Groups gem-diphenyl

Higher enantioselectivities were generally found when 3-mannose thiol 13 was used as hydrogen donor. The extra bulkiness provided by the gem- -diphenyl groups in alkene 14 compared to alkene 11 is indicated to be responsible for the high enantiomeric purity observed (Reaction 5.12) [31]. 

Allylic oxidation of a variety of cyclic alkenes with copper complexes of different pybox ligands (8) and with various peresters shows high enantioselectivity (80-96% ee). Use of phenylhydrazine as an additive and acetone as solvent accelerates the reaction. It has been suggested that the phenylhydrazone is responsible for the observed acceleration. Using EPR spectra, it has been shown that the Cu(II) species is reduced to Cu(I) by phenylhydrazine and phenylhydrazone. It has been found that the presence of a gem-diphenyl group at C(5) and a secondary or tertiary alkyl substituent at the chiral centre at C(4) of the oxazoline rings is crucial for high enantioselectivity. 

The hydrophobicity can also be enhanced by modification of the amino acid into an amide catalyst bearing a gem-diphenyl group, an approach previously developed by Singh et al. [28] Amide 8 [29], with 4-nitrobenzoic acid as additive, and amide 9 [30] were employed in brine (Figure 24.4). The reaction failed when the hydrophilic dihydroxyacetone was used. In contrast, the use of more hydrophobic ketones, such as TBS-protected dihydroxyacetone, was successful. However, the enantiose-lectivity of the reaction (with catalyst 9) decreases with increasing length of the... 

Several prolinamides have been examined (38) catalyses the reactions of acetone with both aliphatic and aromatic aldehydes with ees up to 97%.104 It is proposed that amide and alcohol groups simultaneously donate hydrogen bonds to the aldehyde, while the gem-diphenyl moiety s bulk orients the aldehyde s approach. 

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