Development of Sandwich‐Compound‐Based Planar‐Chiral Six‐Membered Unsaturated N ‐Heterocyclic Carbene Ligands

Sandwich‐compound‐based planar‐chiral six‐membered unsaturated N ‐heterocyclic carbene (NHC) ligands were developed. Their formation was indirectly confirmed through trapping experiments in which carbene precursors were reacted with sulfur or [RhCl(cycloocta‐1,5‐diene (cod))] ₂ in the presence of a base ( t ‐BuOK or lithium bis(trimethylsilyl)amide (LiHMDS)). Determining the Tolman electronic parameter of the Rh dicarbonyl complex [RhCl(CO) ₂ (( R _p )‐ 4b )] revealed slightly stronger electron‐donating ability (2045 cm ^–1 ) than representative classic diaminocarbenes (e.g., SIMes: 2051 cm ^–1 ). Furthermore, the structure of the NHC ligand ( R _p )‐ 4b was unambiguously confirmed by X‐ray diffraction analysis of the related complex [RhI(cod)(( R _p )‐ 4b )], and its utility as a chiral ligand was preliminarily demonstrated in the Rh‐catalyzed asymmetric ring‐opening reaction of oxabenzonorbornadiene with N ‐methylaniline (61% yield, 56% enantiomeric excess (ee)).