozumi, Y. Danjo, H. Hayashi, T. New Amphiphilic Palladium-Phosphine Complexes Bound to Solid Supports Preparation and Use for Catalytic Allylic Substitution in Aqueous Media, Tetrahedron Lett. 1997, 38, 3557. 

Kiji J, Okano T, Nomura T, Saiki K, Sai T, Tsuji J (2001) Mechanistic studies on Pd-catalyzed telomerization and co-cyclization of butadiene amphiphilicity of bis-7i-allylpalladium intermediate in the presence of phosphine ligand. Bull Chem Soc Jpn 74 1939-1945... 

Suzuki-type C-C-coupling reactions (cf. Section 2.11) with Pd phosphine complexes as catalysts can also be promoted by amphiphiles in an aqueous biphasic system (toluene/water). The amphiphile should have a phase-transfer function, with the best effect being observed with micelle-forming amphiphiles [27]. 

An insightful discussion on an alternahve approach is provided in Chapter 6 of this Handbook, where insoluble polymer-supported chiral catalysts (chiral palladium phosphine complexes supported on TentaGel-type amphiphilic polymer bearing PEG chains) are used for heterogeneous asymmetric processes in water. 

Figure 6.1 Amphiphilic PS-PEG resin-supported palladium-phosphine complexes.

Ding, Hanson, and Bakos [39] used an amphiphilic chiral rhodium-phosphine complex in the catalytic asymmetric two-phase hydrogenation of methyl (Z)-a-acetamidocinnamate in ethyl acetate/water and obtained enantioselectivities of up to 69% ee. 

Another approach to water-soluble phosphines with the emphasis on metal recycling was reported by van Leeuwen and co-workers [30], They have synthesized a number of diphosphines that, when coordinated to rhodium, form complexes having an amphiphilic character. The ligands synthesized are based on BISBI and Structures 20-22, and hydroformylation (for example) can be conducted in a homogeneous (organic) phase [30 a]. After it has been used in the hydroformylation of olefins the catalyst can be removed by acidic extraction. It was established that these novel diphosphines form active and highly selective catalysts. This amphiphilic approach, i.e., rhodium recycling abased on the extraction and re-extraction principle, will be discussed in more detail in Section 7.5. 

Ruthenium complexes generated from the amphiphilic phosphines 24 (see also Section 7.5) were used for the selective hydrogenation of 3-methyl-2-butenal (pre-nal) to 3-methyl-2-butenol (prenol) in isopropanol/water mixtures [49]. High conversions of up to 100% and selectivities of 90-96% were achieved with ligands of the type 24 containing long poly ether chains [49]. 

In general, amphiphilic phosphines show both better activity and selectivity than TPPTS when compared in aqueous/solvent mixtures as the reaction medium. The series of phosphines, P[(C6H4)/(CH2)n(C6H4S03Na)]3, n = 3, 6, 10 has been compared with TPPTS for the hydroformylation of 1-octene in aqueous methanol [11,... 

Phosphine 3 has good solubility in both water and methanol. A comparison of 3 and TPPTS for the rhodium-catalyzed hydroformylation of 1-octene in aqueous methanol is shown in Figure 2 [14]. At all ligand/rhodium ratios studied, the amphiphilic phosphine shows both better activity and selectivity than TPPTS. The improved selectivity is attributed to diminished inter- and intra-ligand ionic repulsions with the greater distances between sulfonate groups in complexes of the amphiphilic ligands. 

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