Pan Zhengzheng, Newcomb Martin
Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA.
Inorg Chem. 2007 Aug 6;46(16):6767-74. doi: 10.1021/ic700395j. Epub 2007 Jul 14.
The kinetics of the reactions of three porphyrin-iron(IV)-oxo derivatives with alkenes and benzylic alcohols were measured. The iron-oxo systems studied were 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (2a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(IV)-oxo (2c). Species 2 were stable for hours at room temperature as dilute solutions in acetonitrile and reacted hundreds to thousands of times faster in the presence of high concentrations of substrates. Typical second-order rate constants determined from pseudo-first-order kinetic studies are 1-2 x 10(-2) M(-1) s(-1) for reactions with styrene and 3 x 10(-2) M(-1) s(-1) for reactions with benzyl alcohol. The reactivity order for the iron-oxo species was 2a > 2b > 2c, which is inverted from that expected on the basis of the electron demand of the porphyrin macrocycles, and the oxidation reaction was suppressed when excess porphyrin-iron(III) complex was added to reaction mixtures. These observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be an iron(IV)-oxo porphyrin radical cation, that is the true oxidant in the reactions. Analyses of the kinetics of oxidations of a series of para-substituted benzylic alcohols with Hammett sigma+ -substituent constants and with a dual-parameter method developed by Jiang (Jiang, X. K. Acc. Chem. Res. 1997, 30, 283) indicated that considerable positive charge developed on the benzylic carbons in the oxidation reactions, as expected for electrophilic oxidants, and also that substantial radical character developed on the benzyl carbon in the transition states.
测定了三种卟啉 - 铁（IV） - 氧衍生物与烯烃和苄醇反应的动力学。所研究的铁 - 氧体系分别为5,10,15,20 - 四（2,6 - 二氯苯基）卟啉 - 铁（IV） - 氧（2a）、5,10,15,20 - 四（2,6 - 二氟苯基）卟啉 - 铁（IV） - 氧（2b）和5,10,15,20 - 四（五氟苯基）卟啉 - 铁（IV） - 氧（2c）。2在室温下作为乙腈稀溶液可稳定存在数小时，在高浓度底物存在下反应速度加快数百至数千倍。由准一级动力学研究确定的典型二级速率常数，与苯乙烯反应时为1 - 2×10⁻² M⁻¹ s⁻¹，与苄醇反应时为3×10⁻² M⁻¹ s⁻¹。铁 - 氧物种的反应活性顺序为2a > 2b > 2c，这与基于卟啉大环电子需求所预期的顺序相反，并且当向反应混合物中加入过量的卟啉 - 铁（III）配合物时，氧化反应受到抑制。这些观察结果表明，反应涉及铁（IV） - 氧物种2的歧化反应以生成铁（III）物种和一种氧化程度更高的铁物种，推测为铁（IV） - 氧卟啉自由基阳离子，它是反应中的真正氧化剂。对一系列具有哈米特σ⁺取代常数的对 - 取代苄醇的氧化动力学进行分析，并采用蒋开发的双参数方法（蒋，X. K. Acc. Chem. Res. 1997, 30, 283）表明，氧化反应中苄基碳上产生了相当多的正电荷，这正如亲电氧化剂所预期的那样，并且在过渡态苄基碳上也产生了大量的自由基特征。
