Imidazolium-based N-heterocyclic carbenes (NHCs) are a versatile set of catalyst molecules. Chemists use them as ligands in transition-metal catalysts, and they use them alone as metal-free organocatalysts. They would also like enantioselective versions of these compounds to prepare chiral molecules, but so far only a few successful versions have been reported.

A research team led by Christopher T. Check and Karl A. Scheidt of Northwestern University set out to change that situation by fusing a metal sandwich complex with an NHC framework. In doing so, they created a versatile new class of NHCs with a rigid planar chiral imidazolium ring system that can be tuned to serve as a ligand or organocatalyst by altering the substituents on the imidazolium ring (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201410118)

The Northwestern chemists first synthesized a ferrocene derivative with a cyclopentadienyl ligand on one side of the sandwich and a cyclopentapyridinyl ligand bearing a pseudoephedrine side chain on the other. After separating the resulting enantiomers, they carried out an additional reaction sequence to finalize formation of the planar chiral imidazolium ring system.

“This new molecule represents a creative translation of the notion of planar chirality to the bond-forming business end of the NHC ligand-catalyst scaffold,” says Scott J. Miller of Yale University, whose group develops catalytic methods for synthesizing stereochemically complex molecules. “In doing so, the researchers have provided new dials to turn for optimizing catalysts. It is very thought-provoking and stimulating work.”

With the ferrocene-based chiral NHC in hand, Scheidt and coworkers carried out a series of reactions. In one example, the team used the new NHC as an enantioselective organocatalyst for aryl homoenolate additions to aryl α-ketoesters. In another example, they paired the NHC with nickel for reductive coupling of phenylpropyne with aldehydes to form allylic alcohols in high regioselectivity and enantiomeric excess—among the best ever reported for this reaction, Scheidt notes. The researchers further used the new NHC to prepare and study copper and rhodium complexes.

Sign up for C&EN's must-read weekly newsletter Email Address * Subscribe » Contact us to opt out anytime