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

Abstract

Tetramethyl(alky or aryl)cyclopentadienes were synthesized and the organometallic half-sandwich iridium complexes of the form [(η5-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 [(η5-Cp∗)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.