Available under a Creative Commons Attribution Non-Commercial Share Alike 4.0 International Licence
1.4 CHEMICAL SCIENCES
In this work, the nucleophilic reactions of ironcarbonyl coordinated cyclohexadienyl cations with water and hydroxide to produce the corresponding coordinated arene hydrate analogues was examined. These nucleophilic reactions are a key step in a synthetic route to convert arene cis-dihydrodiols to their trans-isomers via their tricarbonyliron coordinated complexes. The arene trans-dihydrodiols produced have significant potential to be used as chiral buildings blocks in synthetic chemistry and have the advantage that they are more stable then their cis-analogues The first cation studied was (п5-cyclohexadienyl)dicarbonyl-triphenylphosphineiron. An equilibrium constant, pKr, for formation of the coordinated arene hydrate (п4-exo-5-hydroxy-1,3-cyclohexadience)dicarbonyltriphenylphosphine-iron, from this coordinated cation was determined spectrophotometrically to be 9.9. Rate constants were measured for the hydrolysis of the cationic species, (п5-cyclohexadienyl)dicarbonyltriphenylphosphineiron, and for the ionisation of the corresponding hydrate, (п4-exo-5-hydroxy-1,3-cyclohexadiene)dicarbonyltriphenyl-phosphiheiron, allowing a pH-profile (log k versus pH) to be constructed. In the acidic region of the profile, an additional reaction was found to be occurring and was assigned to protonation at the iron atom. Evidence supporting the existence of a protonated iron complex was obtained by examining the reaction with trifluoroacetic acid of a related complex, (п4-cyclohexa-1,3-dienne)-dicarbonyltriphenylphosphineiron, by means of FTIR and 1H NMR spectroscopy. Similarly, the (п5-cyclohexadienyl) tricarbonyliron cation was studied. An equilibrium constant, pKr, of 4.8 was determined by both spectrophotometric and kinetic means. Equilibrium and kinetic solvent isotope effects were observed and the values measured were consistent with those expected for the hydrolysis of a carbocation. A spectrophotometric examination of the (п5-cyclohexadienyl)tricarbonyliron cation was studied. An equilibrium constant, pKr, of 4.8 was determined by both spectrophotometric and kinetic means. Equilibrium and kinetic solvent isotope effects were observed and the values measured were consistent with those expected for the hydrolysis of a carbocation. A spectrophotometric examination of the (n5-cyclohexadienyl)-tricarbonyliron cation in methanol provided a pKr of 1.0 for the methanolyis reaction. A complementary 1H NMR examination in deuterated methanol showed that the changes observed correspond to those observed in the UV spectra. From the 1H NMR study, it can be concluded that that the nucleophilic reaction of the coordinated cyclohexadienyl cations with the nucleophiles in this study provide the kinetically favoured exo-substituted product. A comparison of the rate constant for ionisation of the tricarboyliron coordinated exo-hyrdrate measured in this work with that for the endo-hydrate previously determined 85 showed that the exo-isomer reacted more than 10 6 times more quickly. It can be concluded that the difficult step in the route to convert arene cis-dihydrodiols to their trans isomers will be reaction of the coordinated endo diol to give the corresponding cation. Thus the best strategy for improving the process is to explore and optimise condition for conversion of cis-diol complexes to their carbocations. The equilibrium constants determined for both cations, (п5-cyclohexadienyl)tricarbonylioron, can be compared with that for the uncoordinated cyclohexadienyl cation (benzenonium ion) which is much less stable. The pKr for this cation was previously determined to -2.1 53. The difference in stability amounts to factors of 10 12 and 10 7 respectively. However this large difference in stability fails to reflect the difference in the reactivity of the species. The coordinated cations undergo a different reaction in aqueous solution to the uncoordinated cation, hydrolysis rather then deprotonation. When the acid dissociation constant, pKa of the cyclohexadienyl cation is compared with the pKr values of the coordinated species, differences in reactivity of nearly 48 kcal mol-1 and 40 kcal mol-1 are observed. Comparisons were also made between the cations studied in this work and other coordinated and uncoordinated cations, including the tropylium ion. It can be inferred that if these coordinated cations underwent deprotonation to form the corresponding coordinated benzene complexes, two of the double bonds would be coordinated and the one remaining double bond would be olefinic.
Galvin, Martin.Rate and equilibrium constants for reactions of coordinated cyclohexadienyl cations. Doctoral Thesis. Technological University Dublin. doi:10.21427/D7QG76