Olefin cyclopropanation catalysed by half-sandwich ruthenium complexes

Ruthenium complexes of the type [RuX(Cp′)(PPh3)2] (X=Cl and H; Cp′=Cp, Cp*, indenyl, and carboranyl) efficiently catalyse olefin cyclopropanation with diazoesters, and the cis/trans stereoselectivity of the resulting cyclopropanes strongly depends on the Cp′ ligand. With [RuCl(Cp*)(PAr3)2] complexes, cyclopropanation competes with the formal carbene insertion into C–H vinyl bonds of styrene, whereas ring-opening metathesis polymerisation takes place with norbornene, lending support to the formation of ruthenium–carbene and ruthenacyclobutanes as intermediates in these reactions.