Key Role of the Lewis Base Position in Asymmetric Bifunctional Catalysis:  Design and Evaluation of a New Ligand for Chiral Polymetallic Catalysts

Abstract

New chiral ligands for asymmetric polymetallic catalysts were designed on the basis of the assumption that the higher-order assembly structure is stabilized by modifying the modular unit. The designed ligands 6 and 7 contained a scaffolding cyclohexane ring with a Lewis base phosphine oxide directly attached to the scaffold. A module in the polymetallic complex contains two metals per ligand, and a stable 6-, 5-, 5-membered fused chelation ring system should be generated. Synthesis of these ligands is simple and high yielding, using a catalytic dynamic kinetic resolution promoted by the Trost catalyst as a key step. Ligand function was assessed in a catalytic asymmetric ring-opening reaction of meso-aziridines with TMSCN, a useful reaction for the synthesis of optically active beta-amino acids. The Gd complex generated from Gd(OiPr)3 and the ligand was a highly active and enantioselective catalyst in this reaction. Enantioselectivity was reversed compared to the previously reported d-glucose-derived catalyst containing the same chirality of the individual module. ESI-MS analysis and X-ray crystallographic studies indicate that the assembly state of the modules in the polymetallic catalysts differs depending on the chiral ligand. The difference in the higher-order structure stems from a subtle change (one carbon) in the position of the Lewis base relative to the Gd metal. The change in the higher-order structure of the polymetallic complex led to a dramatic reversal of the enantioselectivity and increased catalyst activity.

Chiral tetranuclear and dinuclear copper(ii) complexes for TEMPO-mediated aerobic oxidation of alcohols: are four metal centres better than two?

“…[17][18][19][20] The construction of metal-organic cluster compounds through self-assembly of predesigned ligands with appropriate metal ions is a prime research topic [21][22][23][24][25] due to the advantages in catalysing useful chemical transformations, in contrast to simple monometallic complexes. [26][27][28][29][30] Schiff base type ligands are excellent ligand candidates for high-nuclearity metallosupramolecular assemblies, in particular in cases where additional O-donor atoms have been introduced to the backbone of a Schiff base ligand scaffold. [31][32][33][34][35] In search of active multimetallic copper catalysts for the aerobic oxidation reactions, we are particularly interested in exploring the catalytic performance of multinuclear copper(II) complexes based on simple Schiff base-type N,O-ligands.…”

“…It is known that in a molecular catalyst containing more than one metal centre the Lewis acidity of a metal centre can be significantly enhanced by other closely linked Lewis acid sites through a Lewis acid-assisted Lewis acidity enhancement effect. [26][27][28] We therefore assume that the substrate accessible Cu II site in (R)-1 as shown in Fig. 2 is more acidic than any one of reactive Cu II centres in (S,R)-2, and therefore better able to coordinate to the substrate benzylic alcohols (upon deprotonation) and TEMPO radical, which was believed to play an important role in initiating the catalytic cycle.…”

Base Induced Chemical Conversion of 3-Carbamoyl-2-isoxazolines

“…In cases of benzoisoxazoles , the similar reactions are often observed, which is applied to development of antibody catalyst 3,4) . On the other hand, the similar type ring opening reaction of monocyclic isoxazolines leading to cyanoalcohols [5][6][7][8][9][10] is not common in organic syntheses except for a few examples ( ) [11][12][13][14] . This different reactivity is due to the stability of the leaving group.…”

Katalytische enantioselektive Ringöffnungen von Aziridinen

“…[5] Durch Modifizierung der Struktur des chiralen Liganden konnte die Enantioselektivität der Reaktion weiter gesteigert werden: Mit lediglich 2 Mol-% des aus Gd(OiPr) 3 und 7 als chiralem Liganden (im Verhältnis 1:2) gebildeten Katalysators wurden die Reaktionsprodukte in fast quantitativen Ausbeuten mit bis zu 99 % ee erhalten, wobei interessanterweise die entgegengesetzten Enantiomere erhalten wurden. [6] Offenbar führt der verkleinerte Abstand zwischen der Phosphinoxideinheit und der benachbarten Hydroxygruppe in 7 zu einer solchen Veränderung des Gadolinium-Ligand-Bindungsmodus, dass sich die asymmetrische Induktion der Reaktion vollständig umkehrt.…”

“…Chrom(III)-katalysierte, enantioselektive Aziridinöffnung mit Trimethylsilylazid nach Jacobsen et al [3] Schema 2. Gadolinium-katalysierte, enantioselektive Aziridinöffnung mit Trimethylsilylcyanid nach Shibasaki et al [5,6] [*] Prof. Dr. C. Clusters abgeleitet werden. [7] Danach verhält sich 5 als vierfach koordinierender Ligand, bindet dabei aber an mindestens zwei Metallatome gleichzeitig.…”