Direct observation of β-fluoro-substituted 4-methoxyphenethyl cations by laser flash photolysis

Abstract

4-Methoxyphenethyl cations bearing β-fluorine substituents [4-MeOC₆H₄C⁺R¹R² with R¹= H, R²= CH₂F (3), R¹= H, R²= CHF₂(4), R¹= H, R²= CF₃(5), R¹R²= CF₃(6)] are observed as transient intermediates following 248 nm laser photolysis of chloride or bromide precursors ArC(–X)R¹R² in 2,2,2-trifluoroethanol (TFE) and TFE–water mixtures. Corresponding phenethyl radicals are also observed; successive fluoro-substitution has remarkably little effect on the Absorbance(cation): Absorbance-(radical) ratio. Rate constants k_s for the decay of the cations have been directly measured, and compared with those of non-fluorinated analogues. In TFE–water mixtures the reactivity order is ArCH⁺CHF₂ > ArCH⁺CH₂F > ArCH⁺CF₃ > ArCH⁺CH₃(1) > ArC⁺(CH₃)₂(2) > ArC⁺(CF₃)₂, but the differences are small, there being little more than a two-order of magnitude decrease across the series. This remarkably small effect of fluorine had previously been observed in an examination of azide:solvent partitioning ratios k_az:k_s. Rate constants k_az have also been obtained in this work, and, with the exception of 2 which reacts somewhat more slowly, these are constant, with values consistent with the azide : cation combination occurring at the diffusion limit for these cations, as had been assumed in the previous work. The small, complex effects of fluoro substitution on the reactivities of these cations have been explained by a model which has, as a key component, an effect of electron withdrawing substituents of forcing the positive charge away from the benzylic carbon to the oxygen of the 4-methoxy group. Evidence for a significant perturbation of the π electron density is available in the present work, in that the absorption maxima of the α-CF₃and bis-α-CF₃ cations 5 and 6 are shifted 20 nm and 50 nm, respectively, to lower wavelengths than the corresponding α-methyl derivatives 1 and 2. Studies of 6 in 100% aqueous solution reveal a second, slower, transformation following the decay of the cation. Product studies have previously indicated that a fraction of the hydration of this cation occurs at the 4-position of the aromatic ring, producing a quinone methide and products arising from such a species. An unobserved hemiacetal is an intermediate in this sequence. From spectral and kinetic considerations the second process observed with flash photolysis has been identified as representing the breakdown of the hemiacetal to the quinone methide. A complete rate–pH profile for this reaction has been obtained, from which a pK_a of 11.0 has been determined for the ionization of the hemiacetal.