Extending the range of pentasubstituted cyclopentadienyl compounds: The synthesis of a series of tetramethyl(alkyl or aryl)cyclopentadienes (Cp*R), their iridium complexes and their catalytic activity for asymmetric transfer hydrogenation

Tetramethyl(alky or aryl)cyclopentadienes were synthesized and the organometallic half-sandwich iridium complexes of the form [(eta(5)-Cp*(R))Ir(aa)Cl], Cp*(R) = tetramethyl(phenyl)cyclopentadienyl (Cp*(Ph)), tetramethyl(benzyl)cyclopentadienyl (Cp*(Bn)), tetramethyl(2-propyl)cyclopentadienyl (Cp*(iPr)), or tetramethyl(cyclohexyl)cyclopentadienyl (Cp*(Cy)) were prepared and characterized. The complexes adopt a piano stool configuration, forming diastereomers, with ratios similar to reported [(eta(5)-Cp*(R))Ir(aa)Cl] complexes. The complexes display an intermolecular hydrogen bonding network in the solid state. These complexes were tested for the asymmetric transfer hydrogenation of several ketones, showing that the R of the Cp*(R) drastically impacts both selectivity and rate of reaction. Additionally, severe solvent effects are displayed when the reaction media is changed from aqueous to organic solvent. (C) 2014 Elsevier Ltd. All rights reserved.