Enantioselectivity and conclusions

In the case of the diphosphites, compounds containing seven-, eight-, and nine-membered rings have been studied. The bis-equatorial coordination mode that involves an eight-membered ring provides the highest enantioselectivities. Chiral cooperativity between the central backbone and the aryl substituents as well as the influence of the terminal groups has been observed for several systems. 

In RhH(BINAPHOS)(CO)2 the ligand coordinates in an equatorial-apical fashion. Both apical-equatorial isomers of the phosphine-phosphite bidentates have been identified. For BINAPHOS and related systems, the unique dissynunetric enviromuent created by phosphine-phosphite seems to be determinant for enantioselectivities to be high. As for diphosphites, phosphine-phosphite systems require a well-tuned cooperation between the chiral centers in the ligand. 

High enantioselectivities have not still been found for rhodium chiral diphosphine systems in asymmetric hydroformylation. 

As pointed out in Chapters 3 and 4 the intermediate alkene adducts of the diphosphite catalysts probably contain bis-equatorial bidentate ligands and the intermediates of BINAPHOS contain an equatorial-apical phosphine-phosphite ligand. Thus the structures involved in the migratory insertion are different for the two systems. 

The models used for rationalizing the enantioselectivities have been discussed in section 5.3.3. The simple four-quadrant model was cited [46], which gave good results for many substrates, but often failed for styrene. Gleich and Herrmann [15,161 have applied computational methods to hydroformylation using a combination of MM and QM. The predictive power of this approach will certainly increase in the near future. 

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