One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer
One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer
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DOI：
10.1021/ja031583q
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发表时间：
2004-10-13
影响因子：
15
通讯作者：
Mayer, JM
中科院分区：
文献类型：
--
作者：
Rhile, IJ;Mayer, JM
用一系列单电子氧化剂在MeCN中氧化制备了氢键苯酚2-（氨基二苯甲基）-4,6-二叔丁基苯酚（HOAr-NH2）。产物是苯氧基，其中酚质子转移到胺，•OAr-NH3+。hoar - nh2与tris(p- toyl) amium （[N(tol)3]•+）反应生成•OAr-NH3++ N(tol)3hasKeq= 2.0±0.5，符合二级动力学，k=（1.1±0.2）× 105M-1s-1（ΔG⧧= 11 kcal mol-1），初级同位素效应kh /kD= 2.4±0.4。hoar - nh2与[N(C6H4Br)3]•+的氧化速度更快，为k × 4 × 107M-1s-1。同位素效应、热化学参数和速率对驱动力的依赖（ΔΔG⧧/ΔΔG°= 0.53）都表明，hoar - nh2的电子转移必须与分子内质子从苯酚到胺的转移（质子耦合电子转移，PCET）一致发生。这些数据排除了由初始电子转移形成苯酚自由基阳离子•+ hoar - nh2或由初始质子转移形成两性离子oar - nh3 +的阶梯路径。电子转移速率常数与驱动力的关系可以用绝热Marcus方程拟合，得到一个大的本态势垒：λ = 34 kcal mol-1。
The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine,•OAr-NH3+. The reaction ofHOAr-NH2and tris(p-tolyl)aminium ([N(tol)3]•+) to give•OAr-NH3++ N(tol)3hasKeq= 2.0 ± 0.5, follows second-order kinetics withk= (1.1 ± 0.2) × 105M-1s-1(ΔG⧧= 11 kcal mol-1), and has a primary isotope effectkH/kD= 2.4 ± 0.4. Oxidation ofHOAr-NH2with [N(C6H4Br)3]•+is faster, withk≅ 4 × 107M-1s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (ΔΔG⧧/ΔΔG° = 0.53) all indicate that electron transfer fromHOAr-NH2must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation•+HOAr-NH2or that involve initial proton transfer to give the zwitterion-OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: λ = 34 kcal mol-1for reactions ofHOAr-NH2with NAr3•+.