Molecular Motors’ Magic Methyl and Its Pivotal Influence on Rotation

Molecular motors have found a wide range of applications,
powering
a transition from molecules to dynamic molecular systems for which
their motion must be precisely tuned. To achieve this adjustment,
strategies involving laborious changes in their design are often used.
Herein, we show that control over a single methyl group allows a drastic
change in rotational properties. In this regard, we present the straightforward
asymmetric synthesis of β-methylated first-generation overcrowded-alkene-based
molecular motors. Both enantiomers of the new motors were prepared
in good yields and high enantiopurities, and these motors were thoroughly
studied by variable-temperature nuclear magnetic resonance (VT–NMR),
ultraviolet–visible (UV–vis), and circular dichroism
(CD) spectroscopy, showing a crucial influence of the methylation
pattern on the rotational behavior of the motors. Starting from a
common chiral precursor, we demonstrate that subsequent methylation
can drastically reduce the speed of the motor and reverse the direction
of the rotation. We show for the first time that complete unidirectionality
can be achieved even when the energy difference between the stable
and metastable states is small, resulting in the coexistence of both
states under ambient conditions without hampering the energy ratcheting
process. This discovery opens the way for the design of more advanced
first-generation motors.