Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 93

Asymmetric transition‐metal catalysis has traditionally relied on the use of chiral ancillary ligands to modulate the metal’s primary coordination sphere to directly influence enantioselectivity. In recent years, a distinct strategy has emerged wherein ion‐pairing interactions at a metal’s secondary coordination sphere can influence enantioselectivity. Given that cationic metal complexes are more common than their anionic counterparts in catalysis, implementation of this strategy using chiral‐anions has seen more progress and will be discussed in detail in Section 3.6. Conversely, ion‐pairing between a chiral cation and an anionic metal complex has also been reported. In 2016, Tan reported the use of a dinuclear peroxomolybdate anionic oxidant ion‐paired to a chiral bisguanidinium dication catalyst for the enantioselective oxidation of various sulfides to the corresponding sulfoxides (Scheme 4.25) [81]. This proposed catalytically relevant ion‐pair was isolable and characterized by single‐crystal X‐ray diffraction. The same year, a related system was reported by the same group using a tungstate catalyst and the identical chiral bisguanidinium catalyst [82]. Prior to this work, chiral cationic catalysts had been reported to ion‐pair with anionic metal complexes such as permanganate anions when used stoichiometrically [83, 84].

Scheme 4.23. Enantioselective α‐amination of oxindoles and lactones using azodicarboxylates.

Scheme 4.24. Enantioselective α‐amination of oxindoles using hydroxylamines.

Source: Based on [80].

Scheme 4.25. Enantioselective oxidation of sulfides using a peroxomolybdate/chiral cation ion pair catalyst.

Source: Based on [81].

Scheme 4.26. Enantioselective transformations via transition metal/chiral cation ion pairing catalysis.

The asymmetric addition of alkynyl nucleophiles to carbonyl‐containing compounds represents a powerful approach to form chiral propargyl alcohols. In 2019, Maruoka demonstrated that a chiral ammonium catalyst could ion‐pair with a catalytically generated Ag‐alkynylide for the enantioselective addition to isatin derivatives (Scheme 4.26) [85]. While chiral ion‐pairing and phase‐transfer catalyses have typically involved ion‐pairing with an enolate nucleophile, this represents a unique example where an alternative carbon‐based nucleophile can be productively used via the use of a transition‐metal co‐catalyst. In 2020, the Phipps group disclosed an elegant system for the desymmetrization of geminal diaryl derivatives via enantioselective Ir‐catalyzed C–H borylation [86]. The key ion‐pairing between a strategically placed sulfonate group on the bipyridine ligand backbone and a chiral cinchoninium cation enables the enantioselective meta‐selective borylation of prochiral diaryl substrates. This work is particularly significant as it represents a rare example of remote asymmetric induction, where the site of the newly formed C–B bond is far from the newly formed chiral center. Looking forward, the strategy of incorporating chiral ion‐pairing interactions in ligand design for metal‐catalyzed transformations is positioned to have significant impact for the development of new reactions with unique selectivity profiles.