Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 96

The first proof of principal example of utilizing ion‐pairing catalysis with a chiral‐anion was demonstrated by the List group in 2006 (Scheme 4.31) [108]. In this example, an ammonium/chiral phosphate salt would catalyze the enantioselective reduction of enals with Hantzsch ester. The key intermediate for imparting enantioselectivity is the iminium formed from condensation of the achiral ammonium and the enal, which remains ion‐paired with a (R)‐TRIP counteranion. This ion‐pair leads to preferential reduction of one face of the alkene by Hantzsch ester, leading to β‐chiral ketones in high yields and enantioselectivities. Following this, the List group utilized this key intermediate for the epoxidation of di‐ and tetrasubstituted enals by using t‐butyl hydroperoxide [109]. It was found that the identity of the achiral ammonium had a profound effect on the observed enantioselectivities (24–90% ee), where dibenzylic ammoniums with electron deficient arenes provided the best enantioselectivities. This demonstrates the importance of tuning both components of the ion‐pair for cooperatively achieving high selectivities, and is a key feature in many examples of chiral‐anion catalysis.

In addition to ammonium/chiral phosphoric acid salts, a variety of catalyst architectures can be utilized for generating iminium intermediates that are amenable to chiral‐anion catalysis. In 2013, the Masson group demonstrated that an iminium intermediate could be generated from protonation and subsequent dehydration of γ‐hydroxy‐γ‐lactams by a chiral phosphoric acid (Scheme 4.32) [110]. The resulting iminium intermediate underwent asymmetric C3‐arylation with indoles in high yields and enantioselectivities. Other catalysts not based on BINOL phosphoric acids have been demonstrated to be competent for chiral‐anion catalysis. Notable examples include a novel conjugate‐based stabilized carboxylic acid that catalyzes an enantioselective three‐component Povarov reaction[111], and an ammonium/chiral BOROX salt, which activates aldehydes toward an asymmetric three‐component Ugi reaction [112].

Scheme 4.31. First example of asymmetric counteranion‐directed catalysis.

Source: Based on [108].

Scheme 4.32. Asymmetric counteranion‐directed catalysis involving iminium ion intermediates.

Source: Based on [110].