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. Issue 14 (22nd February 2019)

• Record Type: Journal Article
• Authors: Baumann, Thomas; Brückner, Reinhard
• Is Part Of: Angewandte Chemie international edition. Volume 58:Issue 14(2019)
• Page Start: 4714
• Page End: 4719
• Publication Date: 2019-02-22
• Subjects: asymmetric allylation -- asymmetric hydrogenation -- atropselectivity -- biaryls -- bromination; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.201806294
• Languages: English
• ISSNs: 1433-7851

Abstract

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. Abstract : Locking a labile atropisomer : The biindolyl ( R, R )‐2 consists of diastereomeric atropisomers, 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.