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
作者:Stephen A. Chandler、Peter Hanson、Alec B. Taylor、Paul H. Walton、Allan W. Timms
DOI:10.1039/b006184k
日期:——
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.
在2-芳酰基苯基自由基的Pschorr环化过程中,通过1,5-氢转移发生可逆的重排,生成2-苯甲酰基芳基自由基。在2-(4-甲基苯甲酰基)苯基和2-苯甲酰基-5-甲基苯基自由基之间的平衡中,正向和反向反应的速率常数估计为(1.2 ± 0.2) × 106 s−1(293 K)。假设苯基自由基从苯中夺取氢的假设速率为1.6 × 10−2 dm3 mol−1 s−1,自由基转移的统计校正有效摩尔浓度为2.2 × 108 mol dm−3。相比之下,竞争性的环化反应尽管发生速率为(8.0 ± 0.9) × 105 s−1,但其有效摩尔浓度仅为5.3 mol dm−3。这些差异的原因通过反应机理进行了分析。