Dendritic diphosphine

Maraval et al 39) synthesized core- and periphery-functionalized ruthenium and palladium dendritic diphosphines (Fig. 12) that were applied in three reactions (Stille coupling, Knoevenagel condensation, and diastereoselective Michael addition). The catalyst was recovered by using the precipitation strategy. 

Fig. 12. Periphery- and core-functionalized ruthenium and palladium dendritic diphosphines.

The increase of the catalyst turnover numbers is indeed one other major area where further improvements could be expected. Such improvements have recently been achieved for the standard Heck reaction by the use of high pressure conditions [86], the use of preformed palladacycles as catalysts [87], or by using a macrocyclic tetraphole as hgand [88].Dendritic diphosphine-palladium complexes as catalysts for Heck reactions have also been reported to possess superior stabihty compared to the monomeric parent compounds [89]. Transferring such iimovations to the AHR remains an important goal. 

Brunner et al. attached chiral branches to non-chiral catalytically active sites. With the aim to influence the enantioselectivity of transition metal catalyzed reactions they synthesized several dendritically enlarged diphosphines such as 81 [101] (Fig. 29). In situ prepared catalysts from [Rh(cod)Cl]2and81 have been tested in the hydrogenation of (a)-N-acetamidocinnamic acid. After 20 hours at 20 bar H2-pressure (Rh/substrate ratio 1 50) the desired product was obtained with an enantiomer ratio of 51 49. 

Fig. 29. Dendritically enlarged diphosphine enantioselective catalytic hydrogenations ai... 

Three generations of dendritic phosphines have been prepared from 3,5-diaminobenzoylglycine and 9-fluorenylmethoxycarbonyl-L-phenylalanine. The dendrimers were then attached to MBHA resin, treated with CH20 and Ph2PH, and converted to their Rh complexes. The polymer-supported complexes are excellent catalysts for the hydroformylation of alkenes, which could be recycled.283 The bidentate diphosphine A,A-bis-(P-(phosphabicyclo[3.3.1] nonan) methyl)aniline was prepared by phosphanomethylation of aniline. It forms a Rh-complex which is a highly regioselective catalyst in the hydroformylation of citronellene.284... 

The last example of a dendritic effect discussed in this chapter is the use of core-functionalized dendritic mono- and diphosphine rhodium complexes by Van Leeuwen el al. [45] Carbosilane dendrimers were functionalized in the core with Xantphos, bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphine (Figures 4.22, 4.32 and 4.33). 

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). 

Better results were obtained by using in situ prepared palladium complexes of a G4 dendrimer (calculated molecular weight 20 564 Da for 100% palladium loading of the 32 diphosphines). After 100 residence times, the conversion had decreased from 100% to approximately 75% (Fig. 3). A small amount of palladium was leached from the catalyst during this experiment (0.14% per residence time), which only partly explains the decrease in conversion. The formation of inactive PdCl2 was proposed to account for the additional drop in activity. A sound conclusion about the effect of this dendritic catalyst requires more experiments. 

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). 

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. 

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... 

Ferrocenyl diphosphine core-functionalized carbosilane dendrimers have been prepared as ligands for homogeneous catalytic reactions applied in a CFMR by Van Leeuwen et al. [20,21,67,68]. The syntheses of these dppf-like ligands (Go-G2)-17 were performed using carbosilane dendritic wedges with an aryl bromide as focal point. These wedges were coupled to the core via quenching of the lithiated species with ferrocenyl phosphonites (Scheme 11). 

The results depicted in Table 4 demonstrate the dendritic effect and strong influence of the nature of the diphosphine substituents on the reactivity and recoverability of the catalysts. 

The clean introduction of clusters onto the termini of polyphosphine dendrimers is a real challenge because of the current interest of dendritic clusters in catalysis and the mixtures usually obtained in thermal reactions of [Ru3(CO)i2] with phosphines.37 The diphosphine CH3(CH2)2N(CH2PPh2)2 (abbreviated P-P) was used as a simple, model ligand. The reaction between P-P and [Ru3(CO)i2] (molar ratio 1/1.05) in the presence of 0.1 equiv. [Fe p 6-C6Me6)] in THF at 20°C led to the complete disappearance of [Ru3(CO)i2] in a few minutes and the appearance of a mixture of chelate [P-P. Ru3(CO)i0], monodentate [P-P. Ru3(CO)n], and bis-cluster [P-P. (Ru3(CO)n 2]. These reactions were reported by Bruce et al. with simple diphosphines.38 On the other hand, the reaction of P-P with [Ru3(CO)i2] in excess (1/4) and only 0.01 equiv. [FeICp(r 6-C6Me6)] in THF at 20°C led, in 20 minutes, to the formation of the air-stable, light-sensitive bis-cluster [P-P. Ru3(CO)n 2] as the only reaction product. Given the simplicity of this characterization of the reaction product by 31P NMR and the excellent selectivity of this model reaction when excess [Ru3(CO)i2] was used, the same reaction between Reetz s dendritic phosphines,39 derived from DSM s dendritic amines,40 and... 

Van Leeuwen et al. have elaborated numerous diphosphine-core-functionalized dendrimers, among which are the carbosilane dendritic analogs of [l,l -bis(diphenylphosphino)ferrocene] (Figure 36, Experiments showed that... 

The dendrimers were more efficient catalysts than the monomeric model complex. This could possibly be due to labilization of metal-phosphine bonds that is facilitated in dendrimers as eompared to the monomer for entropic reasons. Indeed, DFT ealeulation showed that the catalytic process must involve decoordination of a phosphorus atom, since the interaction of the olefin with the diphosphine complex is nonbonding." The dendritic Ru-benzylidene dendrimers were air-sensitive contrary to the monomer model complex, consistent with more rapid dissociation of the alkyl phosphine in the dendrimers than in the monomer. 

---