Neutral chiral cyclopentadienyl Ru(ii)Cl catalysts enable enantioselective [2+2]-cycloadditions† †Electronic supplementary information (ESI) available: Experimental procedures and characterization of all new compounds. CCDC 1499170 and 1499171. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc05092a Click here for additional data file. Click here for additional data file.

Cyclopentadienyl ruthenium(II) complexes with a large number of available coordination sites are frequently used catalysts for a broad range of transformations. To be able to render these transformations enantioselective, we have designed a chiral neutral CpRu(II)Cl complex basing on an atropchiral cyclopentadienyl (Cp) ligand which is accessed in a streamlined C–H functionalisation approach. The catalyst displays excellent levels of reactivity and enantioselectivity for enantioselective [2+2]-cycloadditions leading to strained chiral cyclobutenes, allowing for catalyst loadings as low as 1 mol%. A very strong counterion effect of a bound chloride anion transforms the corresponding unselective cationic complex into a highly enantioselective neutral version. Moreover, by adding norbornadiene at the end of the reaction the catalyst can be recovered and subsequently reused.