Supramolekulare Ansätze zur Erzeugung von Bibliotheken zweizähniger Chelatliganden für die homogene Katalyse

Abstract

Die kombinatorische Katalysatorentwicklung und ‐optimierung leidet bislang an der schlechten Zugänglichkeit strukturell diverser Ligandenbibliotheken, vor allem für die Klasse der strukturell komplexeren Chelatliganden. Ein völlig neuer Ansatz umgeht das Problem der schwierigen Ligandensynthese durch die Selbstorganisation von strukturell einfacheren einzähnigen Liganden zu zweizähnigen Liganden durch anziehende nichtkovalente Ligand‐Ligand‐Wechselwirkungen. Bei Verwendung komplementärer Ligand‐Ligand‐Bindungsmotive führt schon einfaches Mischen von jeweils zwei unterschiedlichen einzähnigen Liganden zum Aufbau von Bibliotheken definierter Chelatligand‐Katalysatoren. Dieser Kurzaufsatz fasst die ersten Ansätze und Ergebnisse in diesem vielversprechenden Forschungsgebiet der homogenen Katalyse zusammen.

PhthalaPhos: Chiral Supramolecular Ligands for Enantioselective Rhodium‐Catalyzed Hydrogenation Reactions

“…In recent years, combinatorial and supramolecular approaches to the development of new ligands for asymmetric catalysis has gained momentum. [1, 2d] The term "supramolecular ligand" encompasses all ligands possessing, besides the atom(s) coordinating to the catalytic metal atom, an additional functionality capable of noncovalent interactions (mainly hydrogen [3] or coordinative bonds [4] ) which can play the following roles: 1) self-assembly of two monodentate ligands to form a so-called supramolecular bidentate ligand; [5] 2) binding the substrate(s) in proximity to the catalytic metal center [2] in analogy to metalloenzymes. [6] Among the different kinds of noncovalent interactions that have been used so far for developing supramolecular ligands, [5] hydrogen bonds are arguably the most practical and efficient [2,3] for several reasons: 1) functional groups capable of hydrogen bonding (e.g., amides, ureas, guanidines) are stable and relatively easy to introduce; 2) hydrogen bonds are created dynamically and reversibly in the reaction medium (where catalysis is to take place), are capable of self-repair when broken, and often coexist with other interactions in a "noninvasive" manner.…”

“…[1, 2d] The term "supramolecular ligand" encompasses all ligands possessing, besides the atom(s) coordinating to the catalytic metal atom, an additional functionality capable of noncovalent interactions (mainly hydrogen [3] or coordinative bonds [4] ) which can play the following roles: 1) self-assembly of two monodentate ligands to form a so-called supramolecular bidentate ligand; [5] 2) binding the substrate(s) in proximity to the catalytic metal center [2] in analogy to metalloenzymes. [6] Among the different kinds of noncovalent interactions that have been used so far for developing supramolecular ligands, [5] hydrogen bonds are arguably the most practical and efficient [2,3] for several reasons: 1) functional groups capable of hydrogen bonding (e.g., amides, ureas, guanidines) are stable and relatively easy to introduce; 2) hydrogen bonds are created dynamically and reversibly in the reaction medium (where catalysis is to take place), are capable of self-repair when broken, and often coexist with other interactions in a "noninvasive" manner.As a result of our continued interest in developing supramolecular ligands, [7] we report herein the design and synthesis of a novel class of chiral monodentate phosphite ligands, named PhthalaPhos, which contain a phthalic acid primary diamide moiety (Scheme 1). The phthalamidic group displays both donor and acceptor hydrogen-bonding properties that, in principle, can give rise to supramolecular interactions both between the ligands and with the reaction substrate.…”

1H‐DOSY‐Spektren von Liganden für hochenantioselektive Reaktionen – eine schnelle und einfache Methode zur Optimierung katalytischer Reaktionsbedingungen

“…[9] Heutzutage müssen derartige rationale Modelle allerdings noch komplexere Fragestellungen bewältigen, da erkannt wurde, dass auch nichtkovalente Ligand-Ligand-Wechselwirkungen in metallorganischen Komplexen entscheidende Beiträge leisten. [10] So können selbst schwache p-p-Wechselwirkungen die Komplexstruktur beeinflussen; [11,12] gezeigt wurde dies z. B. an ciskoordinierten Liganden in einem Bis(phosphonit)-PlatinKomplex und in einem Bis(phosphoramidit)-PalladiumKomplex, deren cis-Anordnung auf schwache intermolekulare Wechselwirkungen zurückgeführt wurde.…”

Combinatorial Transition‐Metal Catalysis: Mixing Monodentate Ligands to Control Enantio‐, Diastereo‐, and Regioselectivity

“…[125] Once a library of appropriate monodentate P ligands of this kind has been prepared, the simple process of mixing generates structural diversity without the need to synthesize new ligands. [126] This is analogous to the situation when mixing structurally simple achiral or chiral monodentate P ligands (see Sections 3-5), except with the added advantage that the heterocombination with a predictable geometry is formed exclusively. Of course, this advantage is achieved at the expense of added synthetic effort.…”