Dendritic effect, positive

To investigate this dendritic effect, a dimeric model compound was synthesized which mimics the tethered relationship of two catalytic units within one branch of the PAMAM dendrimer. All dendritic catalysts were more active in the HKR than the parent complex. Furthermore, the dendritic catalysts also displayed significantly higher activity than the dimeric model compound. The authors proposed that this positive dendritic effect arises from restricted conformation imposed by the dendrimer structure, thereby creating a bigger effective molarity of [Co(salen)] units. Alternatively, the multimeric nature of the dendrimer, may lead to higher order in productive cooperative interactions between the catalytic units. 

Generally these globular dendritic architectures offer several advantages over other kinds of organic polymers, such as the full exposure of the catalytic centers to the environment. In contrast to linear or cross-Hnked polymeric supports, which can partially hide catalytic centers, the functional groups are located on the surface of the dendritic nanoparticle and diffusional Hmitations are less relevant Furthermore the close proximity of the catalytic centers on the surface of the dendritic polymer can enhance the catalytic activity by multiple complexation or even cooperativity. This behavior is described as positive dendritic effect. However, in some cases a negative dendritic effect was observed, which is caused by an undesired interaction or electron transfer between the neighboring catalytic centers on the surface of the dendrimer [70]. 

An instructive example for a positive dendritic effect was reported by Reetz et al. [71]. The authors described a poly(propylenimine) dendrimer, with diphenylphos-phine groups in the periphery (Fig. 7.20). A dendritic [PdMe2]-complex was tested as an efficient catalyst in the Heck reaction of bromobenzene and styrene to yield stilbene (85-90% conversion). The separation technique originally investigated for... 

In a subsequent paper (87), the same authors investigated the palladium-catalyzed allylic aminations with pyrphos-functionlized PPI and PAMAM dendrimers as multidentate ligands. Zero to four generation PPI-(pyrphosPdCl2)x and PAMAM-(pyrphosPdCl2)x neutral dendrimers (97) showed a strong positive dendritic effect on the selectivity of the allylic amination of 1,3-diphenyl-1-acetoxypropene with morpholine (at 45°C in DMSO). 

Interestingly, a family of 1,2,3-triazole-linked dendrimers containing from 9 (compound 4, Fig. 6.4a) up to 243 peripheral ferrocene units give strong recognition of both oxoanions and metal cations with positive dendritic effect.30... 

ATP2 sensing better than in solution and they exhibit a positive dendritic effect not evidenced in solution.32... 

This result, caused by the proximity effect between peripheral catalytic sites, can translate into higher or lower catalytic activity of the metallodendrimer in homogeneous catalysis, and is commonly termed the dendritic effect. In the above case, a negative dendritic effect is observed. An interesting example of a positive dendritic effect on catalyst activity was reported by Jacobsen et al. in the hydrolytic kinetic resolution of terminal epoxides by peripherally Co(salen)-substituted PAMAM dendrimers [39]. 

Sensing Oxo-Anions with Positive Dendritic Effects. 122... 

Recognition of HS04" proceeds best in CH2C12 with the parent Cp series with, again, a positive dendritic effect (i.e. an increase of AE° on going to higher generations, Fig. 1) [64,65]. 

Fig. 12 Variation of nh for the exocyclic amine proton of the 24-Fe dendrimer measured by NMR spectroscopy upon addition of n-Bu4NCl or n-Bu4NBr. Comparison with monometallic [FeCp (7j6-c3H5NHPh)][PF6] (1-Fe) and tripodal PhCFhNFIChCFhbO (CH2)3NH-[)76-PhFeCp ][PF6] 3 (3-Fe) showing the positive dendritic effect...

Gade et al.120 reported a strong positive dendritic effect in the asymmetric catalysis in the allylic amination of 1,3-diphenyl-l-acetoxypropene with morpholine when using pyrphos-palladium-functionalized PPI and PAMAM dendrimers. A remarkable and unprecedented increase in catalyst selectivity was observed as a function of the dendrimer s generation. This steady increase of ee values for these allylic animations was less pronounced for the PPI-derived Pd-catalysts than for the corresponding PAMAM-catalysts, for which an increase in selectivity from 9% ee for a mononuclear reference system to 69% ee for the Pd64-dendrimcr was realized. 

Recognition of HS04 proceeds best in CH2C12 with the parent Cp series with, again, a positive dendritic effect (i.e., an increase of AE° when the generation number increases Figure 11.6).28,34 In DMF, however, recognition and titration are only possible with the permethy-lated dendrimer series. It is subjected to another dramatic dendritic effect the shift of the initial CV wave upon titration (weak interaction) is only observed with Gj and a new wave... 

Dendritic catalysts offer the possibility of recovery due to their size, but dendritic effects are negative, that is, the kinetics drops when the dendritic generation increases due to the bulk problem (access to the metal center). On the other hand, recognition and sensing with dendritic exoreceptors benefits from positive dendritic effects, because the channels become narrower, and thus better suited for interaction with the substrate when the dendritic generation increases. 

Ropartz and coworkers examined the regioselectivity of the hydroformy-lation reaction catalysed by Rh-containing dendrimers of different generations with diverse spacer moieties [114]. They showed that the appearance of a positive dendritic effect is not only generation-dependent but also depended on an exact spacer length and composition between the support and the ligand. 

When the catalytic units are placed in a densely packed surface (of a higher generation dendrimer) they are easily accessible and can interfere with each other either increasing (vide supra positive dendritic effect) or decreasing (vide supra negative dendritic effect) catalytic activity. 

Caminade and Majoral reacted phosphorus-containing dendrimers with various transition metal complexes (Fig. 7) [ 113 ]. Via phosphane, diphosphane, or P=N-P=S ligands, they were able to incorporate several metals (Pd, Pt, Rh, Au, Fe, W, Zr) either in the core or in the periphery of their dendrimers. The dendritic palladium and ruthenium complexes 7 have been introduced in Stille couplings, Knoevenagel condensations and diastereoselective Michael additions. A positive dendritic effect and high stability of dendritic Pd complexes has been observed. Separation occurred by precipitation with ether and reuse was reported. 

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