N-C Axially Chiral Anilines: Electronic Effect on Barrier to Rotation and A Remote Proton Brake.

N-Aryl-N-methyl-2-tert-butyl-6-methylaniline derivatives exhibit a rotationally stable N-C axially chiral structure and the rotational barriers around an N-C chiral axis increased with the increase in electron-withdrawing character of para-substituent on the aryl group. X-ray crystal structural analysis and the DFT calculation suggested that the considerable change of the rotational barriers by the electron effect of para-substituents is due to the disappearance of resonance stabilization energy caused by the twisting of para-substituted phenyl group in the transition state. This structural property of the N-C axially chiral anilines was employed to reveal a new acid-decelerated molecular rotor caused by the protonation at the remote position (remote proton brake).

The synthesis of optically active N-C axially chiral tetrahydroquinoline and its response to an acid-accelerated molecular rotor.

Optically active atropisomeric N-(2,5-di-tert-butylphenyl)-1,2,3,4-tetrahydroquinoline with an N-C chiral axis was prepared via a catalytic enantioselective reaction. The addition of methane sulfonic acid to this axially chiral quinoline dramatically lowered the barrier to rotation around the chiral axis.