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

Abstract

DOSY für die Katalyse: Bei ausgewählten Phosphoramiditen und ihren Übergangsmetallkomplexen gilt, dass für Liganden, deren Komplexe hochenantioselektive Reaktionen katalysieren, ein DOSY‐NMR‐spektroskopisches Screening der freien Liganden die Vorhersage des Verlaufs der Aggregation ihrer Komplexe ermöglicht (siehe Bild). Da diese Methode keine Kenntnis der Komplexstruktur erfordert, ist sie sehr nützlich für Optimierungsprozesse.

Structures and Interligand Interaction Pattern of Phosphoramidite Pd Complexes by NMR Spectroscopy: Modulations in Extended Interaction Surfaces as Stereoselection Mode of a Privileged Class of Ligands

“…For extended interaction areas, however, unex-pectedly high interaction energies have been calculated, [22] unusual C À C bonds are stabilized with the help of a multitude of dispersion interactions, [23] and the concept of "dispersion energy donors" has been developed. [24] Recent studies by our research group on the aggregation behavior of phosphoramidite ligands 1-3 as well as of their transition metal complexes (M = Cu, Ir, Pd) proved the general tendency of these ligands to form intermolecular interactions [25] based on p-p as well as CH-p interactions, in accordance with the results of Pregosin. [17] The intriguing and surprising result of our studies was that the aggregation trends of the highly enantioselective ligands 1 and 2 are almost independent of the transition metal used and/or the structure of the complex.…”

“…Despite the vast structural knowledge about phosphor-A C H T U N G T R E N N U N G amidite Cu complexes acquired by our research group, [25,27] copper was not the metal of choice for the investigation of ligand-ligand interactions because ligand-exchange processes within Cu complexes are too fast on the NMR time scale for detailed structural investigations. Furthermore, basic structural knowledge about the Pd complexes was already available from aggregation studies.…”

“…Both the 31 P and 1 H signals of these different trans complexes show severe spectral overlap and, in addition, aggregation phenomena at low temperature, rotational processes, and conformational exchange cause a significant broadening of the 1 H signals, which additionally impairs signal resolution. [25] Nevertheless, some important structural information concerning these complex species could be gained. As no release of ligand was observed with decreasing temperature, a temperature-dependent interconversion into other complex species, as we observed for phosphoramidite copper complexes, [27b] could be excluded.…”

“…[17] The intriguing and surprising result of our studies was that the aggregation trends of the highly enantioselective ligands 1 and 2 are almost independent of the transition metal used and/or the structure of the complex. [25] This suggests a general interaction pattern of phosphoramidites beyond single and structure-dependent p-p or CH-p interactions.…”

The Supramolecular Balance for Transition‐Metal Complexes: Assessment of Noncovalent Interactions in Phosphoramidite Palladium Complexes

“…This method was tested on Pd II complexes using combinations of the well-known phosphoramidite ligands (S a ,R c ,R c )-1, [7] (S c ,S c )-2*, and (R c ,R c )-2 [8] (see Figure 2 b), which find broad application in many asymmetric catalytic reactions. [7][8][9] Moreover, we could already observe a general affinity of these phosphoramidites to form noncovalent interligand interactions in Cu complexes [10] and in aggregation studies of these ligands and their Cu, Pd, and Ir complexes. [11] In these three ligands, all heteroatoms encompassing the dipoles are located in a very small and structurally rigid inner sphere (O 2 PN moieties, see Figure 2 b).…”