Chiral Rh diene NHC Complexes and their Application in Rh-Catalyzed Cycloisomerization of Enynes

Chiral dienes and N-heterocyclic carbenes (NHC) are very important ligands for transition metal catalysis. Surprisingly, both ligand classes have rarely been employed together in Rh complexes. In order to get more detailed insight into the interaction between NHC and diene ligand simulta-neously 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, (S)-IEtPh), and four cati-onic analogues, [Rh(L1)(NHC)]X (NHC = ICy, IMes, IPr: X = B(ArF)4; R = ICy: X = SbF6) were synthesized from the known precursor [Rh(L1)Cl] by three complementary synthetic routes and following anion exchange. X-Ray crystal structure analyses revealed pronounced NHC-dependent distortions from square-planar geometry in the neutral complexes, whereas the cationic species remain comparatively rigid. Catalytic studies showed a strong dependence on the identity of the NHC ligand: while NHC incorporation generally reduced yields and enantioselectivities in asym-metric 1,2- and 1,4-additions, the cationic complex [Rh(L1)(ICy)]SbF6 delivered markedly im-proved activity and selectivity in 1,6-enyne cycloisomerization, consistent with reduced catalyst deactivation as confirmed by kinetic analysis. DFT calculations identified a concerted ring-closure as the rate-determining step and showed that stereocontrol originates from key interactions en-forced by the chiral diene scaffold, which are disrupted by bulkier NHC ligands. Overall, these findings highlight the decisive influence of NHC sterics on the structure, stability, and asymmetric reactivity of Rh diene catalysts.