Molecular Machines with a Robotic Arm that can Control the Stereochemistry of Catalyzed Reactions
Final Report Abstract
The initially proposed machine could not be realized. The key point of the project was the loading and reloading that was supposed to extend the scope of this kind of machines. It was not possible to introduce a reloading chemistry that is orthogonal to the machine’s operation (iminium/enamine activation). Therefore, the project was discontinued and the two aspects of the project are pursued independently. A machine with an extendable, robotic arm was proposed to realize sophisticated transport on the molecular level. The machine should be able to deposit and pick up a small molecule cargo from four chemically equivalent stations. The targeted positioning is achieved by applying two independent stimuli. Protonation/deprotonation will rotate the robotic arm by 180°. Light switching will extend/contract the robotic arm to reach stations far away or close by, respectively. The majority of the synthetic work has been completed for this project. The loading and operation of the first machine (a half-machine) is currently under investigation. A new concept towards controlling the enantioselectivity of reactions using a switchable rotaxane has been introduced. The rotaxane contains a pseudo-meso pyrrolidine unit for iminium and enamine activation. The position of the macrocycle determines which enantiomer is produced. Equivalent to the initially proposed machine, all four diastereomers of the iminium-enamine cascade reaction should be accessible using the appropriate program of inputs. A reliable synthetic route to obtain the switchable rotaxanes was established. The proposed concept proved to work. The enantiomeric outcome of enamine and iminium activated reactions can be inverted by switching the rotaxane. However, the enantiomeric excess is not yet sufficient to be particularly useful for a cascade process. The current focus is to improve the efficiency of the chiral expression by tweaking the catalyst’s design and the reaction conditions.
