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

Locking a labile atropisomer: The biindolyl (R,R)‐2 consists of diastereomeric atropisomers ((this word should not be hyphenated as in the current pdf‐file)), which interconvert on the NMR timescale at +30 °C but not at −10 °C. (R,R)‐2 was dibrominated providing the biindolyls 1 (M)‐ or (P)‐selectively, depending on the achiral reagent. The two dibromides were elaborated into atropisomerically pure biindolyldiphosphanes, which induced higher ee values than BINAP in some Ru‐catalyzed β‐ketoester hydrogenations.

Abstract

On the ¹H 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.

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