Structural, thermodynamic and kinetic consequences of a spectroscopic study of the equilibrium between isomeric forms of ferrocene-containing β-diketones

Proton NMR showed that, at equilibrium and at 25°C, asymmetric enolization in the direction furthest from the ferrocenyl group is dominant for the β-diketones 1-ferrocenyl-4,4,4-trifluoro-1,3-butanedione (ferrocenoyltrifluoroacetone, Hfctfa), 1-ferrocenyl-4,4,4-trichloro-1,3-butanedione (ferrocenoyltrichloroacetone, Hfctca), 1-ferrocenyl-1,3-butanedione (ferrocenoylacetone, Hfca), 1,3-diferrocenyl-1,3-propanedione (diferrocenoylmethane, Hdfcm) and 1-ferrocenyl-3-phenyl-1,3-propanedione (benzoylferrocenoylmethane, Hbfcm). This finding is considered to be the result of resonance driving forces rather than inductive electronic effects of substituents on the pseudo-aromatic β-diketone core. Lowering of the concentration of the β-diketones from 35 to below 1 mmol dm−3 only slightly shifts the equilibrium more towards the keto side. By increasing the temperature of the solvent (CDCl3) from 20 to 60°C, the percentage keto isomer at equilibrium of Hdfcm (32.9–34.2%), Hfca (22.5–28.4%), Hbfcm (8.8–11.9%) and Hfctca (4.9–9.8%) increased but it decreased the keto percentage of Hfctfa from 3.2 to below 0.7%. Slow conversion kinetics from the keto to the dominant enol isomer explains why, directly after isolation of newly prepared compounds, higher percentages of the keto isomer are observed.