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Optically active overcrowded alkenes were synthesized by employing bis-beta-naphthol as a chiral template during an intramolecular coupling reaction. The major isomer 2 has a unique helical structure with twisted and folded structural moieties. Removal of the chiral template afforded overcrowded thioxanthylidene 3 with 96 % ee, which indicates that no racemization or isomerization of the enantiomers took place.

Attempts to fabricate mechanical devices on the molecular level1,2 have yielded analogues of rotors3, gears4, switches5, shuttles6,7, turnstiles8 and ratchets9. Molecular motors, however, have not yet been made, even though they are common in biological systems10. Rotary motion as such has been induced in interlocked systems11,12,13 and directly visualized for single molecules14, but the controlled conversion of energy into unidirectional rotary motion has remained difficult to achieve. Here we report repetitive, monodirectional rotation around a central carbon–carbon double bond in a chiral, helical alkene, with each 360° rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system. We find that axial chirality and the presence of two chiral centres are essential for the observed monodirectional behaviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180° rotation around the carbon–carbon double bond and are each followed by thermally controlled helicity inversions, which effectively block reverse rotation and thus ensure that the four individual steps add up to one full rotation in one direction only. As the energy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic components for ‘molecular machinery’ driven by light.

Thermal α-H abstraction from (C5H4R)2Ti(CH2CMe3)2(R = H, Me) produces reactive titanocene neopentylidene intermediates under mild conditions, that can either be trapped with PMe3 to yield the alkylidene complexes (C5H4R)2Ti(CHCMe3)PMe3, or add C–H bonds of hydrocarbon substrates R′H (R′H = benzene, p-xylene) to the TiC double bond to produce (C5H4R)2Ti(CH2CMe3)R′; an alternative reaction pathway, insertion of the alkylidene moiety into the cyclopentadienyl C–C bond, has been observed in the presence of THF.