Lansky David E, Goldberg David P
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Inorg Chem. 2006 Jun 26;45(13):5119-25. doi: 10.1021/ic060491+.
High-valent metal-oxo complexes are postulated as key intermediates for a wide range of enzymatic and synthetic processes. To gain an understanding of these processes, the reactivity of an isolated, well-characterized Mn(V)-oxo complex, (TBP8Cz)MnVO (1), (TBP8Cz = octakis(para-tert-butylphenyl)corrolazinato(3-)) has been examined. This complex has been shown to oxidize a series of substituted phenols (4-X-2,6-t-Bu2C6H2OH, X = C(CH3)3 (3), H, Me, OMe, CN), resulting in the production of phenoxyl radicals and the MnIII complex [(TBP8Cz)MnIII] (2). Kinetic studies have led to the determination of second-order rate constants for the phenol substrates, which give a Hammett correlation ((log k''x/k''H) vs sigmap+) with rho = -1.26. A plot of log k versus BDE(O-H) also reveals a linear correlation. These data, combined with a KIE of 5.9 for 3-OD, provide strong evidence for a concerted hydrogen-atom-abstraction mechanism. Substrates with C-H bonds (1,4-cyclohexadiene and 9,10-dihydroanthracene) are also oxidized via H-atom abstraction by 1, although at a much slower rate. Given the stability of 1, and in particular its low redox potential, (-0.05 V vs SCE), the observed H atom abstraction ability is surprising. These findings support a hypothesis regarding how certain heme enzymes can perform difficult H-atom abstractions while avoiding the generation of high-valent metal-oxo intermediates with oxidation potentials that would lead to the destruction of the surrounding protein environment.
高价金属氧配合物被认为是广泛的酶促和合成过程中的关键中间体。为了理解这些过程，人们对一种分离得到的、表征明确的Mn(V)-氧配合物(TBP8Cz)MnVO (1)（TBP8Cz = 八(对叔丁基苯基) 卟嗪(3 -)）的反应活性进行了研究。已证明该配合物能氧化一系列取代酚（4-X-2,6-二叔丁基苯酚，X = C(CH3)3 (3)、H、Me、OMe、CN），生成苯氧自由基和MnIII配合物[(TBP8Cz)MnIII] (2)。动力学研究确定了酚底物的二级速率常数，这些常数给出了哈米特相关性（(log k''x/k''H) 对σp+），其中ρ = -1.26。log k对BDE(O-H)的作图也显示出线性相关性。这些数据，再加上3-OD的动力学同位素效应为5.9，有力地证明了协同氢原子夺取机制。含有C-H键的底物（1,4-环己二烯和9,10-二氢蒽）也通过1的氢原子夺取被氧化，不过速率要慢得多。鉴于1的稳定性，特别是其低氧化还原电位（相对于饱和甘汞电极，为-0.05 V），观察到的氢原子夺取能力令人惊讶。这些发现支持了一个关于某些血红素酶如何在避免生成具有会导致周围蛋白质环境破坏的氧化电位的高价金属氧中间体的情况下进行困难的氢原子夺取的假说。
