A Proline-Squaraine Ligand Framework (Pro-SqEB) for Stereoselective Rhodium(II)-Catalyzed Cyclopropanations.

A proline-squaraine ligand (Pro-SqEB) that demonstrates high levels of stereoselectivity in olefin cyclopropanations when anchored to a Rh scaffold is introduced. High yields and enantioselectivities were achieved in the cyclopropanation of alkenes with diazo compounds in the presence of Rh(Pro-SqEB). Notably, the unique electronic and steric design of this catalyst enabled the use of polar solvents that are otherwise incompatible with most Rh complexes.