New Spin On Stereocontrol

Challenging those who think molecular motor makers are just spinning their wheels, this year chemists in the Netherlands created a rotary motor catalyst molecule, driven by light and heat, that dynamically controls chirality in an asymmetric addition reaction (C&EN, Feb. 14, page 9; Science, DOI: 10.1126/science.1199844). The motor molecule, designed and built by the University of Groning­en’s Ben L. Feringa and Jiaobing Wang, can make an R enantiomer, an S enantiomer, or a racemic mixture on demand, depending upon where the motor is in its rotary cycle. The catalytic motor consists of two arms that turn past one another in a uni­directional manner in four discrete steps. One arm is equipped with a 4-dimethyl­aminopyridine (DMAP) moiety, and the other arm features a thiourea group. When the arms are pointed away from each other, the catalyst generates a racemic mixture of products during the addition of 2-methoxythiophenol to cyclohexenone. But when light shines on the catalyst, its double-bond axle isomerizes, bringing the arms and their cooperative catalytic groups close to each other. Here, the thiourea group holds the cyclohexenone in place while the DMAP unit guides the thiophenol to stereoselectively form the addition product with S stereochemistry. Heating the system forces the arms to pass one another, which causes DMAP to send the thiophenol to the other side of the cyclohexenone and preferentially form the product with R stereochemistry. Light prompts the double bond to isomerize again and sends the arms back to their starting positions.