Enantioselective addition of ZnEt2 to benzaldehyde

On the other hand, sulfur-containing pyridine ligands have been prepared by Chelucci et al These ligands gave only moderate enantioselectivities (<51% ee) in the enantioselective addition of ZnEt2 to benzaldehyde, as shown in Scheme 3.17. 

Enantioselectivities ranging from 18 to 94% ee were obtained by Martens et al. by using C2-symmetric bis-p-amino alcohols derived from D-cysteine for the enantioselective addition of ZnEt2 to benzaldehyde (Scheme 3.27). ... 

The enantioselective addition of ZnEt2 to benzaldehyde was also performed in the presence of chiral 2,2-disubstituted thiaprolinol derivatives as ligands by Liu et al., providing the product with an enantioselectivity of up to 81% ee (Scheme 3.29). ... 

In 2008, Juaristi et al. developed the synthesis of a series of novel chiral thioureas that were further examined as possible ligands for the enantioselective addition of ZnEt2 to benzaldehyde." The expected carbinol was isolated in... 

In 1997, Chelucci et al. developed new chiral 2-(l-p-tolylsulfinyl)alk-ylpyridines and assessed these ligands in the enantioselective addition of ZnEt2 to benzaldehyde." This produced in all cases catalysts in good yields but with low enantioselectivities (< 19% ee). An examination of the results collected in Scheme 3.33 indicates that the enantioselectivity and the configuration of the resulting carbinol were independent of both the substituent and the configuration at the carbon centre. Therefore, this suggests that the stereochemical... 

In 2008, Rykowski et al. reported the synthesis of optically active 2,2 -bipyridine alkyl sulfoxides by asymmetric oxidation of their corresponding readily accessible 2,2 -bipyridine alkyl sulfides. These sulfoxides were further evaluated as ligands for the enantioselective addition of ZnEt2 to benzaldehyde, providing only low enantioselectivities of up to 14% ee (Scheme 3.34). 

In 2003, these authors developed the Ti-catalysed enantioselective addition of ZnEt2 to benzaldehyde in the presence of C2-symmetric bis(camphorsulfo-namides) ligands derived from achiral 1,2- and 1,3-diamines and ( + )-camphorsulfonic acid. These authors showed that the stereochemical outcome of the reaction was highly influenced both by the structure of the... 

On the other hand, several S/O ligands have been successfully applied to the enantioselective addition of ZnEt2 to aldehydes. As an example, Aral et al. have developed isoborneol-derived p-hydroxy-sulfide ligands and employed them in the enantioselective addition of ZnEt2 to benzaldehyde, providing enantio-selectivities of up to 88% ee (Scheme 3.54). These authors showed that the enantioselectivity of the reaction did not depend on the substituent of the sulfur atom. 

In addition, Bonini et al. have shown that a planar chiral sulfur-containing ferrocenyl-oxazoline carbinol ligand, depicted in Scheme 3.59, could also be used to catalyse the addition of ZnEt2 to benzaldehyde with a moderate enantioselectivity (46% ee). ... 

Other excellent results have been reported by Kang et al. for the addition of ZnEt2 to aldehydes by using chiral cyclic amino thiol ligands depicted in Scheme 3.7. A total enantioselectivity was obtained when (lR,25)-l-phenyl-2-piperidinopropane-1-thiol was used as the ligand in the reactions of substituted benzaldehydes. However, -hexanal and traw -cinnamaldehyde could only be... 

Similar polymeric chiral ligand of N-sulfonylated amino alcohol (171) was developed by Gau et al. [74]. The polymeric chiral ligand (171) was prepared by two methods chemical modification method and polymerization method (Scheme 19.35). Chemical modification of chlorosulfonylated polystyrene with chiral amino alcohol, (1R,2S)-2-amino-1,3-dipheny 1-1-propanol, afforded (171). The copolymerization of the chiral N-sulfonylated amino alcohol monomer with styrene in the presence of divinylbenzene also yielded the polymer (171). These polymers were complexed with Ti(O Pr)4 to apply to ZnEt2 addition to benzaldehyde. Higher enantioselectivity was obtained by using (171) prepared by polymerization method. 

---