A kinetic resolution strategy for the synthesis of chiral octahedral NHC-iridium(III) catalysts - Archive ouverte HAL Accéder directement au contenu
Article Dans Une Revue Journal of the Chemical Society, Chemical Communications Année : 2019

A kinetic resolution strategy for the synthesis of chiral octahedral NHC-iridium(III) catalysts

Résumé

The transmetalation reaction of a chiral-bidentate NHC-silver complex to racemic [Ir(-Cl)(ppy)2]2 operates with kinetic resolution leading to chiral octahedral NHC-iridium(III) complexes and enantio-enriched bis-cyclometalated iridium(III) complexes. The iridium(III) complexes demonstrated efficient catalytic activities in intermolecular [2+2] photocycloaddition reactions and in asymmetric Friedel-Crafts alkylations, respectively. In the past two decades, bis-cyclometalated Iridium(III) complexes have been attracting broad interest due to their outstanding photophysical and chemical properties which are ideal in view of applications in photonic and optoelectronic devices, 1 in bioimaging and cancer therapy as potential theranostic agents 2 and in organic synthesis as photosensitizer and photoredox catalysts. 3 More recently, particular attention has been given to N-Heterocyclic Carbenes (NHCs) ligands in the design of highly stable luminescent cyclometalated iridium complexes with high range of emission colors. 4 On the other hand, increasing efforts has been given to the development of efficient methods for the preparation of chiral octahedral Ir(III) complexes that are highly valuable materials to access selective protein inhibitors, circularly polarized (CP) light-emitting devices and chiral photocatalysts. 5 Nevertheless, there is still a need for efficient and practical strategies to synthesize chiral non racemic luminescent NHC-based octahedral Ir(III) complexes. 4c,l,6 Recently, we described a practical multicomponent strategy that provides access to various unsymmetrical NHC ligands precursors, including carboxyalkyl-NHCs that are beyond reach otherwise. 7 NHC ligands containing an additional donor functionality have demonstrated significant benefit in stabilizing a variety of transition-metal complexes. 8 Herein, we describe an efficient strategy for the synthesis of robust chiral luminescent octahedral NHC-iridium(III) complexes and enantio-enriched bis-cyclometalated iridium(III) complexes (Fig. 1). The catalytic potential of the new complexes was respectively demonstrated in a selective [2+2] photocycloaddition reaction and in asymmetric Friedel-Crafts alkylation. Fig. 1 Synthesis of chiral octahedral Iridium complexes. The reaction of racemic [Ir(-Cl)(ppy)2]2 (1) with the chiral NHC-Ag (S)-2 complex in dichloromethane at 40 °C afforded a mixture of the -(S) and -(S) diastereomers with a diastereomeric ratio of 2:1 in favor of the -(S)-3 as determined by 1 H NMR spectroscopy (Scheme 1, Fig. S1, ESI ‡). The strong -donor ability and chelating coordination mode of the chiral NHC-carboxylate ligand conferred high complex stability, 9 allowing efficient separation of the two diastereomers by silica gel chromatography to provide pure -(S)-3 and -(S)-4 in a good 75% combined isolated yields (49 and 26% respectively). Similar results were obtained when reacting racemic [Ir(-Cl)(ppy)2]2 (1) with the chiral NHC-Ag (R)-2 complex of opposite configuration, affording the mirror-imaged diastereomeric pair which was isolated (80% yield) in about 2:1 ratio in favor of the -(R)-3. The absolute configuration at the metal center of the new octahedral NHC complexes was confirmed by X-ray diffraction analysis of single crystals (Fig. 2). Both -(S)-3 and -(S)-4 diastereomers displayed strong structural similarity with high steric crowding around the Ir center. A typical ligand arrangement is observed with the pyridine N atoms of the ppy ligands lying in trans to each other, whereas the two ortho-carbons are respectively in trans to the carbenic carbon and in trans to the carboxylate fragment of the NHC ligand. The main structure difference results from the isobutyl substituent of the asymmetric carbon center that induces enhanced steric constraints in the -(S) conformation by pointing towards the pyridyl ligand.

Domaines

Chimie organique
Fichier principal
Vignette du fichier
KR_Chem comm 2019_proof2604HAL.pdf (645.18 Ko) Télécharger le fichier
Origine : Fichiers éditeurs autorisés sur une archive ouverte
Loading...

Dates et versions

hal-02111808 , version 1 (07-05-2019)

Identifiants

Citer

Romane Manguin, Delphine Pichon, Robert Tarrieu, Thomas Vives, Thierry Roisnel, et al.. A kinetic resolution strategy for the synthesis of chiral octahedral NHC-iridium(III) catalysts. Journal of the Chemical Society, Chemical Communications, 2019, 55 (43), pp.6058-6061. ⟨10.1039/C9CC02434D⟩. ⟨hal-02111808⟩
98 Consultations
105 Téléchargements

Altmetric

Partager

Gmail Facebook X LinkedIn More