Soluble Polymer-Bound Catalysts

Janda, Bolm and Zhang generated soluble polymer-bound catalysts for the asymmetric dihydroxylation by attaching cinchona alkaloid derivatives to polyethylene glycol monomethyl ether (MeO-PEG) [84—87]. Since these polymeric catalysts like (24) are soluble in many common solvents they are often as effective as their small homogenous counterparts. Janda et al. prepared catalyst (24) in which two dihydroquinidine (DHQD) units were linked together by phthalazine and finally were attached to MeO-PEG via one of the bicyclic ring system moieties (Scheme... 

Soluble polymer-bound catalysts for epoxidation reactions have also been explored, with a complete study into the nature of the polymeric backbone performed by Janda [70]. Chiral (salen)-Mn complexes were appended to MeO-PEG, NCPS, Jan-daJeF and Merrifield resin via a glutarate spacer. It was found that for the Jacobsen epoxidation of ds-/ -mefhylstyrene, the enantioselectivities for each polymer-supported catalyst were comparable (86-90%) to commercially available Jacobsen catalyst (88%). Both soluble polymer-supported catalysts could be used twice before a decline in yield and enantioselectivity was observed. However, neither soluble polymer support proved as suitable as the insoluble JandaJel-supported (salen)-Mn complex for the epoxidation because residual impurities during precipitation and leaching of Mn from the complex, resulted in lowered yields. 

Recently a rhodium water-soluble polymer-bound catalyst, based on the commercially available copolymer of maleic anhydride and methyl vinyl ether, was shown to be very active in the hydrogenation of various substrates in basic aqueous media [25]. 

Catalysis with water-soluble polymer-bound catalysts in a single homogeneous aqueous phase, the subject to this section, can be of interest for the conversion of water-soluble organic substrates. With a view to applications, the use of water as a nonhazardous, environmentally benign solvent can be advantageous. 

Soluble polymer-bound catalysts can be expected to receive continued attention as they offer specific advantages. By comparison to aqueous two-phase catalysis, a range of substrates much broader with respect to their solubility can be employed. By comparison to heterogenization on solid supports, the selectivity and activity of homogeneous complexes can be retained better. However, it must also be noted that to date no system has been unambiguously proven to meet the stability and recovery efficiency required for industrial applications. 

The various properties of water in different aspects (being important for the reactivity, reaction kinetics or mechanisms, reaction engineering, or other concerns) are discussed elsewhere. The procedures for tailoring the water-solubility of the catalysts are many-sided and may be generalized much more easily than the corresponding methods for SHOP (cf. Section 7.1), fluorous phase (Section 7.2), supercritical solvents (Section 7.4), water-soluble polymer-bound catalysts (Section 7.6), or NAIL utilization (Section 7.3) no wonder that all other biphasic applications remain singular or are still just proposals. Both the scientific and industrial com-... 

Last but not least, the success of aqueous-phase catalysis has drawn the interest of the homogeneous-catalysis community to other biphasic possibilities such as or-ganic/organic separations, fluorous phases, nonaqueous ionic liquids, supercritical solvents, amphiphilic compounds, or water-soluble, polymer-bound catalysts. As in the field of aqueous-phase catalysis, the first textbooks on these developments have been published, not to mention Job s book on Aqueous Organometallic Catalysis which followed three years after our own publication and which put the spotlight on Job s special merits as one of the pioneers in aqueous biphasic catalysis. Up to now, most of the alternatives mentioned are only in a state of intensive development (except for one industrial realization that of Swan/Chematur for hydrogenations in scC02 [2]) but other pilot plant adaptations and even technical operations may be expected in the near future. 

The subject of soluble polymers as supports in catalysis has been discussed in a number of recent reviews [ 1-5]. These other reviews each focused on a particular polymer or groups of polymers or on some aspect of catalysis (e.g., organic catalysis or asymmetric synthesis) [2, 3, 6-8]. Soluble polymers use as supports in synthesis has also been reviewed, but this topic is not covered below because in synthesis the polymer is used in a stoichiometric amount and is generally not recyclable [5,9-11]. This review takes a general approach, focusing on soluble polymers used as catalyst supports. It discusses these supports within a context of the separation strategies that could be or were used to separate or recover the soluble polymer-bound catalyst from the products. This review emphasizes examples from the last few years where soluble polymers are used but includes, for completeness, earlier examples if a particular... 

Using soluble polymers under conditions where the products and polymer supports both remain in solution during the separation stage is a scheme that is uniquely applicable to soluble polymers. It is, for the most part, the scheme used in nature where enzyme catalysts are used and separation is based on size. To date, hquid/hquid separations remain a less common way of recovering soluble polymer-bound species. However, the earhest examples where soluble polymers were used in catalysis separated the solutions of soluble polymer-bound catalyst from the products with membranes [ 106,107]. While solid/Uq-uid separations (vide infra) stiU predominate, that situation may change as new separation strategies are invented and perfected or as new more durable and improved membranes are developed. Indeed, as can be seen from the discussion below, a variety of new and improved approaches have recently been developed where soluble polymer-bound catalysts are isolated as solutions. 

The use of a sohd or hquid membrane to separate products and reactants is most attractive as it lends itself to continuous operations. The main problems are the efficiency of the separation, the speed of the separation, and the dma-bility of the membrane. Membrane separation was also one of the first ways linear soluble polymer-bound catalysts were separated from products. However, separation efficiencies in those first examples were not as high as present ones, as membrane technology has substantially improved over the past 35 years [135]. Thus, it is not surprising that many recent examples using membranes to recover polymer-bound catalysts and to separate them from products have been reported. This technique seems particularly apt for dendrimers because of their overall globular structure [136]. However, improved membranes can also be useful with hnear soluble polymer-bound catalysts. A recent review summarizes much of this work [137]. 

Water-soluble polymer-bound catalysts represent an interesting alternative [86], in particular when they are attached to smart polymers, which can undergo a complete... 

Thus, it is not surprising that many recent examples using membranes to recover polymer-bound catalysts and to separate them from products have been reported. This technique seems particularly apt for dendrimers because of their overall globular structure [136]. However, improved membranes can also be useful with linear soluble polymer-bound catalysts. A recent review summarizes much of this work [137]. 

Industrial interest in soluble polymer-bound catalysts has been closely linked to the development of ultrafiltration membranes with sufficient long-term stability in organic solvents. Membranes fulfilling these requirements were prepared first in the late 1980s. Today, solvent-stable flat sheet membranes and membrane modules are available from several suppliers. As for the viability of ultrafiltration in organic solvents, rhodium-catalyzed hydroformylation of dicydopentadiene with continuous catalyst recovery and recycling has been demonstrated successfully on a pilot plant scale over an extended period of time [5]. The synthesis of other fine chemicals by asymmetric reduction and other reactions has also been carried out in continuously operated membrane reactors (also cf Section 7.5) [6-9]. The extent of commercial interest in catalysts bound to soluble polymers appears to fluctuate at intervals. Amongst other factors, the price of precious metals can be a driver. 

Homogeneous Catalysis with Soluble Polymer-Bound Catalysts as a Unit Operation I 765... 

Figure 8 Schematic representation of different reactor types for the continuous recovery of soluble polymer-bound catalyst ...

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