All-Photochemical Rotation of Molecular Motors with a Phosphorus Stereoelement

Unidirectional molecular
rotation based on alternating photochemical
and thermal isomerizations of overcrowded alkenes is well established,
but rotary cycles based purely on photochemical isomerizations are
rare. Herein we report three new second-generation molecular motors
featuring a phosphorus center in the lower half, which engenders a
unique element of axial chirality. These motors exhibit unusual behavior,
in that all four diastereomeric states can interconvert solely photochemically.
Kinetic analysis and modeling reveal that the behavior of the new
motors is consistent with all-photochemical unidirectional rotation.
Furthermore, X-ray crystal structures of all four diastereomeric states
of two of these new motors were obtained, which constitute the first
achievements of crystallographic characterization of the full 360°
rotational cycle of overcrowded-alkene-based molecular motors. Finally,
the axial phosphorus stereo­element in the phosphine motor can
be thermally inverted, and this epimerization enables a “shortcut”
of the traditional rotational cycle of these compounds.