Abstract The gas-phase pyrolysis kinetics of tetrahydropyranyl phenoxy ethers: 2-(4-cyanophenoxy)tetrahydro-2H-pyran, and 2-(4-bromophenoxy)tetrahydro-2H-pyran were studied in a static system, with allyl bromide as surface-effect deactivator, and in the presence of toluene to inhibit chain reactions. The temperature and pressure were 350–440 °C and 25–89 Torr, respectively. The pyrolysis is homogeneous, unimolecular, and consistent with the first-order rate law yielding 3,4-dihydro-2H-pyran (DHP) and the corresponding 4-substituted phenol. The Arrhenius equations were found as follows: 2-(4-cyanophenoxy)tetrahydro-2H-pyran log k1 (s−1) = (14.76 ± 0.12) − (232.1 ± 1.3) kJ mol−1 (2.303 RT)−1 2-(4-bromophenoxy)tetrahydro-2H-pyran log k1 (s−1) = (14.08 ± 0.15) − (209.1 ± 1.8) kJ mol−1 (2.303 RT)−1 DFT theoretical calculations were performed to 2-(4-substituted-phenoxy) tetrahydro-2H-pyranes with groups H, OCH3, C(CH3)3, CN and Br. The calculated values for thermodynamic parameters using the WB97XD/6–311 + +G(d,p) level have a good agreement with the experimental values except for the cyano compound. Experimental and theoretical data suggest a mechanism with four-membered cyclic transition state (TS). The results indicate a small increment of rate constants values with raising electron donating ability groups in the phenoxy ring, and a slight decrease effect with electron withdrawing substituents. The effect of resonance structures giving by the phenyl ring seems to be the main factor in the TS stabilization. The assistance of the oxygen atom belonging to the phenoxy group in the pyranyl hydrogen abstraction exerts influence on the rate constants, but the polarization of Cδ+⋯δ−OPh bond is proposed as the rate determining process.

中文翻译：

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气相中 2-(4-苯氧基取代)四氢-2H-吡喃热解动力学和机理的实验和 DFT 研究

摘要 在静态系统中研究了四氢吡喃基苯氧基醚：2-(4-氰基苯氧基)四氢-2H-吡喃和 2-(4-溴苯氧基)四氢-2H-吡喃的气相热解动力学，以烯丙基溴为表面。 -效应钝化剂，并在甲苯存在下抑制链式反应。温度和压力分别为 350–440 °C 和 25–89 Torr。热解是均相的、单分子的，符合一级速率定律，产生 3,4-二氢-2H-吡喃 (DHP) 和相应的 4-取代苯酚。Arrhenius 方程如下： 2-(4-cyanophenoxy)tetrahydro-2H-pyran log k1 (s−1) = (14.76 ± 0.12) − (232.1 ± 1.3) kJ mol−1 (2.303 RT)−1 2 -(4-溴苯氧基)四氢-2H-吡喃 log k1 (s-1) = (14.08 ± 0.15) - (209.1 ± 1.8) kJ mol-1 (2. 对具有 H、OCH3、C(CH3)3、CN 和 Br 基团的 2-(4-取代苯氧基)四氢-2H-吡喃进行 303 RT)-1 DFT 理论计算。使用 WB97XD/6–311 + +G(d,p) 水平计算的热力学参数值与除氰基化合物外的实验值非常吻合。实验和理论数据表明了一种具有四元循环过渡态 (TS) 的机制。结果表明，随着苯氧基环中给电子能力基团的增加，速率常数值略有增加，吸电子取代基的影响略有下降。苯环产生的共振结构的影响似乎是 TS 稳定的主要因素。