Highly Selective Ortho-Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO 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] [C H (CO ) H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO Me (Cat=NR , PR , Im ) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR , PR , Im ) with CO . 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 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 activating role for the second kinetically accelerated CO addition step exclusively in ortho position. The same and related thiocarboxylates [Cat] [C H (COS) H] are obtained by reaction of COS with Cat[Cp] (Cat=NR , PR , Im ). A preliminary study on [Cat] [C H (CO ) H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo , Ru ) and hard Lewis acids (Al , La ).