Changing the absolute configuration of atropisomeric bisnaphthols (BINOLs)

Bisnaphthol (BINOL) is a ubiquitous core skeleton in versatile chiral catalysts and ligands for transition metals, and this representative atropisomeric structure also serves as a building block in various optically active natural products. Due to the high rotational barrier (∼40 kcal mol^-1) in the neutral form of BINOL, it displays stable atropisomerism. Asymmetric catalysis using racemic BINOL substrates generally exhibits a kinetic resolution process, with reaction yields having to be below 50%. Dynamic kinetic resolution (DKR) combines kinetic resolution with a racemization process to push the ideal yield to 100%. A changing of the absolute configuration of BINOL has been observed since Brussee et al. did so in 1985, but its mechanism remains unknown. Recently, racemization strategies and a mechanism based on single-electron oxidation, producing a released radical-anion species as the key intermediate, have been clearly disclosed. In particular, deracemization of BINOLs achieved by using stochiometric amounts of a chiral amine or ammonium salt, and dynamic kinetic resolution in cooperation with biocatalysis, have been well established for accessing enantioenriched BINOL derivatives, as summarized and discussed in this review.