Structual, thermodynamic and kinetic consequences of a spectroscopic study of the equilibrium between isomeric forms of ferrocene-containing #beta#-diketones

摘要

Proton NMR showed that, at equilibrium and at 25°C, asymmetric enolization in the direction furthest from the ferrocenyl group is dominant for the !3-diketones I-ferrocenyl-4,4,4-trifluoro-l,3-butanedione (ferrocenoyltrifluoroacetone, Hfctfa), I-ferro- cenyl;.4,4,4-trichloro-l ,3-butanedione (feliocenoyltrichloroacetone, Hfctca), I-ferrocenyl-l,3-butanedione (ferrocenoylacetone, Hfca), 1 ,3-diferrocenyl-l ,3-propanedione ( diferrocenoylmethane, Hdfcm) and I-ferrocenyl-3-phenyl-l,3-propanedione (benzoylfer- rocenoylmethane, Hbfcm). This finding is considered to be the result of resonance driving forces rather than inductive electronic effects of substittlents on the pseudo-aromatic !3-diketone core. Lowering of the concentration of the !3-diketones from 35 to below I mmol dm -} only slightly shifts the equilibrium more towards the keto side. .By increasing the temperature of the solvent (CDC1}) 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.