Mechanical chirality gives new shape to catalysis

Researchers in the UK have used a mechanically planar chiral rotaxane as the basis for an enantioselective gold catalyst (Chem 2020, DOI: 10.1016/j.chempr.2020.02.006). Stephen Goldup and Andrew Heard at the University of Southampton say their work is a proof of concept that mechanical bonds can be used to build “pockets” for selective synthesis akin to those found in enzymes. Heard started by building and separating two mechanically chiral rotaxanes. Rotaxanes are mechanically interlocked molecules in which a macrocycle is threaded onto a dumbbell-shaped compound or axle. Heard’s chiral rotaxanes differ from one another by the direction that the macrocyles loop around the central axles. By adding gold to a phosphine ligand at one end of the axle, Heard created chiral catalysts for a cyclopropanation reaction (shown). Each catalyst gave a different enantiomeric product and worked about as well as current catalysts. However, Goldup doesn’t expect that synthetic chemists will be using his new catalyst just yet—not least because these chiral rotaxanes are hard to make and isolate. The team now hopes to show that chiral rotaxanes can help build enantioselective catalysts that are currently impossible to create otherwise—opening up new vistas of synthetic chemistry would make it well worth dealing with the fussy rotaxanes.