Membrane Reactors for Homogeneously Soluble Catalysts

Especially for chiral ligands, which are often more expensive than the noble metals with which they coordinate, easy recovery and repeated use in order to increase the total turnover number are highly desirable [45, 64]. Attempts to couple chiral ligands to insoluble polymers often result in a decrease in the enantiomeric excess of the product, as described for the addition of diorganozinc to aldehydes (eq. (1)) [47, 48]. 

For the example of the addition of diethylzinc 2 to benzaldehyde 1 (eq. (1)), a continuous asymmetric synthesis in a membrane reactor using a homogeneously soluble catalyst [38, 39] has been developed for the first time. a,a-Diphenyl-t-pro-linol, which is used as the chiral ligand, has been coupled to a copolymer made from 2-hydroxyethyl methacrylate and octadecyl methacrylate [49] (molecular weight 96000 g mol ), resulting in the polymer-enlarged chiral ligand 4 [50]. 

The use of membrane reactors is favorable not only with respect to an increase in the total turnover number. In certain cases the selectivity can also be increased by applying high concentrations of the soluble catalyst together with making use of the behavior of a continuously operated stirred-tank reactor. Basically, this is also possible with a catalyst coupled to an insoluble support, but here the maximum volumetric activity is limited by the number of active sites per mass unit of the catalyst. This has been shown for the enantioselective reduction of ketones (eq. (2)) such as acetophenone 5 with borane 6 in the presence of polymer-enlarged oxazaborolidines 8 and 9 [65-67]. 

Besides the use of homogeneously soluble polymethacrylates or poylstyrene, as for the examples described above, other soluble supports may be used in order to yield a catalyst which can be retained by ultra- or nanofiltration membranes. Several groups have introduced catalysts (chiral and nonchiral) coupled to dendrimers and dendrimer-like structures [54, 59-76]. Compared with catalysts coupled to polymers, such complexes offer the advantage of a more defined structure. Thus, the number of active sites can be controlled more accurately. As these will be present at the surface of a globular structure they will be easily accessible. 

For some of them, the use of membrane reactors for their recovery or application in continuously operated reactors has been demonstrated. Examples include the use of dendrimer-bound nickel catalysts for the Kharasch addition [54, 59] and dendritic palladium catalysts for an allylic substitution [73, 60]. The membrane reactor concept has also been transferred to reactions at higher pressure, as shown for the hydrovinylation of styrene (cf. Section 3.3.3) [75]. Modem ultra-and nanofiltration membranes allow an effective recovery of the homogeneously soluble catalyst. However, in some cases the long-term stability of the catalyst under operating conditions has to be improved. 

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