Goh Yeong Mee, Nam Wonwoo
Department of Chemistry and Center for Cell Signaling Research, Ewha Womans University, Seoul 120-750, Korea.
Inorg Chem. 1999 Mar 8;38(5):914-920. doi: 10.1021/ic980989e.
High-valent iron(IV) oxo porphyrin cation radical complexes containing a series of substituents at the meso position of the porphyrin ring (i.e., electron-donating and -withdrawing substituents on phenyl groups) were prepared and used in oxygen atom transfer reactions to elucidate the electronic effect of porphyrin ligands on the reactivities of iron porphyrin complexes. The reactions that we studied with the in situ generated high-valent iron oxo porphyrins were (1) the relative reactivities of the intermediates toward oxygen atom transfer and ROOH disproportionation (ROOH = hydrogen peroxide and tert-butyl hydroperoxide), (2) the mechanism of heterolytic versus homolytic O-O bond cleavage of hydroperoxides, (3) the dependence of oxidizing power of the intermediates on the electronic nature of porphyrin ligands, and (4) the relative rates between oxygen atom transfer and oxygen exchange with labeled H(2)(18)O. We found from these reactivity studies that (1) a high-valent iron oxo porphyrin complex containing electron-donating substituents reacts fast with ROOH in a competitive reaction performed with a mixture of olefin and ROOH, whereas a high-valent iron oxo porphyrin containing electron-withdrawing substituents transfers its oxygen atom to olefin to give an epoxide product at a fast rate, (2) the O-O bond of hydroperoxides is homolytically cleaved by iron porphyrin complexes in aprotic solvent, (3) a high-valent iron oxo complex of electron-deficient porphyrin ligand is a more powerful oxidizing species than that of electron-rich porphyrin ligand in alkane hydroxylation reactions, and (4) the presence of electron-donating substituents on a porphyrin ligand gives a relatively high (18)O incorporation from labeled H(2)(18)O into an oxygenated product when a mixture of olefin and H(2)(18)O is added to a reaction solution containing a high-valent iron oxo intermediate, whereas only a small amount of (18)O incorporation is observed with iron porphyrin complexes containing electron-withdrawing substituents. These results clearly demonstrate that the electronic nature of iron porphyrin complexes is an important factor in determining the reactivities of iron porphyrin complexes in oxygen atom transfer reactions.
制备了在卟啉环中卟啉阳离子自由基络合物的中位含有一系列取代基（即苯基上的供电子和吸电子取代基）的高价铁（IV）氧代卟啉阳离子自由基络合物，并将其用于氧原子转移反应，以阐明卟啉配体对铁卟啉络合物反应活性的电子效应。我们用原位生成的高价铁氧代卟啉研究的反应包括：（1）中间体对氧原子转移和ROOH歧化反应（ROOH = 过氧化氢和叔丁基过氧化氢）的相对反应活性；（2）氢过氧化物的异裂与均裂O - O键断裂的机理；（3）中间体氧化能力对卟啉配体电子性质的依赖性；（4）氧原子转移与用标记的H₂¹⁸O进行氧交换之间的相对速率。我们从这些反应活性研究中发现：（1）含有供电子取代基的高价铁氧代卟啉络合物在与烯烃和ROOH的混合物进行的竞争反应中与ROOH快速反应，而含有吸电子取代基的高价铁氧代卟啉以快速速率将其氧原子转移到烯烃上以生成环氧化物产物；（2）在非质子溶剂中，氢过氧化物的O - O键被铁卟啉络合物均裂；（3）在烷烃羟基化反应中，缺电子卟啉配体的高价铁氧代络合物比富电子卟啉配体的高价铁氧代络合物是更强的氧化物种；（4）当将烯烃和H₂¹⁸O的混合物加入到含有高价铁氧代中间体的反应溶液中时，卟啉配体上供电子取代基的存在使得从标记的H₂¹⁸O到氧化产物中的¹⁸O掺入相对较高，而含有吸电子取代基的铁卟啉络合物仅观察到少量的¹⁸O掺入。这些结果清楚地表明，铁卟啉络合物的电子性质是决定铁卟啉络合物在氧原子转移反应中反应活性的一个重要因素。
