3-Aryl Substituted 1-Methylquinolinium Salts as Carbene Precursors

Abstract

1-Methylquinolinium salts substituted at the C3 position with phenyl, naphthalen-1-yl, phenanthren-9-yl, and pentaphenyl phenyl as well as aryl ethynyl substituents were prepared. The phenyl, naphthalen-1-yl, and phenanthren-9-yl substituents are in conjugation with the quinolinium ring, and the sterically crowded pentaphenyl phenyl residue causes a propeller-type molecule possessing a calculated dihedral angle between aryl substituent and the quinolinium ring of approximately 54 degrees. The different influences of the substituents including the aryl ethynyl residues on the pi-architecture is well reflected in their frontier orbital profiles, their spectroscopic properties, and in their ability to form N-heterocyclic carbenes. The latter were generated from the C3-aryl substituted quinolinium salts and trapped as sulfur and selenium adducts, whereas the aryl ethynyl substituted derivatives failed to undergo the NHC generation under the conditions applied. Se-77 NMR measurements of the corresponding selenones reveal that quinolin-2-ylidenes are electron poor carbenes with strong pi-acceptor character.