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.

Abstract

On the 1 H 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 °C and interconvert at >30 °C. A 2,2'-biindolyl (R,R)-17 a of that kind and achiral (!) brominating reagents gave the atropisomerically stable 3,3'-dibromobiindolyls (M)- and/or (P)-18 a at best atropselectively-because of point-to-axial asymmetric inductions-and 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)-18 a 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 β-ketoester rac-25 c was hydrogenated trans-selectively with 98 % ee; this included a dynamic kinetic resolution.