Understanding hydrogen atom transfer: from bond strengths to Marcus theory.
Understanding hydrogen atom transfer: from bond strengths to Marcus theory.
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DOI：
10.1021/ar100093z
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发表时间：
2011-01-18
影响因子：
18.3
通讯作者：
Mayer, James M.
中科院分区：
文献类型：
--
作者：
Mayer, James M.
氢原子转移（HAT）是最基本的化学反应之一：A - h + B→A + H-B。它是广泛的化学、环境和生物过程的关键步骤。传统的HAT涉及到像buo•这样的p区自由基从有机分子中提取H•。最近，人们认识到许多过渡金属物种经历HAT。这导致了一个更广阔的前景，HAT被视为一种质子耦合电子转移（PCET）。当过渡金属配合物通过去除H•（≡e -和H+）来氧化底物时，通常电子转移到金属上，质子转移到配体上。图中显示了两个例子：铁咪唑酸盐和钒氧配合物。虽然这些试剂不“看起来像”主基团自由基，但它们具有相同的反应性模式。例如，它们的HAT速率常数与一系列类似反应中的a - h键强度平行。就像主基团自由基一样，它们从O-H键中提取H•的速度比从相同强度的C-H键中提取H•的速度快得多。这表明驱动力不是反应性的唯一决定因素。我们发现HAT反应性可以用马库斯理论方法很好地描述。在最简单的模型中，交叉关系kAH/B = (kAH/AkBH/BKeqf)½，用自交换速率常数（AH + A的kAH/A）和平衡常数来预测AH + B的速率常数。对于各种过渡金属氧化剂，除了少数例外，kAH/B的预测值在一到两个数量级内。对于36个氧自由基有机反应，预测的kAH/B平均偏差为3.8倍，除6个反应外，其余反应的偏差均在5倍以内。这些反应包括O-H键或C-H键，发生在水或有机溶剂中，在Keq和kAH/B中超过1028和1013的范围。对O-H键有机反应的处理包括对HAT反应的动力学溶剂效应。这是简单交叉关系不包括的一些次要效应之一，如氢隧穿和前体和后体配合物的参与。描述了各种案例研究，应用交叉关系并说明了一些这些附加问题。交叉关系的成功，尽管它是一个相当简化的处理，表明马库斯方法基于自由能和本征势垒捕获了HAT反应的大部分基本化学。从分析中得出的见解之一是，反应与ΔG°相关，而不是像长期以来假设的那样与键焓相关。同样，与一般的直觉相反，自由基的性质或氧化剂的自旋状态并不是HAT抽象能力的主要决定因素。HAT反应的内在障碍可以理解为，至少部分地，马库斯型内球重组能。交叉反应的本征势垒精确地由HAT自交换速率常数推导而来，这对于除电子转移以外的任何类型的化学反应都是一个显著的和前所未有的结果。因此，马库斯交关系为理解和预测HAT反应性提供了一个有价值的新框架。
Hydrogen atom transfer (HAT) is one of the most fundamental chemical reactions: A–H + B → A + H–B. It is a key step in a wide range of chemical, environmental, and biological processes. Traditional HAT involves p-block radicals such as tBuO• abstracting H• from organic molecules. More recently, it has been recognized that many transition metal species undergo HAT. This has led to a broader perspective, with HAT viewed as one type of proton-coupled electron transfer (PCET). When transition metal complexes oxidize substrates by removing H• (≡ e– and H+), typically the electron transfers to the metal and the proton transfers to a ligand. Two examples are shown in the Figure: iron-imidazolinate and vanadium-oxo complexes. Although such reagents do not “look like” main group radicals, they have the same pattern of reactivity. For instance, their HAT rate constants parallel the A–H bond strengths within a series of similar reactions. Just like main group radicals, they abstract H• much faster from O–H bonds than from C–H bonds of the same strength. This shows that driving force is not the only determinant of reactivity. We have found that HAT reactivity is well described using a Marcus-theory approach. In the simplest model, the cross relation, kAH/B = (kAH/AkBH/BKeqf)½, predicts the rate constant for AH + B in terms of the self-exchange rate constants (kAH/A for AH + A) and the equilibrium constant. For a variety of transition metal oxidants, kAH/B is predicted within one or two orders of magnitude with only a few exceptions. For 36 organic reactions of oxyl radicals, kAH/B is predicted with an average deviation of a factor of 3.8, and within a factor of 5 for all but six of the reactions. These reactions involve both O–H or C–H bonds, occur either in water or organic solvents, and over a range of 1028 in Keq and 1013 in kAH/B. The treatment of organic reactions of O–H bonds includes the well-established kinetic solvent effect on HAT reactions. This is one of a number of secondary effects that the simple cross relation does not include, such as hydrogen tunneling and the involvement of precursor and successor complexes. Various case studies are described, applying the cross relation and illustrating some of these additional issues. The success of the cross relation, despite it being a quite simplified treatment, shows that the Marcus approach based on free energies and intrinsic barriers captures much of the essential chemistry of HAT reactions. Among the insights derived from the analysis is that reactions correlate with ΔG°, not with bond enthalpies as has long been assumed. Also in contrast to common intuition, the radical character or spin state of an oxidant is not found to be a primary determinant of HAT abstracting ability. The intrinsic barriers for HAT reactions can be understood, at least in part, as Marcus-type inner-sphere reorganization energies. The intrinsic barriers for cross reactions are accurately derived from the HAT self-exchange rate constants, which is a remarkable and unprecedented result for any type of chemical reaction other than electron transfer. The Marcus cross relation thus provides a valuable new framework for understanding and predicting HAT reactivity.
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影响因子：
18.3
作者：
Hammes-Schiffer, Sharon
通讯作者：
Hammes-Schiffer, Sharon
影响因子：
15
作者：
Mayer, JM;Hrovat, DA;Borden, WT
通讯作者：
Borden, WT
影响因子：
4.6
作者：
Bryant, JR;Matsuo, T;Mayer, JM
通讯作者：
Mayer, JM
影响因子：
16.6
作者：
Eckert, Nathan A.;Vaddadi, Sridhar;Holland, Patrick L.
通讯作者：
Holland, Patrick L.
影响因子：
3.3
作者：
Marcus, R. A.
通讯作者：
Marcus, R. A.