Chiral ligands salans

Subsequently, Kee and co-workers recognised the importance of steric and coordination flexibility in the chiral ligand scaffold and have switched their attention more recently to chiral SALAN systems [37], the reduced version of... 

A similar observation is noted for complexes containing partially (salalen) and completely (salan) saturated nitrogen centers (Fig. 8) [28]. Metal complexes of these ligands generally exhibit a c -p configuration and the metal center of these derivatives is chiral (A or A) (Fig. 9) [29, 30], Figure 10 depicts the structure of a (salalen)CrCl(H20) complex as determined by X-ray analysis. 

Organogallium reagents, RsGa, have been added enantioselectively to aldehydes, using TiCl4 as a Lewis acid catalyst, with a chiral salan ligand.258... 

Group 4 metals have also been used widely in conjunction with salen-type ligands (Figure 25). In 2006 Gregson et reported several chiral and achiral titanium salen alkoxide complexes for the ROP of lactide. All catalysts reported were modestly active and heteroselective (P 0.51-0.57). Several achiral Ti and Zr salan catalysts were reported by Gendler et for melt polymerization of lactide. While no stereoselectivity has been reported for either system, the Zr complexes were more active towards lactide ROP than the Ti analogs. 

The utilization of organogallium as an alkylation reagent for asymmetric alkylation of aldehydes has been realized by using titanium complex as a chiral Lewis acid catalyst. The titanium catalyst was obtained by the reaction of TiCU with a salan [N,N-bis(o-hydroxybenzyl)-l,2-diaminoethane] type ligand. Moderate to good chemical yields were obtained with up to 84% ee [53] (Scheme 14.8). 

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