Dendritic wedges, catalysts

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

Fig. 1. Different dendritic architectures catalyst located at the periphery (a), core (b), focal point of a wedge (c) and periphery of a wedge (d).

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). 

BINAP core-functionalized dendrimers were synthesized by Fan et al. (36), via condensation of Frechet s polybenzyl ether dendritic wedges to 5,5 -diamino-BINAP (26—28). The various generations of BINAP core-functionalized dendrimers were tested in the ruthenium-catalyzed asymmetric hydrogenation of 2-[p-(2-methyl-propyl)phenyl]acrylic acid in the presence of 80 bar H2 pressure and in a 1 1 (v/v) methanol/toluene mixture. As later generations of the in situ prepared cymeneruthe-nium chloride dendritic catalysts were used, higher activities were observed (TOF values were 6.5, 8.3, and 214 h respectively). Relative to those of the BINAP... 

Complexes 88—91 catalyzed the polymerization of ethene upon activation with MAO. Every dendritic catalyst displayed a lower activity than its reference complex Cp2ZrCl2 or Cp2TiCl2. It was found that the replacement of a C5HJ ligand by a dendritic cyclopentadienide (88 or 89) caused a moderate decrease in activity (4,320 2,064 and 1,720 kg/mol/h for CpTiCls, 88, and 89, respectively), which could be attributed to steric hindrance. The bis-dendritic Cp system 90 gave a lower activity (576kgmor h ). Interaction between a pair of dendritic wedges may restrict the conformations in which they are kept far away from the metal center. 

Pyrphos-Based Catalysts Bearing Dendritic Wedges. 82... 

Dendrimer fixation may be achieved by attachment of catalysts at the periphery of dendrimers (Fig. la) in the way first established by van Koten, van Leeuwen and co-workers in their pioneering work on the Karasch reaction [3,4]. The second possibility is the attachment of one or more dendritic wedges to the catalysts, which are then located at the core of the result-... 

Brunner s concept of attaching dendritic wedges to a catalytically active metal complex represented the first example of asymmetric catalysis with metal complex fragments located at the core of a dendritic structure [5,6]. Important early examples of catalysts in core positions were Seebach s TAD-DOL systems (TADDOL = 2,2-dimethyl-a,a,a/,a/-tetraphenyl-l,3-dioxolane-4,5-dimethanol) [38,39]. In general, the catalytic performance of such systems was either unchanged with respect to the simple mononuclear reference system or significantly lower. In no case has the potential analogy of this core fixation and the existence of efficient reactive pockets in enzymes been vindicated. This may be due to the absence of defined secondary structures in the dendrimers that have been employed to date. 

These polymeric Ru(BINAP) catalysts exhibited high catalytic activity and enantioselectivity (up to 92%) in the hydrogenation of simple aryl ketones, which is very similar to the results obtained with the corresponding parent Ru(BINAP) as well as the Ru(BINAP)-cored dendrimers referred to above. Unsurprisingly, they found that the pendant dendritic wedges have a major impact on the solubility and the catalytic properties of the polymeric catalysts, which could be easily recovered from the reaction mixture by simple precipitation. 

BINOL ligands 41a-c bearing dendritic wedges at the 6,6 positions were used in titanium-catalysed reaction of tributylallyl stannane and benzaldehyde (Scheme 7.33). Whereas the yield was low, the enantioselectivity was similar to the reaction with BINOL (87% enantiomeric excess), whatever the size of the dendritic moieties. However, no attempt to recycle the catalyst or the ligand was described by the authors. 

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