Enantioselective Reduction of Aromatic and Aliphatic Ketones Catalyzed by Ruthenium Complexes Attached to β‐Cyclodextrin

Abstract

Wasserlösliche chirale Ru‐Komplexe mit einer β‐Cyclodextrin‐Einheit katalysieren in Gegenwart von Natriumformiat die Reduktion aliphatischer Ketone (siehe Schema) in guten bis ausgezeichneten Ausbeuten und mit bis zu 97 % ee. Die β‐Cyclodextrin‐Einheit ist für die Katalyse essenziell, denn sie trägt durch Präorganisation des Substrats im hydrophoben Hohlraum zu den außergewöhnlichen Enantioselektivitäten bei.

Enantioselective Transfer Hydrogenation of Aliphatic Ketones Catalyzed by Ruthenium Complexes Linked to the Secondary Face of β‐Cyclodextrin

“…High enantiomeric excesses (ee) have been accomplished for a number of aromatic and aliphatic ketones in the presence of HCOONa as the hydrogen donor in water and water/ DMF mixtures, respectively. [13] We convey here an alternative strategy employing the modification of the secondary face of b-cyclodextrin 1 ( Figure 2). …”

“…Surprisingly 1 H NMR experiments of this Ru complex 8 revealed no chemical shifts of the À O À CH 2 À CH 2 À NH À unit as observed when this moiety was linked to C-6 at the primary face of b-cyclodextrin. [13] Nevertheless, 8 displayed very good reactivity and high enantiomeric excess in the hydrogen transfer reaction to aliphatic ketones (Table 1).…”

“…It is important to note that selectivities relying on the enantioface differentiating capacity of b-cyclodextrin alone rarely exceed 50% ee. [13,22,23] Unfortunately, the metal complexes 8 and 11 refused to gave crystals suitable for X-ray analysis, accordingly we have no precise information on the structure of these pre-catalysts, the substrate inclusion complexes and the corresponding catalytically active hydride-ruthenium complexes. The NMR spectra of 8 and 11 provided no evidence that the phenyl ring co- 89% [8] 99% [11] 86% [11] 2 98% [8] 92% [8] 99% [11] 93% [11] 3 98% [11] 96% [11] 4 35% [8] 98% [8] 76% [11] 93% [11] 5 95% [8] 90% [8] 99% [11] 86% [11] 6 60% [8] 89% [8] 7 72% [8] 70% [8] 98% [11] 74% [11] 8 80% [8] 89% [8] 99% [11] 89% [11] 9 55% [8] 91% [8] 99% [11] 93% [11] 10 56% [8] 91% [8] 80% [11] 93% [11] 11 37% [11] 93% [11] 12 55%…”

Cyclodextrins in Asymmetric and Stereospecific Synthesis

“…Scheme 2. Potential mechanism of hydrogen transfer from the Ru-hydride intermediate to geranyl acetone encapsulated in β-CD, adapted from [10]. Shen and Ji [11] proposed a group of catalysts based on [RuCl2(benzene)]2 and [RuCl2(mesitylene)]2 cores with eight amino alcohol-modified β-CD-based ligands as another way for the reduction of aromatic ketones.…”

“…Thus, Schlatter et al suggested [10] the application of ruthenium complexes of the amino alcohol β-CDs as catalysts in enantioselective reduction of aromatic and aliphatic ketones. The synthetic pathway for obtaining these complexes is presented in Scheme 1.…”