Enantiopure chiral concave N-heterocyclic carbene precursors

Chiral concave imidazolinium salts 4·Cl^- with different ring sizes were synthesized as precursors of chiral concave N-heterocyclic carbenes, starting from di-ortho-substituted anilines 5 and 2,7-disubstituted aminonaphthalenes 6. The terminal alkene groups of the substituents were cyclized by ring-closing metathesis to give bimacrocyclic axially chiral concave imidazolinium salts 14·Cl^-, and, after hydrogenation of the double bonds, 4·Cl^-. The overall yields of bimacrocycles 4b·Cl^- and 4c·Cl^- were 54 and 39 %, starting from amino-substituted arenes 5 and 6, respectively. Racemic mixtures of the three concave NHC precursors 4a-c·Cl^- were separated by chiral HPLC techniques, and their absolute stereochemistry was assigned by comparison of simulated and experimental CD spectra. Axially chiral concave N-heterocyclic carbene precursors of different sizes were synthesized and separated into the corresponding enantiomers. The determination of the absolute configuration was achieved by comparison of measured and simulated circular dichroism spectra.