Asymmetric counteranion-directed catalysis amines

The transfer hydrogenation of ketones was subsequently attempted, although for these more challenging substrates chiral primary amines such as L-valine terf-butyl ester were required in order to obtain good yields (68-99%) and enantioselectivities (70-98% ee) [168]. The reaction most likely involves an iminium-phosphate hydrogen bond. Another important application of asymmetric counteranion-directed catalysis developed by the List group is the chiral phosphate anion-directed epoxidation of a, 3-unsamrated carbonyls [169]. 

In 2008, List s group developed an epoxidation using asymmetric counteranion-directed catalysis (ACDC) [177]. In this work, the epoxidation of 1,2-disubstituted enals (33) and (3,(3-disubstituted, a, 3-unsaturated aldehydes (115) was explored. Instead of using a chiral amine (e.g. Jprgensen-Hayashi s catalyst), an achiral amine and a chiral counteranion (a phosphoric acid derived from BINOL), were employed. 

Besides the utilization of chiral secondary amines to achieve a LUMO-lowering activation as well as face discrimination, the use of achiral secondary amines in combination with a chiral counterion also proved to be highly promising for such transformations. This strategy resembles the use of achiral metal catalysts in combination with a chiral ligand to achieve a stereoselective transformation (206-208). It is due to Benjamin List that the elegant concept of asymmetric counteranion-directed catalysis (ACDC) has found widespread applications in organocatalysis at the present time (209-212). 

While asymmetric counteranion-directed catalysis (ACDC) has been estab-hshed as a powerful strategy in iminium catalysis, enamine-based asymmetric counteranion-directed catalysis has not yet been developed. Recently, Lu et al. [32] demonstrated that the combination of a cinchona alkaloid-derived primary amine and chiral camphorsuhnnic acid (CSA) results in an effective ion-pair catalyst for the directed asymmetric amination of a-branched aldehydes through enamine activation (Scheme 43.21). 

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