Atropselective Dibrominations of a 1,1’‐Disubstituted 2,2’‐Biindolyl With Diverging Point‐to‐Axial Asymmetric Inductions. Deriving Diphosphane Ligands for Asymmetric Catalysis

On an 1H‐NMR time‐scale, 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)‐17a of that kind and achiral (!) brominating reagents gave the atropisomerically stable 3,3'‐dibromobiindolyls (M)‐ and/or (P)‐18a at best atropselectively – because of point‐to‐axial asymmetric inductions – and atropdivergently, exhibiting up to 95% (M)‐ and as much (P)‐atropselectivity. Such a 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)‐14a without atropisomerization. These syntheses entailed no resolution of a racemic mixture, which makes them different than virtually all biaryldiphosphane syntheses known to date. (M)‐ and (P)‐14a acted as ligands in catalytic asymmetric allylations and hydrogenations. Remarkably, the beta‐ketoester rac‐25c was hydrogenated trans‐selectively with 98% ee due to a dynamic kinetic resolution.

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