Artificial Biomacromolecules for Asymmetric Catalysis

The strategy considered in the following section is characterized by the incorporation of biomacromolecules into the supramolecular catalyst. Mechanistic effects include metal-mediated catalysis and chiral induction. 

FIGURE 27 Self-assembly of diphosphine catalyst for asymmetric rhodium-complex-catalyzed hydrogenation the catalyst contains titanium as the assembly metal (96). (For a color version of this figure, the reader is referred to the Web version of this chapter.) 

The approach of Wilson and Whitesides was not further developed for 20 years, until, in 1999, Chan and coworkers (99) extended the work by linking a chiral diphosphine to biotin. The resulting complex was tested for the hydrogenation of itaconic acid. Using this strategy, the authors were able to prepare methylsuccinic acids with moderate enantioseiectivity. 

Surprisingly, with n = 2 or n = 3, the sense of stereoinduction is reversed. These results reflect the subtleties of the interactions among the DNA, the Cu(II) complex of the 9-aminoacridine derivative, and the diene and dienophile. The highest enantioselectivity was observed for the ligand having an ethylene linker (n = 2), R = 3,5-dimethoxybenzyl, and a methoxy substituent on the dienophile— with — 57% ee for the endo-isomer and —90% ee for the exo one (104,105). 

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