Cleaner Continuous Photo-Oxidation Using Singlet Oxygen in Supercritical Carbon Dioxide
Cleaner Continuous Photo-Oxidation Using Singlet Oxygen in Supercritical Carbon Dioxide
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DOI：
10.1002/anie.200901731
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
George, Michael W.
中科院分区：
文献类型：
--
作者：
Bourne, Richard A.;Han, Xue;George, Michael W.
单线态氧（1O2）在当代有机合成中得到了广泛的应用。[1]o2，也是一个有效的电子受体，引导[2]形成超氧自由基O2C À。1O2和O2C À都与一些天然产物的生物合成和生化过程有关，利用单线态氧来帮助构建天然产物中发现的复杂分子基序是当前研究的主题将氧引入碳氢化合物底物的两种最方便、最“干净”的方法是将氧与烯烃反应生成烯丙醇，或与1,3 -二烯反应生成内过氧化物。这两种合成转化已被用作许多天然产物合成和香水成分合成的关键步骤此外，O2的加入是100%的原子效率，不像更传统的氧化，一半的氧原子通常被浪费，通常以H2O或CO2的形式，尽管一些原子效率可能会在随后的步骤中损失。在实验室中产生1O2最常用的方法是通过分子氧的光激发，通常在光敏剂存在的情况下。也可以用次氯酸钠和过氧化氢[7]或Ph3PO3[8]或oxone[9]热生成o2光化学在工业上大规模的广泛应用尚未被证明是可行的光化学反应器存在严重的放大限制，随着试剂浓度增加到与工业规模工艺相适应的高浓度条件，需要新的反应器设计来克服光化学反应效率的下降。氧的高反应性和相对较短的寿命带来了严重的技术问题；也就是说，需要一种不可燃的溶剂，它能相对缓慢地将1O2松弛回其三态基态。传统上CCl4一直是溶剂的选择，但从环境的角度来看，它不再是可接受的。其他溶剂，如异丙醇也被使用[11,12]，但存在潜在可燃性的问题。实验证明，在超临界二氧化碳（scCO2）中可以产生具有相当长的寿命（在14.7 MPa和314 K下5.1 ms）的1O2。[13-16]最近，我们通过使用可溶于scCO2的全氟光敏剂演示了如何将1O2用于均相反应。此外，CO2的不可燃性和O2在压缩气体溶剂中的高溶解度[18,19]都具有潜在的优势。O2与CO2的完全混溶使系统能够安全地保持在爆炸极限以下，显著降低了反应的风险。与传统溶剂相比，scCO2具有更低的粘度和更高的扩散率，从而克服了传统多相体系中存在的传质限制
Singlet oxygen, 1O2, is widely used in contemporary organic synthesis.[1] 1O2, is also an efficient electron acceptor leading [2] to the formation of superoxide radical ion O2C À. Both 1O2 and O2C À have been implicated in the biosynthesis of some natural products and in biochemical processes and harnessing singlet oxygen to aid in the construction of the complex molecular motifs found in natural products is a subject of current research.[3] Two of the most convenient and “clean” methods for introducing oxygen into hydrocarbon substrates involve reactions of 1O2 either with alkenes, leading to allylic alcohols, or with 1, 3-dienes leading to endoperoxides. Both of these synthetic transformations have been used as pivotal steps in a number of natural-product syntheses [4] and also in the synthesis of perfume ingredients.[5] Furthermore, the addition of O2 is 100% atom efficient, unlike more conventional oxidations where half of the oxygen atoms are usually wasted, often in the form of H2O or CO2, although, some of this atom efficiency may be lost in subsequent steps. The most common method used to generate 1O2 in the laboratory, is by photoexcitation of molecular oxygen, generally in the presence of a photosensitizer.[6] 1O2 can also be generated thermally with sodium hypochlorite and hydrogen peroxide [7] or from Ph3PO3[8] or oxone.[9] The widespread application of photochemistry on a large scale in industry has not yet proved feasible.[10] There are severe scale-up limitations with photochemical reactors and new reactor designs are required to overcome the decrease in photochemical reaction efficiency as the concentration of reagents is increased to the high-concentration conditions compatible with industrial-scale processes. The higher reactivity and relatively short lifetime of 1O2 gives rise to a serious technical problem; namely a solvent is required which is non-flammable and which is relatively slow in relaxing the 1O2 back to its triplet ground state. Traditionally CCl4 has been the solvent of choice but it is no longer acceptable from an environmental standpoint. Other solvents, such as iso-propanol, have been used [11, 12] but there are problems of potential flammability. Experiments have demonstrated that it is possible to generate 1O2 with a reasonably long lifetime (5.1 ms at14.7 MPa and 314 K) in supercritical carbon dioxide (scCO2).[13–16] Recently we demonstrated [17] how 1O2 can be used in homogeneous reactions by using a perfluorinated photosensitizer which is soluble in scCO2. In addition, both the non-flammability of CO2 and high solubility of O2 in the compressed-gas solvent [18, 19] give potential advantages. The complete miscibility of O2 with CO2 allows the system to be safely kept below the explosion limits, significantly reducing the risks of the reaction. scCO2 also has much lower viscosity and higher diffusivity than traditional solvents thus overcoming mass-transfer limitations that are found in traditional multiphasic systems.[20]