Konformativ flexible Biphenyl-Phosphanliganden für Ru-katalysierte enantioselektive Hydrierungen

Abstract

Die Inversion eines BIPHEP/RuCl2/Diamin‐Komplexes (im Bild schematisch gezeigt) wird durch die Konformationsflexibilität der BIPHEP‐Liganden ermöglicht. Die Folge ist eine asymmetrische Aktivierung in der Ru‐katalysierten Hydrierung von Carbonylverbindungen zu optisch aktiven Alkoholen. Während ein racemischer BINAP/RuCl2‐Komplex mit einem chiralen Diamin als Aktivator ein 1:1‐Diastereomerengemisch liefert, erhält man mit einem BIPHEP/RuCl2‐Komplex und einem chiralen Diamin die Diastereomere in ungleichen Mengen. Ar = 3,5‐Dimethylphenyl; BINAP = 2,2′‐Bis(diphenylphosphanyl)‐1,1′‐binaphthyl; BIPHEP = 2,2′‐Bis(diarylphosphanyl)biphenyl.

The Stereodynamics of 5,5’‐Disubstituted BIPHEPs

“…An advantage of tropos ligands like BIPHEP (2,2’‐bis(diphenylphosphino)‐biphenyls) over atropos ligands is their dynamic rotational behavior, allowing to use synthetically easier accessible racemic mixtures of the ligand, which can be deracemized at low temperature to yield the desired enantiomerically pure catalyst. Mikami and Noyori reported first about a novel concept: the use of a racemic BIPHEP‐Ru catalyst bearing a chiral amine to dynamically deracemize the BIPHEP‐Ru complex . Treatment of the racemic BIPHEP‐Ru catalyst with a chiral amine led to the deracemization of the catalyst, which was used in hydrogenations of ketones to chiral alcohols with excellent enantioselectivities .…”

“…Mikami and Noyori reported first about a novel concept: the use of a racemic BIPHEP‐Ru catalyst bearing a chiral amine to dynamically deracemize the BIPHEP‐Ru complex . Treatment of the racemic BIPHEP‐Ru catalyst with a chiral amine led to the deracemization of the catalyst, which was used in hydrogenations of ketones to chiral alcohols with excellent enantioselectivities . Mikami and colleagues made further contributions in enantioselective catalysis with the tropos BIPHEP‐ligand by using either enantiopure diamine coligands in Pd‐catalyzed hetero‐Diels‐Alder reactions or enantiomerically pure counterions to deracemize BIPHEP‐Au complexes, which were applied in intramolecular hydroaminations of allenes and enantioselective intermolecular carbon–carbon bond formations .…”