Proton-coupled electron transfer versus hydrogen atom transfer in benzyl/toluene, methoxyl/methanol, and phenoxyl/phenol self-exchange reactions
Proton-coupled electron transfer versus hydrogen atom transfer in benzyl/toluene, methoxyl/methanol, and phenoxyl/phenol self-exchange reactions
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DOI：
10.1021/ja012732c
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发表时间：
2002-09-18
影响因子：
15
通讯作者：
Borden, WT
中科院分区：
文献类型：
--
作者：
Mayer, JM;Hrovat, DA;Borden, WT
用密度泛函理论（DFT）计算了苯甲基自由基与甲苯，苯氧基自由基与苯酚，甲氧基自由基与甲醇的简并氢原子交换反应.第一和第三反应通过氢原子转移（HAT）机制发生。苄基/甲苯氢交换的过渡结构（TS）具有C-2 h对称性，对应于自由基的苄基碳上的2 p-pi轨道接近甲苯的苄基氢。在该TS中，以及在用于甲氧基/甲醇氢交换的类似的C-2 TS中，SOMO在位于前一反应中的沿着C-H矢量和几乎沿着O-H矢量的原子轨道中具有显著的密度。相比之下，在苯氧基/苯酚TS的SOMO是一个π对称轨道内的每个C6 H5 O单元，涉及2 p原子轨道上的氧原子，基本上是正交的O…H... O向量。该反应中的转移氢是质子，其是典型氢键的一部分，涉及苯氧基自由基的氧上的σ孤对和苯酚的O-H键。因为质子在氧的a轨道之间转移，而电子在氧的π轨道之间转移，所以该反应应该被描述为质子耦合电子转移（PCET）。PCET机制需要形成氢键，因此不能用于苄基/甲苯交换。苯氧基/苯酚发生的PCET，而甲氧基/甲醇交换发生的HAT的偏好被追溯到更大的PI的能力苯基甲基。与甲氧基/甲醇的PCET山顶相比，这导致苯氧基/苯酚的PCET过渡结构中的氧上的更大的电子密度，并且氧上的更大的电子密度通过提供转移质子的更大的结合能来选择性地稳定苯氧基/苯酚TS。
Degenerate hydrogen atom exchange reactions have been studied using calculations, based on density functional theory (DFT), for (i) benzyl radical plus toluene, (ii) phenoxyl radical plus phenol, and (iii) methoxyl radical plus methanol. The first and third reactions occur via hydrogen atom transfer (HAT) mechanisms. The transition structure (TS) for benzyl/toluene hydrogen exchange has C-2h symmetry and corresponds to the approach of the 2p-pi orbital on the benzylic carbon of the radical to a benzylic hydrogen of toluene. In this TS, and in the similar C-2 TS for methoxyl/methanol hydrogen exchange, the SOMO has significant density in atomic orbitals that lie along the C-H vectors in the former reaction and nearly along the O-H vectors in the latter. In contrast, the SOMO at the phenoxyl/phenol TS is a pi symmetry orbital within each of the C6H5O units, involving 2p atomic orbitals on the oxygen atoms that are essentially orthogonal to the O...H...O vector. The transferring hydrogen in this reaction is a proton that is part of a typical hydrogen bond, involving a sigma lone pair on the oxygen of the phenoxyl radical and the O-H bond of phenol. Because the proton is transferred between oxygen a orbitals, and the electron is transferred between oxygen pi orbitals, this reaction should be described as a proton-coupled electron transfer (PCET). The PCET mechanism requires the formation of a hydrogen bond, and so is not available for benzyl/toluene exchange. The preference for phenoxyl/phenol to occur by PCET while methoxyl/methanol exchange occurs by HAT is traced to the greater pi donating ability of phenyl over methyl. This results in greater electron density on the oxygens in the PCET transition structure for phenoxyl/phenol, as compared to the PCET hilltop for methoxyl/methanol, and the greater electron density on the oxygens selectively stabilizes the phenoxyl/phenol TS by providing a larger binding energy of the transferring proton.