Organocatalytic Enantioselective Synthesis of Axially Chiral Molecules: Development of Strategies and Skeletons

The growing importance of axially chiral architectures in different scientific domains has unveiled shortcomings in terms of efficient synthetic access and skeletal variety. This account describes our strategies in answering these challenges within the organocatalytic context where the emergence of bifunctional catalysts such as chiral phosphoric acids (CPAs) has proven invaluable in controlling the sense of axial chirality. The wide occurrence of bi(hetero)aryl skeletons in privileged structures constitutes a strong motivation to devise more effective arylation methods. Our design revolves around modulating the intrinsic nucleophilicity of aromatic amines and alcohols. The first approach involves the design of an electron-withdrawing activating group which could associate with the catalyst for reactivity enhancement and selectivity control. The resonance of arenes offers the unique mechanistic possibility to select between activating sites. C2-Azo- and nitroso-substituted naphthalenes undergo atroposelective ortho C- or N-arylation with (hetero)aromatic nucleophiles. For monocyclic benzenes, programmable charge localization leads to regioselective activation by catalytic control alone or aided by substrate design. For instance, selective addition to nitroso nitrogen enables successive annulation initiated by the amine to yield axially chiral N-arylbenzimidazoles. In a biomimetic manner, a finely tuned catalyst could direct a para-selective nucleophilic approach in the atroposelective arylation of azobenzenes. The second strategy employs electrophilic arene precursors for arylation which occurs via rearomatization with central-to-axial chirality transfer. This enabled the arylation of (imino)quinones with indoles to access phenylindole atropisomers. By adapting this chemistry with an additional oxidation event to liberate the carbonyl functionalities, aryl-o-naphthoquinone and aryl-p-quinone atropisomers were attained. Along with the development of new arylation strategies, deriving new axially chiral structures has been another consistent theme of our research program. The atroposelective functionalization of alkynes provides broad entry to atropisomeric alkenes. The monofunctionalization of alkynes through the interception of an electrophilic vinylidene-quinonemethide (VQM) intermediate with 2-naphthols yielded the new EBINOL scaffolds. By designing an internal directing group, the atroposelective dihalogenation of alkynes was realized using abundant alkali halides despite their weak nucleophilicities and poor solubilities. The atroposelective N-alkylation of alkenes was pursued to prepare multifunctionalized alkene atropisomers that could be converted into 2-arylpyrroles with chirality transfer. The synthesis of B-aryl-1,2-azaborines containing a C-B chiral axis was accomplished where the CPA catalyst effects the desymmetrization and defines the configuration of the distal C-B bond. Inspired by the axially chiral scaffold of allenes, we leveraged the developed arene activation strategy to achieve para-addition and dearomatization of judiciously designed azobenzenes, which led to structurally novel cyclohexadienylidene-based hydrazones. To complement these structures, axially chiral cyclohexadienyl oxime ethers were also attained through CPA-catalyzed condensation between hydroxylamines and spiro[4.5]trienones.

Automated summary

This paper describes the strategies to address the challenges in efficient synthetic access and skeletal variety within the organocatalytic context. The approaches include modulating nucleophilicity, designing electron-withdrawing activating groups, and using electrophilic arene precursors for arylation. These strategies have led to the synthesis of structurally diverse axially chiral compounds.