Fluorine-Substituted Molecular Motors with a Quaternary Stereogenic Center.

A series of unprecedented second generation molecular motors featuring a quaternary stereogenic center substituted with a fluorine atom has been synthesized. It is demonstrated that a seemingly benign replacement of the stereogenic hydrogen for a fluorine atom, regarded as a common substituent in pharmacology, resulted in a dramatic change in the energetic profile of thermal helix inversion. The barrier for the thermal helix inversion was found to increase considerably (by 20-30 kJ mol-1 ), presumably due to destabilization of the transition state by increased steric hindrance when the fluorine atom is forced to pass over the lower half of the motor. This results in the activation barrier for the thermal helix inversion to be higher than the barrier for backward thermal E-Z isomerization, impairing the motor function. A fluorine-substituted motor capable of performing unidirectional rotation is successfully prepared when these limitations are considered in the design phase.