Bassan Arianna, Blomberg Margareta R A, Siegbahn Per E M, Que Lawrence
Department of Physics, SE-106 91, Stockholm, Sweden.
Chemistry. 2005 Jan 7;11(2):692-705. doi: 10.1002/chem.200400383.
The reactivity of [HO-(tpa)Fe(V)=O] (TPA=tris(2-pyridylmethyl)amine), derived from O-O bond heterolysis of its [H(2)O-(tpa)Fe(III)-OOH] precursor, was explored by means of hybrid density functional theory. The mechanism for alkane hydroxylation by the high-valent iron-oxo species invoked as an intermediate in Fe(tpa)/H(2)O(2) catalysis was investigated. Hydroxylation of methane and propane by HO-Fe(V)=O was studied by following the rebound mechanism associated with the heme center of cytochrome P450, and it is demonstrated that this species is capable of stereospecific alkane hydroxylation. The mechanism proposed for alkane hydroxylation by HO-Fe(V)=O accounts for the experimentally observed incorporation of solvent water into the products. An investigation of the possible hydroxylation of acetonitrile (i.e., the solvent used in the experiments) shows that the activation energy for hydrogen-atom abstraction by HO-Fe(V)=O is rather high and, in fact, rather similar to that of methane, despite the similarity of the H-CH(2)CN bond strength to that of the secondary C-H bond in propane. This result indicates that the kinetics of hydrogen-atom abstraction are strongly affected by the cyano group and rationalizes the lack of experimental evidence for solvent hydroxylation in competition with that of substrates such as cyclohexane.
通过杂化密度泛函理论研究了由[H₂O-(tpa)Fe(III)-OOH]前体的O-O键异裂衍生出的[HO-(tpa)Fe(V)=O]（TPA = 三(2-吡啶甲基)胺）的反应活性。研究了在Fe(tpa)/H₂O₂催化中作为中间体的高价铁氧物种使烷烃羟基化的机理。通过遵循与细胞色素P450血红素中心相关的反弹机理，研究了HO-Fe(V)=O对甲烷和丙烷的羟基化作用，结果表明该物种能够进行立体选择性烷烃羟基化。所提出的HO-Fe(V)=O使烷烃羟基化的机理解释了实验观察到的产物中溶剂水的掺入情况。对乙腈（即实验中使用的溶剂）可能的羟基化作用的研究表明，HO-Fe(V)=O夺取氢原子的活化能相当高，实际上与甲烷的活化能相当，尽管H-CH₂CN键强度与丙烷中仲C-H键的强度相似。这一结果表明，夺取氢原子的动力学受到氰基的强烈影响，并解释了在与环己烷等底物竞争时缺乏溶剂羟基化实验证据的原因。
