Chiral dendritic imines

Scheme 22. Chiral dendritic imines with cyclophane periphery.

These dendritic boxes (Figure 13.7) were synthesized by the conjugation of a chiral shell of protected amino acids onto a flexible polypropylene imine) dendrimer with 64 amino end groups. In solution, the shell was highly hydrogen-bonded and dense-packed, displaying a solid-phase behavior, which was indicated by the low NMR relaxation time of the surface groups [11]. 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

A stochiometric approach was applied by Van Koten and co-workers [29], who used chiral carbosilane dendrimers as soluble supports in the in situ ester enolate-imine condensation in the synthesis of /Mactams (e.g. 19, Scheme 20). The formation of the /Mactam products proceeded with high trans selectivity, and with the same level of stereoinduction as was earlier established in reactions without the dendritic supports, (i.e. the use of the enantiopure dendritic support did not affect the enantioselectivity of the C-C bond formation). After the reaction, the dendrimer species could be separated from the product by precipitation or GPC techniques and reused again. 

Synthesis, reactivity, properties and use as building blocks for various dendritic architectures of phosphorus-containing dendrons and the use of chiral phosphoric acids as powerful organocatalysts for asymmetric addition reactions to imines have... 

---