Dendritic catalyst recycling

Mizugaki et al. 74) have recently utilized thermomorphic properties of Pd(0)-complexed phosphinated dendrimers for dendritic catalyst recycling. Using the method developed by Reetz 16), they prepared dendritic ligands containing, respectively, 2, 8, 16, and 32 chelating diphosphines. Palladium dichloride was com-plexed to the dendrimers, and a reduction in the presence of triphenylphosphine gave the Pd(0)-complexed dendrimers (80—83). The dendritic complexes were active... 

Here we review recent progress and breakthroughs in research with promising, novel transition metal-functionalized dendrimer catalysts and discuss aspects of catalyst recycling and unique dendritic effects in catalysis. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

In the first part of this overview, we focus on the recycling of dendritic catalysts. This part of the review is divided according to the various recycling approaches, and the sections are organized by way of the reactions catalyzed. In the second part, we describe examples in which attachment of the catalyst to the dendrimer framework results in modified performance. (Although we attempted to make a clear division between catalyst recycling and dendritic effects, these two properties cannot always be addressed separately.)... 

Kragl 13) pioneered the use of membranes to recycle dendritic catalysts. Initially, he used soluble polymeric catalysts in a CFMR for the enantioselective addition of Et2Zn to benzaldehyde. The ligand a,a-diphenyl-(L)-prolinol was coupled to a copolymer prepared from 2-hydroxyethyl methyl acrylate and octadecyl methyl acrylate (molecular weight 96,000 Da). The polymer was retained with a retention factor > 0.998 when a polyaramide ultrafiltration membrane (Hoechst Nadir UF PA20) was used. The enantioselectivity obtained with the polymer-supported catalyst was lower than that obtained with the monomeric ligand (80% ee vs 97% ee), but the activity of the catalyst was similar to that of the monomeric catalyst. This result is in contrast to observations with catalysts in which the ligand was coupled to an insoluble support, which led to a 20% reduction of the catalytic activity. 

Reetz et al. 16) were the first to recover and recycle a dendritic catalyst through a precipitation procedure. The dimethylpalladium complex of the phosphine-functionalized DAB-dendr-[N(CH2PPh2)2]i6 dendrimer (la) is an active catalyst for the Heck reaction of bromobenzene and styrene to give trara-stilbene (89% trans-stilbene and 11% 1,1-diphenylethylene, at a conversion of 85—90%, Scheme 8). 

Application of the same dendrimer 31 in the Stille coupling of iodobenzene with tributylvinyltin in DMF (Scheme 10 5mol% catalyst) showed an activity equal to that of the tetraphosphole macrocycle complex (Fu3P)4Pd(OAc)2 (31b 100% conversion after 15 min). In contrast to the monomer, the dendritic catalyst could be recycled, but when recycled it showed a decrease in activity (95% conversion after 15 min). A better performance was achieved with the in situ prepared catalyst 32 by mixing the third-generation diphosphine dendrimer with Pd(OAc)2 (P/Pd ratio = 4/1). An activity similar to that of the monomeric complex (Fu3P)4Pd(OAc)2 was observed (100% conversion after 15 min), even after three consecutive runs. 

The phosphine-containing ruthenium dihydride dendrimer 33 was found to be an active catalyst for the diastereoselective Michael addition of ethyl cyanoacetate to diethyl ethylidenemalonate in THF (Scheme 12). The dendritic catalyst showed an activity and selectivity similar to those of the reference compound RuH2(PPh3)4 (100% conversion after 24 h and a diastereoselectivity of 7/3) (40). The dendritic catalyst was recycled twice by precipitation with diethyl ether without loss of activity or selectivity. Complex 34 showed similar activity and recoverability over three runs like that of complex 33. 

The monomeric catalyst fraction showed similar R[ values as the metathesis products, which complicated the chromatographic separation and recycling procedure. Immobilization of the ruthenium catalyst on a dendrimer was anticipated to facilitate the chromatographic separation. Indeed, the presence of multiple (polar) organometallic sites on the dendrimer periphery resulted in stronger adsorption interactions between the dendritic catalyst and the silica and thus a better separation from the product. Two types of dendritic catalysts were prepared in which... 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

Already at an early stage in the research with dendritic catalysis, these novel systems were proposed to form a promising class of recyclable catalysts. Furthermore, new, interesting properties were proposed to arise by catalyst attachment to these large, structurally well-defined polymers. In the previous section we summarized the results obtained so far in the recycling of dendritic catalysts, and here we describe some of the dendritic effects observed in catalysis. Both negative and positive effects are discussed, in an attempt to provide a balanced view of the current state of affairs. 

The common problems involved in recycle of soluble catalysts pertain to dendritic catalysts one has to contend with decomposition of the dendrimer or the catalytic groups attached to it, dendrimer leaching, metal leaching, and catalyst... 

Direct catalytic Michael addition of aldehydes to nitrostyrenes proceeds in good yield, syn diastereoselectivity, and enantioselectivity (up to 82/90/99%, respectively) using a recyclable dendritic catalyst bearing chiral pyrrolidine moieties.200 High-yielding enantio- and diastereo-selective direct Michael addition of ketones to nitroalkenes to give aldol products employ modular acyclic primary amino acid derivatives as catalysts.201... 

While a number of dendritic catalysts have been described, catalyst recyclization in most cases is an unsolved problem. Diaminopropyl-type dendrimers bearing Pd-phosphine complexes have been retained by ultra- or nanofiltration membranes, and the constructs have been used as catalysts for allylic substitution in a continuously operating chemzyme membrane reactor (CMR) (Brinkmann, 1999). Retention rates were found to be higher than 99.9%, resulting in a sixfold increase in the total turnover number (TTN) for the Pd catalyst. 

Liming successfully attached a concave arrangement of pyridine units to Fre-chet-type dendrimers in homogeneous phase. A remarkable selectivity was thus achieved in base-catalysed addition of ketenes to alcohols and polyols (e.g. monosaccharides). The functionalised dendrimer catalysts exhibit a greater molar mass than conventional non-dendritic catalysts, thus permitting subsequent recycling of the catalyst by nanofiltration. These dendrimers are thus suitable as reagents for selective acylation of polyols [4]. 

Besides the hitherto described dendritic effects, the main aspect of interest regarding the use of dendritic supports in catalysis is represented by the possibility of recovering the catalyst. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycling of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as cross-linked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. For dendritic catalysts separation with these traditional techniques, the function of the dendrimer is not always clear. In contrast, the large... 

Using the first-generation dendritic ALB as catalyst, the Michael adduct was obtained with 91% ee and in 63% yield after 48 h. Under similar conditions, the G2 dendritic ALB gave the product with 91% ee in 59% yield. The dendritic catalysts could be recycled and reused twice, giving comparable results. It is notable that a catalyst derived from randomly introduced BINOLs on polystyrene resin only gave an essentially racemic product. 

In a related approach, Fan et al. synthesized a series of dendritic BINAP-Ru/chiral diamine ((R,R)-l,2-diphenylethylenediamine DPEN) catalysts for the asymmetric hydrogenation of various simple aryl ketones (Fig. 15) [42]. The resulting systems displayed high catalytic activity and enantioselectivity and allowed facile catalyst recycling. In the case of 1-acetonaphthone and... 

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