Ligands for stereoselective reactions

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in 

Fluorine substitution is obviously valuable for improved enantioselection in the ammonium-catalyzed alkylation in the biphase system. However, it is not clear to estimate which factor, the electronic or steric effect, is more important in each reaction. The remarkable enhancement of both yield and enantioselectivity by the substitution of two phenyl groups on the 3- and 5-position of the phenyl group (R = Ph in 46b and R = 3,5-diphenyl in 46d in Table 5.5) suggests an importance of the steric effect and so the substitution of three fluorine atoms in 3,4,5-trifluoro compound (46e) may affect the enantioselection through the steric effect that arises from a Coulombic repulsion between the enolate anion of 47 and the lone pair electrons on fluorine atoms. The related non-spiro quaternary ammonium salt (53) is also usable for asymmetric alkylation of 47 [24]. 

It is obvious that fluorine or fluoroalkyl modification on the aryl moiety of ligands is useful for improving the stereoselectivity, but it is yet unclear which effect, steric or electronic, is preferentially operative in each catalytic reaction. 

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