Atropselective Dibrominations of a 1,1′-Disubstituted 2,2′-Biindolyl with Diverging Point-to-Axial Asymmetric Inductions. Deriving 2,2′-Biindolyl-3,3′-diphosphane Ligands for Asymmetric Catalysis

On the H-1 NMR timescale, 2,2-biindolyls with (R)-configured (1-alkoxyprop)-2-yl, (1-hydroxyprop)-2-yl, or (1-siloxyprop)-2-yl substituents at C-1 and C-1 are atropisomerically stable at <0 degrees C and interconvert at >30 degrees C. A 2,2-biindolyl (R,R)-17a of that kind and achiral (!) brominating reagents gave the atropisomerically stable 3,3-dibromobiindolyls (M)- and/or (P)-18a at best atropselectivelybecause of point-to-axial asymmetric inductionsand atropdivergently, exhibiting up to 95% (M)- and as much (P)-atropselectivity. This route to atropisomerically pure biaryls is novel and should extend to other substrates and/or different functionalizations. The dibromobiindolyls (M)- and (P)-18a furnished the biindolyldiphosphanes (M)- and (P)-14 without atropisomerization. These syntheses did not require the resolution of a racemic mixture, which distinguishes them from virtually all biaryldiphosphane syntheses known to date. (M)- and (P)-14 acted as ligands in catalytic asymmetric allylations and hydrogenations. Remarkably, the beta-ketoester rac-25c was hydrogenated trans-selectively with 98% ee; this included a dynamic kinetic resolution.