Joint Institute of Computational Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
J Org Chem. 2011 Apr 1;76(7):2195-203. doi: 10.1021/jo2000385. Epub 2011 Mar 7.
The phenyl-shift reaction for the β-radical of phenethyl phenyl ether (PhCH(2)ĊHOPh, β-PPE) is an integral step in the pyrolysis of PPE, which is a model compound for the β-O-4 linkage in lignin. We investigated the influence of natural occurring substituents (hydroxy, methoxy) on the reaction rate by calculating relative rate constants using density functional theory in combination with transition state theory, including anharmonic correction for low-frequency modes. The phenyl-shift reaction proceeds through an oxaspiro[2.5]octadienyl radical intermediate and the overall rate constants were computed invoking the steady-state approximation (its validity was confirmed). Substituents on the phenethyl ring have only little influence on the rate constants. If a methoxy substituent is located in the para position of the phenyl ring adjacent to the ether oxygen, the energies of the intermediate and second transition state are lowered, but the overall rate constant is not significantly altered. This is a consequence of the dominating first transition from reactant to intermediate in the overall rate constant. In contrast, o- and di-o-methoxy substituents significantly accelerate the phenyl-migration rate compared to β-PPE.
苯位迁移反应是苯乙基苯基醚（PhCH(2)ĊHOPh，β-PPE）β-自由基热解的重要步骤，β-PPE 是木质素 β-O-4 键的模型化合物。我们通过计算相对速率常数，使用密度泛函理论结合过渡态理论，包括低频模式的非谐校正，研究了天然取代基（羟基、甲氧基）对反应速率的影响。苯位迁移反应通过氧杂螺[2.5]辛二烯基自由基中间体进行，整体速率常数通过稳态近似法（其有效性已得到验证）计算。苯乙基环上的取代基对速率常数影响很小。如果甲氧基取代基位于苯环上与醚氧相邻的对位，中间体和第二个过渡态的能量降低，但整体速率常数没有明显改变。这是因为在整体速率常数中，从反应物到中间体的第一个过渡态占据主导地位。相比之下，对位和邻位二甲氧基取代基显著加速了苯环迁移速率。
