Josiphos units

Figure 4.25 Chiral dendrimer bearing Josiphos units located at the periphery.

Metallocenes have frequently been used as terminal moieties in dendrimer chemistry - as already demonstrated in previous sections. They are of interest primarily because of their potential application in catalysis [123]. An unusual metallodendrimer with peripheral ferrocene entities and optically active ferro-cenyldiphosphine ligands (josiphos ligands) was prepared by Togni et al. (Fig. 4.58) [124]. Adamantanetetracarboxlic acid was one of the core units employed. 

The introduction of chirafity into NHCs will therefore follow different strategies than those that have proved to be successful in phosphine-based asymmetric catalysis. For example, N-heterocydic carbene units will not create an edge-to-face arrangement of their aryl substituents, a structural feature common to many chiral diphosphines, such as the derivatives of Diop, Binap, Josiphos, Chiraphos and others. Results obtained in asymmetric catalysis, using chiral phosphine ligands, are therefore not directly transferable to the respective NHC-analogues. 

A second strategy is to place mnltiple chiral Josiphos-type (329) units along the exterior of dendrimers, and snccessful apphcations for rhodium-catalyzed asynunetric hydrogenation ntUize cores composed of benzene-l,3,5-tricarboxyhc acid esters (381), adamantane-l,3,5,7-tetracarboxylic acid esters (382), and cyclophosphazenes see Phosphazenes) (383)." Enantioselectivities are excellent and these dendrimeric materials can be recovered by filtration throngh a nanofiltration membrane. 

The role played by the growing chain in the microstructure of the copolymer was studied by investigating the chiroptical properties of terpolymers containing ethene and propene, which were prepared with the above-mentioned [ (/ )(5 p)-Josiphos palladium(II)bisaquo]-triflate catalyst precursor. With this catalytic system, the reactivity ratio between ethene and propene is close to 15 and the comonomer distribution seems to be random. Therefore, at low propene concentration, only isolated propene units are present in the chain for these units, enantioface discrimination is probably controlled only by the enantiomorphic site. The higher intensity of the aforementioned circular dichroism band (and the higher rotatory power) observed for the copolymer rather than for the terpolymer should be associated with a more efficient enantioface discrimination due to the growing chain. This conclusion relies on the reasonable assumption that the contribution to the optical activity normalized to... 

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