Highly Selective Ortho-Directed Dicarboxylation Of Cyclopentadiene By Methylcarbonates And Co2 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)[C5H3(CO2)(2)H] 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 H-1 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)(2)H] are obtained by reaction of COS with Cat[Cp] (Cat=NR4+, PR4+, Im(+)). A preliminary study on [Cat](2)[C5H3(CO2)(2)H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo-0, Ru2+) and hard Lewis acids (Al3+, La3+).