Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

Abstract

During the Pschorr cyclisation of 2-aroylphenyl radicals, a rearrangement occurs reversibly by 1,5-hydrogen transfer to give 2-benzoylaryl radicals. Rate constants of (1.2 ± 0.2) × 10⁶ s⁻¹ at 293 K are estimated for both the forward and back reactions in the equilibrium between 2-(4-methylbenzoyl)phenyl and 2-benzoyl-5-methylphenyl radicals. Assuming an empirical estimate of 1.6 × 10⁻² dm³ mol⁻¹ s⁻¹ for the hypothetical rate of abstraction of hydrogen from benzene by phenyl radicals, the radical translocation is calculated to occur with a statistically corrected effective molarity of 2.2 × 10⁸ mol dm⁻³. By contrast, the competing cyclisation, though occurring at a rate of (8.0 ± 0.9) × 10⁵ s⁻¹, exhibits an effective molarity of only 5.3 mol dm⁻³. The causes of these differences are analysed in terms of reaction mechanism.