Highly Selective Ortho-Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO2 or COS – First Insight into Co-ordination Chemistry of New Ambident Ligands

Abstract

This research presents the highly regioselective syntheses of 1,2-dicarboxylated cyclopentadienide salts [Cat]2[C5H3(CO2)2H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO2Me (Cat=NR4+, PR4+, Im+) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR4+, PR4+, Im+) with CO2. One characteristic feature of these dianionic ligands is the acidic proton delocalized in an intramolecular hydrogen bridge (IHB) between the two carboxyl groups, as studied by 1H NMR spectroscopy and XRD analyses. The reaction cannot be stopped after the first carboxylation. Therefore, we propose a Kolbe-Schmitt phenol-carboxylation related mechanism where the acidic proton of the monocarboxylic acid intermediate plays an ortho-directing and CO2 activating role for the second kinetically accelerated CO2 addition step exclusively in ortho position. The same and related thiocarboxylates [Cat]2[C5H3(COS)2H] are obtained by reaction of COS with Cat[Cp] (Cat=NR4+, PR4+, Im+). A preliminary study on [Cat]2[C5H3(CO2)2H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo0, Ru2+) and hard Lewis acids (Al3+, La3+).