Highly Selective Ortho‐Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO 2 or COS – First Insight into Co‐ordination Chemistry of New Ambident Ligands

This research presents the highly regioselective syntheses of 1,2‐dicarboxylated cyclopentadienide salts [Cat] 2 [C 5 H 3 (CO 2 ) 2 H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO 2 Me (Cat=NR 4 + , PR 4 + , Im + ) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR 4 + , PR 4 + , Im + ) with CO 2 . 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 1 H 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 CO 2 activating role for the second kinetically accelerated CO 2 addition step exclusively in ortho position. The same and related thiocarboxylates [Cat] 2 [C 5 H 3 (COS) 2 H] are obtained by reaction of COS with Cat[Cp] (Cat=NR 4 + , PR 4 + , Im + ). A preliminary study on [Cat] 2 [C 5 H 3 (CO 2 ) 2 H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo 0 , Ru 2+ ) and hard Lewis acids (Al 3+ , La 3+ ).