NEWS OF THE WEEK
CATALYSIS: Molecular motor takes
turns at making each enantiomer, racemate in addition reaction
HALLENGING THOSE WHO think molecular
C
S to R enantiomeric ratio of 75 to 25. Heating the system up to 70 °C forces the arms to pass one another, so that the DMAP now sends the thiophenol to the other side of the cyclohexenone, preferentially forming the product with R stereochemistry in an S to R enantiomeric ratio of 23 to 77. Light prompts the double bond to isomerize again and sends the arms back to their starting positions. Feringa notes that other chiral catalysts give higher enantioselectivities for similar reactions, “but that is not the issue. Our goal was not to demonstrate yet another organocatalytic reaction but to test a concept and show that different rates and stereoselectivities can be reached as a catalyst adapts itself in response to an ex-
motor makers are just spinning their wheels, chemists in the Netherlands have created a rotary motor catalyst capable of dynamically controlling the chiral space in a catalytic asymmetric addition STEREOSELECTIVE MOTORING In one position (left), the molecular motor catalyst reaction (Science, DOI: 10.1126/ guides reactants to a product with S stereochemistry. In another position (right), adducts with R science.1199844). The motor stereochemistry form. (C = turquoise, N = blue, F = purple, O = red, S = gray, H = white) molecule can make a racemate, R enantiomer, or S enantiomer on demand, depending upon where it is in its rotary cycle. “As far as we know the system is unique,” says Ben L. Feringa, the University of Groningen chemistry professor who created the motor with coworker Jiaobing Wang. Previous examples of chiral catalysts with this kind of switching ability rely on solvent changes, temperature variation, or addition of a Lewis acid, Feringa says, and their catalytic behavior is usually difficult to predict. This molecular motor catalyst, he continues, allows his team to control the ternal signal,” he says. “The unidirectional light-driven stereoselectivity of the reaction through noninvasive motor gave us a nice opportunity to realize such adapexternal signals—light and heat. tive functional behavior.” “Molecular motors provide us with a great starting “The fact that Wang and Feringa have designed an point for a multistage chiral catalyst design,” Feringa integrated supramolecular system that brings together points out. “They have the intrinsic ability to change molecular recognition, chirality chirality in response to light.” As a result, he says, retransfer, catalysis, stereoelectronic searchers can control how the motors turn in order to control, and enantioselectivity is “Molecular motors provide make products of different stereochemistries. remarkable to say the least,” comus with a great starting The molecular motor catalyst consists of two arms ments Northwestern University point for a multistage that turn past one another in a unidirectional manchemistry professor and molecular chiral catalyst design.” ner in four discrete steps. One arm is equipped with machine expert Sir J. Fraser Stoda 4-dimethylaminopyridine (DMAP) moiety while dart. “The realization, however, that —BEN L. FERINGA the other features a thiourea group. When these two all of these processes can be enabled groups are pointed away from each other, the catalytic or disabled by attenuating catalytic activity in one fell motor generates a racemic mixture of products in the swoop in ways that are internally consistent in an abconjugate addition of 2-methoxythiophenol to cyclosolute, as well as a relative, stereochemical sense raises hexenone. the creativity bar in synthetic chemistry onto a whole Shine light on the catalyst and its double-bond axle new level of sophistication.” isomerizes, bringing the arms and their cooperative “I found this combination of motor and catalytic catalytic groups into close proximity. Here the thiofunction ingenious,” adds Boston College chemistry urea group holds the cyclohexenone in place while the professor and molecular motor maker T. Ross Kelly. “It DMAP unit guides the thiophenol to stereoselectively is especially gratifying when an idea this pretty actually form the addition product with S stereochemistry in an works.”—BETHANY HALFORD WWW.CEN-ONLINE.ORG
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FEBRUARY 14, 2011
SCIENCE
NEW SPIN ON STEREOCONTROL
