Water:: The ideal hydrogen-atom source in free-radical chemistry mediated by TiIII and other single-electron-transfer metals?
Water:: The ideal hydrogen-atom source in free-radical chemistry mediated by TiIII and other single-electron-transfer metals?
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DOI：
10.1002/anie.200600831
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发表时间：
2006-01-01
影响因子：
16.6
通讯作者：
Oltra, J. Enrique
中科院分区：
文献类型：
--
作者：
Cuerva, Juan M.;Campana, Araceli G.;Oltra, J. Enrique
水与碳离子和碳正离子的反应性是众所周知的，但到目前为止，人们普遍认为水对自由基是惰性的这种假设的钝性归因于强大的HÀ OH键，其键解能为117.59 Æ 0.07 kcal molÀ1，[2]将阻止任何潜在的氢原子从水中转移。然而，几年前，我们偶然观察到叔自由基在双（环戊二烯基）氯化钛[3]（[Cp2TiCl]）和水的存在下被有效地还原这一观察结果进一步促进了二茂钛催化的自由基环化反应的最后一步的控制，这对于直接合成复杂的多环萜类化合物是有用的然而，在当时，水作为氢原子源的想法似乎是违反直觉的，这一现象通过调用烷基- tiiv配合物的形成和随后的水解或通过相当复杂的循环过渡态实际上的分子内氢转移来合理化。[4,5]我们现在有确凿的证据表明，水确实可以作为一个完整的氢原子源，而不是一个简单的质子供体，用于TiIII介导的自由基还原，可能还有其他通过单电子转移反应的金属。许多高度选择性的自由基反应已被开发出来，并已被证明在复杂有机化合物的全合成中非常有用，因为所需的温和条件和它们与许多官能团的相容性在此背景下，RajanBabu和Nugent提出了一个新概念：由[Cp2TiCl]诱导的均溶氧环打开该反应生成取代度最高（即最稳定）的β-钛氧自由基，在其他转化中，它可以被第二种[Cp2TiCl]捕获，通过“TiO”消除产生烯烃（环氧化物脱氧），或者在氢原子供体如环己- 1,4 -二烯（1,4 - chd）存在的情况下，还原成醇，其区域化学性质与金属氢化物还原（方案1）相反
The reactivity of water with both carbanion and carbocation intermediates is well known, but until now it has generally been believed that water is inert towards free radicals.[1] This hypothetical passivity has been attributed to the strong HÀ OH bond, which, with a bond-dissociation energy of 117.59 Æ 0.07 kcal molÀ1,[2] would impede any potential hydrogen-atom transfer from water. Some years ago, however, we chanced to observe that tertiary radicals were reduced effectively in the presence of bis (cyclopentadienyl) titanium (III) chloride [3]([Cp2TiCl]) and water.[4] This observation further facilitated the control of the final step in titanocene-catalyzed radical cyclizations, which are useful for the straightforward synthesis of complex polycyclic terpenoids.[5] However, as, at the time, the idea of water acting as a hydrogen-atom source seemed to be counterintuitive, this phenomenon was rationalized by invoking either the formation and subsequent hydrolysis of alkyl–TiIV complexes or a virtually intramolecular hydrogen transfer via a quite sophisticated cyclic transition state.[4, 5] We now have solid evidence to show that water really can act as a complete hydrogen-atom source rather than as a simple proton donor for radical reductions mediated by TiIII and, presumably, other metals that react by single-electron transfer.Many highly selective free-radical reactions have been developed and have proved to be very useful in the total synthesis of complex organic compounds owing to the mild conditions required and their compatibility with many functional groups.[6] Within this context, RajanBabu and Nugent introduced a novel concept: homolytic oxirane opening induced by [Cp2TiCl].[7] This reaction generates the most substituted (ie, most stable) β-titanoxy radical, which, among other transformations, could be either trapped by a second [Cp2TiCl] species to provide an alkene (epoxide deoxygenation) by “TiO” elimination or, in the presence of a hydrogen-atom donor such as cyclohexa-1, 4-diene (1, 4-CHD), reduced to an alcohol with the opposite regiochemistry to that expected from the reduction with metal hydrides (Scheme 1).[7]