Molecular motor acts as a switch for enantiomer-selective reactions
FOM workgroup leader Professor Ben Feringa and Dr Jiaobing Wang of the University of Groningen have discovered how to use a molecular motor as a catalyst system for enantiomer-selective synthesis reactions. The system can be adjusted to produce different types of chiral molecules. The research was published this month in the journal Science.
Many organic chemical molecules exist in two mirror-image variants, similar to a left and right hand. In chemical parlance, these are called enantiomer (opposing) or chiral (from the Greek word for hand) forms.
In an ordinary chemical synthesis, both enantiomeric forms occur in equal quantities, a so-called racemic mixture. Using catalysts, however, it is possible to make mixtures in which one of the two forms is present in far greater quantities, so-called enantiomer-selective synthesis. Feringa's department has done a lot of research in this area.
Medicines
Enantiomer-selective synthesis reactions are of major economic importance, particularly for the manufacture of medicines. Often, just one of the mirrored forms has a healing effect, whereas the other form is unnecessary or may even cause harmful side effects. In Feringa and Wang's newly published system, the position of the molecular motor determines the mirrored form produced.
Molecular motor
The molecular motor is an invention of Feringa dating from 1999. It is a molecule consisting of a fixed part (stator) and a rotating part (rotor) connected by a spindle. The rotor can be in any of four positions, three of which are important for the catalytic system.
Combination
The catalyst system described in the recent Science article connects the two lines of research in Feringa's group - enantiomer-selective catalysis and molecular motors. In the system two catalysts, which have long been known to have an enantiomer-selective effect, are joined at the ends of the stator and rotor. At the position where the catalysts are as far away from each other as possible (the so-called trans-position) a racemic mixture is produced. In a subsequent, nearly superimposed position (a cis-position) a given mirror-image variant is preferably produced, whereas in the next step in the rotation of the rotor (also a cis-position), the other mirror-image variant is produced.
Yield
In the Science article, the Groningen chemists demonstrate this using a model reaction: a Michael addition in which 1-methoxy thiophenol is linked to cyclohexenone. The reaction yield ranges from 49/51 percent (trans, racemic) to 75/25 (M-cis) and 23/77 (P-cis) percent of the enantiomeric forms.
The selectivity arises because the two components are each taken up separately by a catalytic end during the reaction process and so end up opposing each other at a particular angle. This angle varies with the position of the motor.
Feringa and Wang expect that the molecular motor - which in this case is used more as a switch than as a motor - can become an important design tool that will be used in the future to perform successive, multiple, enantiomerically selective reactions in which the catalyst can be adjusted at will.
Information
For more information please contact Prof. Ben Feringa, telephone +31 50 363 4278/8569.
Reference
http://www.sciencemag.org/content/early/2011/02/09/science.1199844.abstract