Thermodynamic and kinetic parameters associated with the fluxional behavior of 2-methyl- and 2,6-dimethyltroponeiron tricarbonyl complexes

The kinetic and thermodynamic parameters for regioisomerisation of 2-methyl- and 2,6-dimethyl-derivatives of tricarbonyl[η4-tropone]iron complexes have been studied by 1H NMR spectrometry over a range of 40 °C. Regioisomerisation of these complexes proceeds by an intramolecular first-order process and results in the almost complete conversion of the less stable complexes (4, 8) to more stable regioisomers (1, 5). The activation energies and half lifes for the conversion (4 → 1) and (8 → 5) were found to be ΔG#=92 kJ mol−1; τ1/2=12.8 h, and ΔG#=107 kJ mol−1; τ1/2=26.8 h, respectively, at 23 °C. Complex 1 reacts with (1R,2S,5R)-menthol in sulphuric acid solution, followed by neutralisation with sodium carbonate to give a separable mixture of diastereomeric tricarbonyl[(2,3,4,5-η)-(1R′,2S′,5R′)-6-menthyloxy-2-methyltropone]iron complexes, 9 and 10. The corresponding dimethylated complex 5 fails to react under these conditions.