Mechanically axially chiral catenanes and noncanonical mechanically axially chiral rotaxanes

Abstract

Chirality typically arises in molecules because of a rigidly chiral arrangement of covalently bonded atoms. Less generally appreciated is that chirality can arise when molecules are threaded through one another to create a mechanical bond. For example, when two macrocycles with chemically distinct faces are joined to form a catenane, the structure is chiral, although the rings themselves are not. However, enantiopure mechanically axially chiral catenanes in which the mechanical bond provides the sole source of stereochemistry have not been reported. Here we re-examine the symmetry properties of these molecules and in doing so identify a straightforward route to access them from simple chiral building blocks. Our analysis also led us to identify an analogous but previously unremarked upon rotaxane stereogenic unit, which also yielded to our co-conformational auxiliary approach. With methods to access mechanically axially chiral molecules in hand, their properties and applications can now be explored.

Despite mechanically axially chiral (MAC) catenanes being recognized in 1961, their stereoselective synthesis had not been disclosed until now. Closer inspection of the MAC stereogenic unit has also led to the identification of an analogous, but unremarked upon, form of rotaxane stereochemistry and the conceptualization of a general approach to prepare MAC molecules stereoselectively.