Dendritic ligand

Recently Togni et al. [19] focussed on the preparation of asymmetric dendrimer catalysts derived from ferrocenyl diphosphine ligands anchored to dendritic backbones constructed from benzene-1,3,5-tricarboxylic acid trichloride and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (e.g. 11, Scheme 11). In situ catalyst preparation by treatment of the dendritic ligands with [Rh(COD)2]BF4 afforded the cationic Rh-dendrimer, which was then used as a homogeneous catalyst in the hydrogenation reaction of, for example, dimethyl itaconate in MeOH. In all cases the measured enantioselectivity (98.0-98.7%) was nearly the same as observed for the ferrocenyl diphosphine (Josiphos) model compound (see Scheme 11). 

Complexation of dendritic ligands 1 and 2 with lanthanide ions (Nd3+, Eu3+, Gd3+, Tb3+, Dy3+) [17f] leads to results qualitatively similar to those obtained upon Zn2+ complexation (see above) an increase in the monomer naphthalene emission band at 337 nm and a complete disappearance of the exciplex band at 480 nm. However, the complex stoichiometry is different. Emission data were best fitted considering the formation of 1 3 and 1 2 (metal/ligand) complexes (log f 1 2 = 14.1 and log [ivi = 20.0) in the case of 1 and a 1 3 (metal/ligand) complex (log / 1 3 = 20.3) for compound 2. Therefore, at low metal ion concentration, only the [M(2)3]3+ species is present, as also demonstrated by NMR titration. It is likely that in this complex... 

Cross J-P, Lauz M, Badger PD et al (2004) Polymetallic lanthanide complexes with PAMAM-naphthalimide dendritic ligands luminescent lanthanide complexes formed in solution. J Am Chem Soc 126 16278-16279... 

So far, few data are available which allow the comparison of differences in efficacy and selectivity of one catalytic system attached to different supports. As far as the TADDOLate complexes are concerned, no clear rules can be drawn. Polystyrene-based catalysts derived from (8) and (10) show similar enantioselectivities and reaction rates. Differences appear, however, when comparing them with a polystyrene-embedded dendritic ligand system, generated by co-polymerization from TADDOL-derivative (32) (Scheme 4.18) which is described in Section 4.3.2.1. Re-cydabihty seems to be easier for the dendritic catalyst and the enantioselectivity. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Fig. 4. Application of dendritic ligand 2 in the continuous allylic alkylation of allyl trifluoroacetate and sodium diethyl 2-methyhnalonate in a membrane reactor (Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da) 18a).

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

The same dendritic ligands were used for the addition of disopropylzinc and diethylzinc to aldehydes (phenyl, 2-naphthyl, / -tolyl) 46). The two ligands were equally selective for the (J )-alcohol product (77% < ee 86%, depending on the substrate). Dendrimer 53 was recovered after the reaction between diethylzinc and benzaldehyde and was reused in a consecutive run, giving the same enantioselec-tivity. 

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... 

Rhodium complexes of the phosphine-functionalized carbosilane dendrimers are active for the hydroformylation of alkenes. The influence of the flexibility of the dendritic backbone on the catalytic performance was characterized by comparing dendritic ligands 84a-84c (conditions toluene, 80°C, 20 bar CO/H2) 49). 

To improve the solubility of the dendritic catalysts, other series of dendritic ligands with 2-methoxyethyl groups instead of benzyl groups were synthesized (101-103). 

Transfer hydrogenation of ketones using metal complexes with a chiral water-soluble [97,98] and a dendritic ligand [99] was investigated for use in recycling catalysts. The reaction with immobilized catalysts has also been reported [100]. 

In a second variant, growth of metallodendrimers can proceed via complexa-tion of a metal cation with dendritic ligands. In this way, Balzani, Vogtle, De Cola et al. [34] obtained photoactive ruthenium complexes by spontaneous self-assembly of the components starting from various dendritically substituted bi-pyridines. Fig. 2.9 shows a representative example (see Sections 5.1.2.3 and... 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

The same dendritic ligands, but used in combination with rhodium, were utilized in hydroformylation reactions [46]. Preliminary experiments with this catalytic system in a nanofiltration membrane reactor, however, showed that this membrane set-up was not compatible with the standard hydroformylation conditions because of its temperature and solvent restrictions. 

Following up this work, the same group recently published the synthesis of hybrid dendritic ligands containing a combination of dendritic chiral DPEN and Frechet polyether dendrons (Scheme 7) [50]. The solubility of these hybrid dendrimers was found to be controlled by the polyether den-dron. Compared with the simple core-functionalized systems displayed in Fig. 21, the hybrid dendrimers showed similar catalytic activity but reduced recyclability. 

A series of ruthenium(II) phthalocyanines with one or two pyridyl dendritic olig-othiophene axial substituent(s) have also been reported (compounds 50 and 51) [50], The dendritic ligands absorb in the region from 380 to 550 nm, which complements the absorptions of the phthalocyanine core. This combination results in better light harvesting property and enhancement in efficiency of the corresponding solar cells. The solution-processed photovoltaic devices made with these compounds and fullerene acceptor give efficiencies of up to 1.6%. These represent the most efficient phthalocyanine-based bulk heterojunction solar cells reported so far. 

PHOTOCHEMISTRY AND PHOTOPHYSICS OF METAL COMPLEXES WITH DENDRITIC LIGANDS... 

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