Synthesis and aromatisaion reactions of arene hydrates and cis-dihydrodiols

Abstract

The aim of this work was to synthesis a range of substituted arene hydrates, determine their second-order rate constant for dehydration and generate a Hammett plot for comparison with other published p-values for similar carbocation-forming reactions. The first hydrate presented is the methyl benzoate hydrate (see section 2.1). A number of first order rate constants for the aromatisation (dehydration) reaction were determined in dilute perchloric acid at ionic strength 0.5 maintained with sodium perchlorate. The reaction was followed by UV-Vis spectrophotometry and (^1)H NMR spectroscopy. The second order rate constant was determined to be 9.32 X l0(^-2) М(^-1)s(^-1) which corresponds to a half-life of 7 seconds in 1M HCIO(_4). The arene hydrate of biphenyl was also synthesised (see section 2.2). The solubility issues discussed in section 2.2.2 meant the aromatisation reaction could not be followed by (^1)H NMR spectroscopy. The first order rate constants were determined in acetate and phosphate buffers, at ionic strength 0.5, maintainted with sodium Perchlorate, from pH 5 to 7. The second order rate constant was determined to be 2.11 X 10(^2) M(^-1)s(^-1) which corresponds to a half-life of 3.3 X 10(^-4)s in 1M HCIO(_4). As presented in sections 2.3-2.7, the synthetic route to the alkyl substituted hydrates gave two products - the ortho and the ipso hydrates. In the case of the ethyl substituted hydrate only the ortho hydrate was synthesised. For R = tBu, only a small proportion of the products formed was the ipso hydrate due to Its reactivity. The ortho and ipso hydrates of toluene and cumene were both synthesised. These could not be separated and so the aromatisation kinetics were followed in situ and fitted to a double exponential equation. The aromatisation reactions were followed by UV-vis spectrophotometry in acetate and phosphate buffers at 25 С and ionic strength 0.5 M, maintained with sodium perchlorate. The second-order rate constants for aromatisation for the ortho-hydrates were determined to be for R = Me, Et, iPr, tBu and are 514, 538, 642 and 949 M (^-1)s(^-1), respectively. The second-order rate constants for aromatisation for the ipso-hydrates isolated were determined to be for R = Me and iPr are 9.81x10(^3) and 1.47 X 10(^4) (^-1)s(^-1). A number of linear free energy correlations were attempted and the best correlation was found with σ(^+), this is consistent with a reaction involving a planar carbocation with through-bond stabilisation. The p-value was determined to be -6.5. The published p- value for the cis dihydrodiols, where a better correlation with Op was reported, is -8.2(^1). The magnitude of p suggests the hydrates have an earlier transition state to carbocation formation than the diols with less positive charge build-up. The computational results show that the carbocation intermediate formed during the aromatisation reaction of the hydrates is planar whereas the carbocation intermediate generated from the diols is puckered. This corroborates the results from the kinetic analysis and also the magnitude and sign of the p-values from the Hammett correlations. When the carbocation intermediate is puckered, the through-bond stabilisation is hindered and so a poor correlation with σ(^+) is observed. The rate constant for acid-catalysed isomerisation of optically active cis-indene dihydrodiol was determined. This represents the rate constant for formation of the corresponding carbocation intermediate. The second-order rate constant for carbocation formation (k(_H)) was determined by (^1)H NMR spectroscopy in concentrated perchloric acid to be 1.11 X 10(-6) M (^-1)s(^-1). This is comparable with the second-order rate constant for carbocation formation in teri-butanol which is 1.4 X 10(^-6) M(^-1)s(^-1). The rate constant for reaction of the carbocation intermediate with water k(_H20) determined using the azide- trapping technique, is 4.99 X 10(^8) s(^-1). Combining k(_H20) with k(_H) allows the (_p)K(_R) of the indene dihydrodiol carbocation to be calculated. The (_p)K(_R) was determined to be -14.6. This is greater than the (_p)K(_R) of the indanol carbocation. The effect of the adjacent hydroxyl group counteracts the stabilising effect of the benzylic substituent.