Chlorine-Radical-Mediated Photocatalytic Activation of C-H Bonds with Visible Light
Chlorine-Radical-Mediated Photocatalytic Activation of C-H Bonds with Visible Light
复制标题
氯自由基介导的可见光光催化活化 C-H 键
DOI：
10.1002/anie.201207904
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发表时间：
2013-01-01
影响因子：
16.6
通讯作者：
Fu, Xianzhi
中科院分区：
文献类型：
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作者：
Yuan, Rusheng;Fan, Shaolong;Fu, Xianzhi
烷烃中C(SP3)±H键的选择性活化和转化以产生更高价值的产物在基础化学和应用化学中都是非常重要的。[1]由于这些键既是热力学上的强键，又是动力学上的惰性键，传统的催化方法需要在相当苛刻的条件下使用有毒、侵略性和昂贵的试剂。[2]多相半导体光催化被认为是在温和的条件下激活C±H键的一种有前途的替代方法。[3]使用廉价而丰富的分子氧作为氧化剂和光作为驱动力，使得这种过程特别适合于工业应用的绿色化学反应。然而，大多数光催化反应需要高能紫外光辐射，且可见光活性极低。[4]因此，开发一种环境友好、高效的光催化体系，在可见光下将惰性烷烃转化为功能化产物具有重要意义。尽管烷烃中的C(SP3)±H键很强，很难直接裂解，但它们很容易与极具活性的物种反应，如自由基。例如，光催化中通常产生的侵略性羟基自由基一直是C±H裂解的典型引发剂，但由于其较差的选择性，几乎所有的有机化合物都会矿化。Ollis研究小组报告说，氯自由基表现出类似的吸氢行为，以提高紫外光照射下气体污染物的光催化降解活性，尽管没有提供形成它的直接证据。然而，氯辅助光催化的概念并没有引起太大的关注，因为它的实际应用仅限于通过紫外光照射对气相痕量污染物的光降解。最近，我们的研究小组发现，表面吸附了氯的二氧化钛基光催化剂在可见光照射下对气态有机底物的降解显示出显著的光催化活性。[7]这一发现使光催化反应更加实用，因为可再生太阳能的可获得性。最有趣的是，这些氯化二氧化钛基光催化剂在降解芳香烃方面的行为取决于是否使用紫外光或可见光。在紫外光照射下，甲苯的转化率较高，矿化程度较高；在可见光照射下，甲苯的转化率较高，矿化程度较低。在后一种情况下，低矿化度表明甲苯已转化为二氧化碳以外的氧化产物。这一结果使我们推断，这种氯自由基介导的策略在烷烃的选择性转化方面可能更有前景：这是有机合成中一个广泛寻求但难以捉摸的过程。本文报道了在可见光照射下，表面氯化BiOBrTiO2光催化剂(CBT)用于烷烃中C(SP3)±H键的选择性活化。首次揭示了氯自由基的产生及其在这些多相光催化过程中的关键作用。廉价、环保的无机半导体光催化剂和太阳能的使用使这种方法比传统的C？H键活化策略更环保、更可持续。在中等光强的可见光(λ>；420 nm)照射下，在BiOBr/Ti02(BT)催化剂上，碳氢化合物的氧化最初在三氟化苯中进行。…
The selective activation and transformation of C (sp3) ÀH bonds in alkanes to produce higher-value products is of great importance in both fundamental and applied chemistry.[1] Since these bonds are both thermodynamically strong and kinetically inert, conventional catalytic strategies for the activation step require the use of toxic, aggressive, and expensive reagents under rather stringent conditions.[2] Heterogeneous semiconductor photocatalysis is considered to be a promising alternative for the activation of CÀH bonds under mild conditions.[3] The use of cheap and abundant molecular oxygen as the oxidant and light as the driving force makes such processes especially appealing for green chemical reactions for industrial applications. However, most photocatalytic reactions require high-energy ultraviolet radiation and show extremely low visible-light activity.[4] Therefore, the development of an environmentally friendly and highly efficient photocatalytic system that can transform inert alkanes into functionalized products under visible light is significant and highly desirable. Although the C (sp3) ÀH bonds in alkanes are strong and difficult to cleave directly, they react readily with extremely reactive species, such as free radicals.[5] For example, the aggressive hydroxyl radical usually generated in photocatalysis has been the typical initiator of CÀH cleavage, but mineralizes nearly all organic compounds owing to its poor selectivity. The chlorine radical, which shows similar hydrogen-abstraction behavior, was reported by the Ollis research group to enhance activity in the photocatalytic degradation of gaseous contaminants under UV irradiation, although no direct evidence for its formation was provided.[6] However, the idea of chlorine-assisted photocatalysis has not drawn much attention, since its practical application has been restricted to the photodegradation of gas-phase trace pollutants by irradiation with UV light. Recently, our research group found that TiO2-based photocatalysts with chlorine chemisorbed on their surface showed a remarkable improvement in photocatalytic activity in the degradation of gaseous organic substrates under irradiation with visible light.[7] This finding makes photocatalytic reactions more practical owing the availability of endlessly renewable solar energy. Most interestingly, the behavior of these chlorinated TiO2-based photocatalysts in the degradation of aromatic alkanes depended on whether UV or visible light was used. Under UV irradiation, both high conversion and extensive mineralization of toluene were observed, whereas under irradiation with visible light, high conversion and rather low mineralization were observed. In the latter case, the low mineralization suggested that toluene had been transformed into oxygenated products other than CO2. This result led us to reason that this chlorine-radical-mediated strategy might be more promising for the selective transformation of alkanes: a widely sought yet elusive process in organic synthesis. Herein, we report the application of surface-chlorinated BiOBr/TiO2 (CBT) for the selective activation of C (sp3) ÀH bonds in alkanes under irradiation with visible light. The generation of chlorine radicals and their pivotal role in these heterogeneous photocatalytic processes were revealed for the first time. The use of a cheap, environmentally friendly inorganic semiconductor photocatalyst and solar energy make this method more green and sustainable than conventional CÀH bond-activation strategies. The oxygenation of hydrocarbons was initially carried out in benzotrifluoride (Btf) over BiOBr/TiO2(BT) under irradiation with visible light (λ> 420 nm) with a moderate light intensity …