Thermoregulated ligand

The development of thermoregulating ligands might well make a decisive contribution to solving the problems of aqueous, homogeneously catalyzed hydroformylations of higher olefins. 

Thermoregulated Ligands and Thermoregulated Phase-Transfer Catalysis... 

Chiral thioureas have been synthesized and used as ligands for the asymmetric hydroformylation of styrene catalyzed by rhodium(I) complexes. The best results were obtained with /V-phenyl-TV -OS )-(l-phenylethyl)thiourea associated with a cationic rhodium(I) precursor, and asymmetric induction of 40% was then achieved.387,388 Chiral polyether-phosphite ligands derived from (5)-binaphthol were prepared and combined with [Rh(cod)2]BF4. These systems showed high activity, chemo- and regio-selectivity for the catalytic enantioselective hydroformylation of styrene in thermoregulated phase-transfer conditions. Ee values of up to 25% were obtained and recycling was possible without loss of enantioselectivity.389... 

Keywords Immobilization Thermoregulated catalysis Thermomorphic ligands Hydroformylation Polyethylene glycol modified phosphites... 

A series of water-soluble polyether-substituted triphenyl phosphines (PETPPs) la-c has been successfully employed by Jin et al. [11] in the thermoregulated hydroformylation of 1-dodecene in the biphasic water/toluene system. The catalysts exhibit very good catalytic properties with conversions up to 93% and about 85% selectivity for aldehyde formation. The catalyst derived from rhodium(III) chloride and ligand Ic could be reused in four consecutive cycles without significant loss of activity or chemoselectivity. The n-selectivity of the product aldehydes was not determined. 

Octylpolyglycol phenylene phosphite 4 was investigated as ligand in the thermoregulated phase-transfer hydroformylation of styrene [ 14]. The cata-... 

The opposite temperature-dependent solubility of ligands in organic solvents is applied in the thermoregulated phase-separable catalysis (TRPSC) first published by Jin and coworkers [17] in 2000. The general principle is shown in Fig. 2 [10]. 

Hermanns E, Hasenjager J, Driefien-Holscher B (2008) PEG-Modified Ligands for Catalysis and Catalyst Recycling in Thermoregulated Systems. 23 53-66 Hermanns J, see Schmidt B (2004) 13 223-267 Herrmann W, see Freimd C (2007) 22 39-77... 

An alternative approach was reported in 1997. A series of novel water-soluble polyether-substituted triphenylphosphanes was prepared by means of the ethoxylation of mono-, bis-, and tris(p-hydroxyphenyl)phosphanes (47). They manifest inverse temperature-dependent solubility in water that enables them to act as thermoregulated phase-transfer ligands. 

Because of their low solubilities in the aqueous phase, the hydroformylation of higher alkenes (>C2) is still a challenging problem. In addition to fluorous biphasic catalysis, possible solutions, which have been addressed, include the addition of surfactants240,241 or the use of amphiphilic ligands242-244 to enhance mutual solubility or mobility of the components across the phase boundary and thereby increase the rate of reaction. The use of polar solvents such as alcohols,245 p-cyclodextrin,246 cyclodextrin ligands,247 248 thermoregulated phase-transfer... 

Another interesting technique to solve the problem of mass transport in bipha-sic reactions was developed by Fell and Jin, the catalysis with so-called thermo-regulated phase transfer ligands [177-185] (details see Section 4.6.3). In the examples given so far, a smart solvent system controlled the phase behaviour of the reaction mixture. Lemaire et al. have found that also polyether-functionalized chiral mono- or diphosphites are active in thermoregulated conversion [186]. The hy-droformylation of styrene yields conversions of 99%, n/i-ratios of 85/15 and an enantioselectivity of up to 25%. A recycling of the catalyst proved to be possible, however, with a certain leaching of the rhodium catalyst. 

More recently, the application scope of thermoregulated phase-separable transition metal complex with nonionic phosphine ligand has been expanded from hy-droformylation to hydrogenation, and the central metal varied from Rh to Ru. The first experimental study is the hydrogenation of styrene catalyzed by thermoregulated phase-separable Ru3(CO)12/PETPP complex catalyst. Under the conditions of Ph2 = 2.0 MPa, T=90°C, catalyst/substrate (mol/mol) = 1/1000, 3 hours, the Ru3(CO)12/PETPP complex catalyst exhibited good activity (Table 5). Compared with other catalysts, Ru3(CO)12/PETPP complex showed the same catalytic activity compared to the lipophilic Ru3(CO)9(TPP)3, while the hydrophilic Ru3(CO)9-(TPPTS)3 and Ru3(CO)9(TPPMS)3 are less active (Table 6). 

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