Fast Racemisation of Chiral Amines and Alcohols by Using Cationic Half-Sandwich Ruthena- and Iridacycle Catalysts

The lipase-catalysed resolution of alcohols and amines yields only 50% of the desired enantiopure product. However, addition of a racemisation catalyst leads to 100% yield in what is called a dynamic kinetic resolution (DKR). There is a need for new racemisation catalysts that are fast and compatible with the conditions of the enzymatic reaction. We show that cationic half-sandwich ruthena- and iridacycle complexes are highly active and efficient in the racemisation of chiral alcohols and amines. Upon activation with base, these complexes are able to selectively racemise alcohols, whereas the non-activated complexes are selective catalysts for the racemisation of amines. We have applied the iridacycles in the DKR of racemic beta-chloroalcohols to produce chiral epoxides in a biphasic system in good yields and high ee (ee = enantiomeric excess).