Kinetic studies on sec-alcohol racemization with dicarbonylchloro(pentabenzylcyclopentadienyl)- and dicarbonylchloro(pentaphenylcyclopentadienyl)ruthenium catalysts

Pentasubstituted cyclopentadienyl complexes of ruthenium R5CpRu(CO)(2)Cl (R=Ph, benzyl) form, upon activation with tBuOK, highly active catalysts for racemization of chiral sec-alcohols. In combination with suitable resolving enzymes, such catalyst systems can efficiently be utilized for dynamic kinetic resolution reactions providing chiral alcohols, after hydrolysis of the corresponding acetates, in high yields and high enantiomeric purities. Here, three such ruthenium complexes were first characterized by NMR spectroscopy and cyclic voltammetry analysis (CVA) for elucidating their electronic characteristics in detail. 5CpRu(CO)(2)Cl (Bn=benzyl) racemizes electron-rich substrates more efficiently and in most cases faster than its pentaphenyl substituted analogue, formerly often considered as the leading catalyst candidate for dynamic kinetic resolution applications. 5CpRu(CO)(2)Cl, in turn, is more efficient for electron-poor substrates.