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

Proton NMR showed that, at equilibrium and at 25 degreesC, asymmetric enoli zation in the direction furthest from the ferrocenyl group is dominant for the beta -diketones 1-ferroclnyl-4,4,4-trifluoro-1,3-butanedione (ferroceno yltrifluoroacetone, Hfctfa), 1-ferrocenyl-4,4,4-trichloro-l,3-butanedione ( ferrocenoyltrichloroacetone, Hfctca), 1-ferrocenyl-1,3-butanedione (ferroce noplacetone, Hfca), 1,3-diferrocenyl-1,3-propanedione (diferrocenoylmethane , Hdfcm) and 1-ferrocenyl-3-phenyl-1,3-propanedione (benzoylferrocenoylmeth ane, Hbfcm). This finding is considered to be the result of resonance drivi ng forces rather than inductive electronic effects of substituents on the p seudo-aromatic beta -diketone core. Lowering of the concentration of the be ta -diketones from 35 to below 1 mmol dm(-3) only slightly shifts the equil ibrium more towards the keto side. By increasing the temperature of the sol vent (CDCl3) from 20 to 60 degreesC, the percentage keto isomer at equilibr ium of Hdfcm (32.9-34.2%), Hfca (22.5-28.4%), Hbfcm (8.8-11.9%) and Hfetca (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 eno l isomer explains why, directly after isolation of newly prepared compounds , higher percentages of the keto isomer are observed. (C) 2001 Elsevier Sci ence B.V. All rights reserved.