Development of Sandwich Compound-Based Planar Chiral Six-Membered NHC Ligands with Quaternary Carbon at α-Position of Nitrogen

Following the development of the seven- and five-membered ring versions, we created a six-membered cyclic (amino)(ferrocenylene)carbene (CAFeC)/Ir complex. To overcome the issue of poor yield of the six-membered CAFeC/Ir complexes in previous studies, we introduced two methyl groups on the carbon atom adjacent to the nitrogen. This modification, which was based on the design of cyclic (amino)(alkyl)carbenes (CAACs), improved the yield of the corresponding Ir complex. Applying this modification to a recently developed NHC ligand having a six-membered ring fused to a bulky η5-cyclopentadienyl-cobalt-η4-tetraphenylcyclobutadiene core improved the yield of its Rh complex in a similar manner. Finally, we applied the Rh complex to a preliminary experiment on the asymmetric ring-opening reaction of oxabenzonorbornadiene with 3,5-dimethylphenol and found it promising as an asymmetric catalyst.