Synthesis of a Mechanically Planar Chiral Rotaxane Ligand for Enantioselective Catalysis

Rotaxanes are interlocked molecules in which a molecular ring is trapped on a dumbbell-shaped axle because of its inability to escape over the bulky end groups, resulting in a so-called mechanical bond. Interlocked molecules have mainly been studied as components of molecular machines, but the crowded, flexible environment created by threading one molecule through another has also been explored in catalysis and sensing. However, so far, the applications of one of the most intriguing properties of interlocked molecules, their ability to display stereogenic units that do not rely on the stereochemistry of their covalent subunits, termed “mechanical chirality,” have yet to be properly explored, and prototypical demonstration of the applications of mechanically chiral rotaxanes remain scarce. Here, we describe a mechanically planar chiral rotaxane-based Au complex that mediates a cyclopropanation reaction with stereoselectivities that are comparable with the best conventional covalent catalyst reported for this reaction. Molecules that exist in non-identical mirror image forms are referred to as chiral. Chirality can arise because of various molecular features in which atoms are held in fixed orientations that are themselves chiral, and typically such “stereogenic units” are maintained by direct bonds between atoms. Molecular chirality can also arise by threading a dumbbell-shaped molecule through a molecular ring to generate a rotaxane. However, these molecules have not been investigated significantly because until recently they were extremely hard to make in one mirror image form. Here, we report the first example of a catalyst based on such a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in healthcare and other areas. Our results demonstrate that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We report an enantioselective catalyst based on a “mechanically chiral” rotaxane. Catalysis with chiral molecules is extremely important in modern chemistry because it is one of the most efficient ways to make chiral molecules for applications in many areas. Our results demonstrate, for the first time, that mechanically chiral molecules are a promising and underexplored platform for generating such catalysts. We achieve enantioselectivities for the Au^I-catalyzed Ohe-Uemura cyclopropanation of benzoate esters comparable to previously reported covalent catalysts.