Chiral Rh Diene NHC Complexes and Their Application in Rh-Catalyzed Cycloisomerization of Enynes

Abstract

Chiral dienes and N-heterocyclic carbenes (NHC) are important ligands for transition-metal catalysis. Surprisingly, both ligand classes are rarely employed together in Rh complexes. To get more detailed insight into the interaction between NHC and diene ligand simultaneously coordinated to the same Rh center, a series of eight neutral Rh diene NHC complexes, [Rh(L1)(NHC)Cl] (NHC = ICy, IMes, IⁱPr, IPr, SIPr, IMe, (R)-IEtPh, and (S)-IEtPh), and four cationic analogues, [Rh(L1)(NHC)]X (NHC = ICy, IMes, IPr: X = B(Ar^F)₄; NHC = ICy: X = SbF₆), were synthesized from [Rh(L1)Cl] by three complementary synthetic routes and anion exchange. X-ray crystal structure analyses revealed pronounced NHC-dependent distortions from a square-planar geometry in neutral complexes, whereas cationic species remain comparatively rigid. Catalytic studies showed a strong dependence on the NHC ligand’s identity: while NHC incorporation generally reduced yields and enantioselectivities in asymmetric 1,2- and 1,4-additions, the cationic complex [Rh(L1)(ICy)]SbF₆ delivered improved activity and selectivity in 1,6-enyne cycloisomerization, consistent with reduced catalyst deactivation as confirmed by kinetic analysis. DFT calculations identified that stereocontrol originates from key interactions enforced by the chiral diene scaffold, disrupted by bulkier NHC ligands. These findings highlight the decisive influence of NHC sterics on the structure, stability, and asymmetric reactivity of Rh diene catalysts.