Optimizing dirhodium(ii) tetrakiscarboxylates as chiral NMR auxiliaries

Abstract

Thirteen enantiopure paddlewheel-shaped dirhodium(II) tetrakiscarboxylate complexes have been checked for their efficiency in the dirhodium method (differentiation of enantiomers by NMR spectroscopy); six of them are new. Their diastereomeric dispersion effects were studied and compared via so-called key numbers KN. Adducts of each complex were tested with five different test ligands representing all relevant donor properties from strong (phosphane) to very weak (ether). Only one of them, the dirhodium complex with four axial (S)-N-2,3-naphthalenedicarboxyl-tert-leucinate groups (N23tL), showed results significantly better for all ligands than the conventional complex Rh* [Rh(II) 2 [(R)-(+)-MTPA] 4 ; MTPA = methoxytrifluoromethylphenylacetate]. On the basis of 1 H{ 1 H} NOE spectroscopy and X-ray diffraction, a combination of favourable anisotropic group orientation and conformational flexibility is held responsible for the high efficiency of N23tL in enantiodifferentiation. Both complexes, Rh* and N23tL, are recommended as chiral auxiliaries for the dirhodium experiment.

Chiral Dirhodium(II) Carboxylates: New Insights into the Effect of Ligand Stereo-Purity on Catalyst Structure and Enantioselectivity

“…This fact was also previously observed through many other complexes from the same family [35,[49][50][51] including Rh 2 (S-PTAD) 4 which carry the bulkiest adamantyl groups at the ligands' α-positions [35]. This observation can be also applied to carbene ligands during asymmetric catalysis which contradicts with the major assumption of Fox mechanistic 4 in asymmetric cyclopropanations [47,48].…”

“…This is in contrast with the clockwise twist reported for [Rh 2 (S-PTTL) 4 . (EtOAc)] [47,48] and other related complexes [35,[49][50][51]. As a result, the daisy-chain type docking of the N-phthaloyl units around the cavity seen in [Rh 2 (S-PTTL) 4 .…”

“…In the case of published X-ray crystal structures of various solvated adducts of Rh 2 (S-PTTL) 4 [47,48], Rh 2 (S-tert PTTL) 4 [35], Rh 2 (S-NTTL) 4 [49], Rh 2 (S-PTAD) 4 [35] and others [35,50,51], the incorporation of a ligand stereogenic center begets a "chiral cavity" or a "chiral pocket". Figure 2 [47,48] depicts the two most important features that are responsible for the establishment of the chiral binding pocket in Rh 2 (S-PTTL) 4 and related complexes; C-C bond torsion (bond connecting carboxylate carbon to α-carbon) that is positioned so as to guide the C-N single bond leading to a clockwise twist of the chiral cavity walls (Figure 2a), and C-N bond torsion (bond units (featuring alternating O ... H-C closest contacts, Figure 2c).…”

On the Structure of Chiral Dirhodium(II) Carboxylate Catalysts: Stereoselectivity Relevance and Insights

“…For example, 2D heteronuclear NOESY data reported by the groups of Charette [54] and Duddeck [99], independently, confirmed that Rh2(S-PTTL)4 and analogues have a mobile conformation in solution. This confirms the existence of other conformers with at least one N-phthalimido group is flipped downward.…”

“…In fact, carboxylate ligands contain conformationally mobile units [13,54,87,98,99] which might give the ability to the dirhodium(II) catalyst to adapt its conformation to the particular substrate undergoing the reaction. Then enantioselection mechanisms can mainly originate from the different preferred reaction channels as a function of the different steric interactions taking place between the approaching substrate and the formed dirhodium(II)-carbene complex.…”