Zr-Catalyzed Enantioselective C—N Bond-Forming Reactions

Related catalytic enantioselective processes [115] Two catalytic procedures for asymmetric addition of cyanides to meso epoxides have been reported [116]. One is the result of work carried out in these laboratories, shown in Eq. 6.24, promoted by Ti-peptide chiral complexes, while the other, developed by Jacobsen and Schaus, is a Yb-catalyzed enantioselective reaction that is effected in the presence of pybox ligands (Eq. 6.25) [117]. Although the Shibasaki method (Eq. 6.21) is not as enantioselective as these latter methods, it has the advantage that it accomplishes both the epoxidation and subsequent desymmetrization in a single vessel. 

The marked difference in reaction efficiency that arises when different sources of metallocene are employed in the (ebthi)Zr-catalyzed hydrogenation is illustrated in Table 6.5. The mechanism by which MAO converts the zirconium(IV) salts to the active zirconium hydride species remains unclear [122]. However, it has been proposed that a chlorine atom may form an r]2-bridge between aluminum and zirconium when the dichloride salt is used, thereby preventing formation of the active cationic metal center. 

As already mentioned, and as illustrated in Table 6.6, an important prerequisite for efficient cationic (ebthi)Zr-catalyzed hydrogenation is that the alkene should not be able to polymerize in a facile manner. Whereas with 1-decene as the substrate, only 28% of the hydrogenation product is obtained (Table 6.6, entry 1 the remainder of the product is the derived polymer), 2-methyl-1-pentene affords 97% of the hydrogenation adduct 

The data in Table 6.7 illustrate that when the non-racemic (ebthi)Zr system is used to catalyze the hydrogenation of prochiral alkenes, moderate levels of enantiofacial differentiation are observed (23—65% ee). Enantioselective deuteration of pentene occurs in low yield but shows noticeable enantioselection (23% ee). The same reaction with styrene proceeds in 61% yield and with moderate enantioselectivity (65% ee). Hydrogenation of 2-phenyl-l-pentene proceeds in excellent yield but with poor control of stereochemistry (95% yield, 36% ee). 

Side Approach Side Approach Side View Front View 

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