Structural, thermodynamic and kinetic consequences of a spectroscopic study of the equilibrium between isomeric forms of ferrocene-containing β-diketones
作者:W.C.(Ina) du Plessis、Wade L. Davis、Sarina J. Cronje、Jannie C. Swarts
DOI:10.1016/s0020-1693(01)00292-4
日期:2001.3
Proton NMR showed that, at equilibrium and at 25 degreesC, asymmetric enolization in the direction furthest from the ferrocenyl group is dominant for the beta -diketones 1-ferroclnyl-4,4,4-trifluoro-1,3-butanedione (ferrocenoyltrifluoroacetone, Hfctfa), 1-ferrocenyl-4,4,4-trichloro-l,3-butanedione (ferrocenoyltrichloroacetone, Hfctca), 1-ferrocenyl-1,3-butanedione (ferrocenoplacetone, 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 beta -diketone core. Lowering of the concentration of the beta -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 degreesC, the percentage keto isomer at equilibrium 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 enol isomer explains why, directly after isolation of newly prepared compounds, higher percentages of the keto isomer are observed. (C) 2001 Elsevier Science B.V. All rights reserved.