Catalytic Asymmetric Synthesis. Группа авторов
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that these were engaging in noncovalent interactions with the substrate, leading to high enantioselectivities. A follow‐up mechanistic study focusing on a data‐intensive approach revealed that a π‐stacking interaction between the triazole and the substrate was the primary factor in controlling enantioselectivity [156]. The oxopiperidinium/chiral‐anion system was later utilized in a deracemization reaction of indolines via oxidation to the 3‐H indole scaffold proceeding, and subsequent reduction to the indoline [157]. In 2016, the Toste group reported the asymmetric arylation of benzopyrylium with phenols under phase‐transfer conditions (Scheme 4.47) [158].
Scheme 4.44. Overview of chiral anion phase‐transfer catalyzed bromination and iodination.
Source: Based on [147].
Scheme 4.45. Chiral anion phase‐transfer catalyzed diazenation.
Source: Based on [153].
Scheme 4.46. Asymmetric transformations mediated by oxopiperidinium/chiral anion salts.
Source: Based on [155].
Scheme 4.47. Asymmetric arylation of benzopyrylium with phenols under phase‐transfer conditions.
Source: Based on [158].
4.3.6. Transition‐Metal/Chiral‐Anion Dual Catalysis
With the prevalence of cationic transition‐metal complexes in catalysis, there was a strong interest in rendering these reactions asymmetric by utilizing ion‐pairing with chiral counterions. In 2007, the Toste group demonstrated this principle with an enantioselective Au(I) catalyzed hydro‐cyclization of allenes (Scheme 4.48) [159]. The chiral‐anion strategy was crucial for success in this system, as the linearity of Au(I) complexes rendered attempts to use chiral ligands for enantioselectivity unsuccessful. By combining an achiral gold complex with a chiral phosphate counter anion, the enantioselective cyclization of alcohols, sulfonamides, and carboxylic acids could be achieved in high yields. Solvent choice was found to be a key factor for achieving selectivity, as more polar solvents resulted in weaker ion‐pairing, leading to diminished selectivities. This strategy was later adapted to the desymmetrization of 1,3‐diols via intramolecular cyclization of allenes [160]. In the succeeding decade, the robustness of this strategy was demonstrated by the compatibility of chiral‐anions with a wide variety of transition‐metal‐catalyzed reactions.
The combination of Pd catalysis with chiral‐anions has led to many asymmetric methodologies. Of particular initial interest was the generation of cationic Pd‐allyl complexes with a chiral counteranion for asymmetric Tsuji–Trost‐type allylations. This was first demonstrated by the List group in 2007, with an asymmetric α‐allylation of aldehydes (Scheme 4.49) [161]. An intramolecular version of this reaction was later reported by the Toste and Sigman labs, where pyrrolidines and benzomorpholines were accessed in high yields and enantioselectivities [162]. In addition to ion‐pairing with a cationic transition‐metal center, the Ooi lab demonstrated that enantioselectivity can be achieved by utilizing an achiral cationic ligand ion‐paired with a BINOL‐derived anion [163]. In addition to Pd(II) allyl complexes, the Toste group demonstrated an enantioselective 1,1‐arylborylation of alkenes, which proceeds through an enantiodetermining migratory insertion, followed by β‐hydride elimination and reinsertion [164]. By changing the coupling partner to aryl boronic acids, an enantioselective 1,1‐diarylation was achieved [165]. Additionally, exclusion of a coupling partner was found to result in an asymmetric Heck–Matsuda arylation, generating cyclic arylated stereocenters in high yield and enantioselectivity [166].
Scheme 4.48. First example of a transition‐metal/chiral anion catalyzed transformation.
Source: Based on [159].
Scheme 4.49. Asymmetric transformations catalyzed by a Pd/chiral phosphate ion pair.
Source: Based on [161].
Scheme 4.50. Asymmetric transformations catalyzed by other transition‐metals ion‐paired with chiral anions.
Source: Based on [167].
Expansion to other transition metals and reaction manifolds was successful. One such important contribution came from the List group in 2010, by utilizing an achiral Mn‐salen complex in conjunction with a chiral‐anion (Scheme 4.50) [167]. The chiral‐anion was proposed to stabilize one enantiomorph of the achiral Mn‐salen complex, leading to high selectivity for the epoxidation of alkenes. This was later extended to the enantioselective sulfoxidation of sulfides with an Fe‐salen complex [168]. In 2018, the Matsunaga group demonstrated an Rh‐catalyzed enantioselective C–H functionalization using a binaphthyl‐derived bis‐sulfate chiral‐anion [169].
4.3.7. Anion‐Binding Catalysis
Rather than utilizing an anionic chiral catalyst to ion‐pair cationic reagents, an alternate strategy was developed that focuses on a neutral chiral catalyst that can bind achiral‐anions, forming an in situ chiral‐anion.
4.3.7.1. Nonaromatic Cations
In 2004, the Jacobsen group demonstrated that an anion‐binding catalyst could be used to affect an enantioselective acyl‐Pictet‐Spengler reaction (Scheme 4.51) [170]. A variant that features activation of a hydroxylactam by trimethylsilyl chloride (TMSCl) was later demonstrated as well [171]. Mechanistically this reaction proceeds by formation of an acyl iminium cation, where the chloride counteranion is hydrogen bound to the chiral urea catalyst. This keeps the chiral information close to the cationic center, allowing selective nucleophilic attack by the indole.