Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors

The enantiomeric
homogeneity of light-driven molecular motors based
on overcrowded alkenes is crucial in their application as either unidirectional
rotors or as chiral multistate switches. It was challenging to obtain
these compounds as single enantiomers via the established synthetic
procedures due to loss of optical purity in the key step, i.e., the
Barton–Kellogg olefination reaction. Searching for strategies
to avoid racemization, a new class of light-driven molecular motors
was designed, synthesized, and studied. The stereochemical integrity
was fully preserved throughout the synthesis, and on the basis of
photochemical and kinetic studies using UV/vis, CD, and 1H NMR spectroscopy, it was established that they still function properly
as unidirectional molecular motors.