Photooxidation of olefins sensitized by .alpha.-diketones and by benzophenone. A practical epoxidation method with biacetyl
Photooxidation of olefins sensitized by .alpha.-diketones and by benzophenone. A practical epoxidation method with biacetyl
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α-二酮和二苯甲酮敏化的烯烃的光氧化。
DOI：
10.1021/ja00430a031
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发表时间：
1976
影响因子：
15
通讯作者：
P. Bartlett
中科院分区：
文献类型：
--
作者：
N. Shimizu;P. Bartlett
9种烯烃在苯溶液中用氧气和苯偶酰光氧化，导致环氧化合物作为主要产物，产率从5%，在60分钟的三苯乙烯到98%，在15分钟的异戊烯。从顺式或反式无环烯烃的环氧化物总是反式的。竞争过程包括从烯烃（2，3-二甲基-2-丁烯和1，2-二甲基环己烯）形成烯丙基氢过氧化物，这些烯烃对单线态氧非常活泼，以及具有较低三重态（芪）的烯烃的顺式、反式异构化的能量转移。苯偶酰的回收率很高，表明它既不消耗化学计量的过程中，也没有直接氧化的光环氧化的有效竞争。丁二酮作为光敏剂的作用类似于苯偶酰，但它对二酮的破坏更大，并产生具有更广泛烯烃的环氧化物。表III中的18种烯烃中的7种以90%或更高的产率转化为环氧化物，使其成为有用的制备方法。苯偶酰和丁二酮与tr，tr* 光敏剂芴酮和n-rr* 光敏剂二苯甲酮进行了比较，前者能产生单线态氧，但不发生环氧化反应，后者与烯烃环加成生成氧杂环丁烷是重要的竞争过程，并成为与异戊烯反应的主要过程。染料敏化光氧化反应已被广泛研究，其结果与单线态氧机理一致。[1]然而，用n-tr* 三重态敏化剂的光氧化反应却不那么简单。在二苯甲酮敏化的2-丙醇氧化反应中，发现了自由基机理。2 Gollnick及其同事指出，n-tr* 三重态敏化剂也可以产生单线态氧。3.本文报道了以苯偶酰和双乙酰为敏化剂的各种烯烃的光氧化反应，结果表明，这种光氧化反应与单线态氧氧化反应和自由基链自氧化反应有很大的不同。用苯偶酰光氧化。用Hanovia 450-W中压汞灯在0 ℃下照射4 1，2-二甲基环己烯（1，0.5M）和苯偶酰（0.1M）的苯溶液15分钟，在照射过程中鼓入氧气，分别以58%和12%的产率得到烯丙基氢过氧化物（2a）和环氧化物（3），以及30%的再包覆烯烃。苯偶酰几乎定量回收。表1给出了这些结果与染料敏化氧化反应和自氧化反应的对比。7，8染料敏化和苯偶酰敏化之间的重要区别也出现在芳香烯烃的氧化中。如表II所示，五种芳族烯烃的苯偶酰敏化氧化得到环氧化物，而已知这些芳族烯烃通过长时间暴露于单线态而非常缓慢地转化为酮。
Photooxidation of nine olefins in benzene solution with oxygen and benzil leads to epoxides as the main product in yields ranging from 5% in 60 min for trazis-stilbene to 98% in 15 min for norbornene. From cis or trans acyclic olefins the epoxide is always trans. Competing processes include formationof allylic hydroperoxides from olefins (2, 3-dimethyl-2-butene and 1, 2-dimethylcyclohexene) that are quite reactive toward singlet oxygen, and energy transfer with cis, trans isomerization of olefins with lower lying triplet states (stilbene). Recovery of benzil is high, showing that it is neither consumed stoichiometrically in the process nor oxidized directly in effective competition with the photoepoxidation. The action of biacetyl as a photosensitizer is similar to that of benzil but is attended by much more destruction of the diketone and yields epoxide with a wider range of olefins. Seven of the 18 olefins in Table III are converted to epoxides in yields of 90% or more, making this a useful preparative method. Benzil and biacetyl are compared with the tr, tr* photosensitizer fluorenone, which can produce singlet oxygen but gives no epoxidation, and with the n-rr* sensitizer benzophenone, in which cycloaddition to an olefin to give oxetane is an important competing process and becomes the major course of the reaction with norbornene. The nature of the mechanism is discussed.Dye-sensitized photooxidation reactions have been extensively investigated and the results are consistent with the singlet oxygen mechanism. 1 Photooxidation with n-tr* triplet sensi-tizers, however, presents a less simple picture. A free-radical mechanism is indicated in benzophenone-sensitized oxidation of 2-propanol. 2 Gollnick and co-workers have pointed out that n-tr* triplet sensitizers can also generate singlet oxygen. 3 We report here the results of photooxidation of various olefins with benzil and biacetyl as sensitizers and show that the present photooxidation is quite different from the singlet oxygen oxidation andalso from radical-chain autoxidations. Photooxidation with Benzil. The irradiation4 of a benzene solution of 1, 2-dimethylcyclohexene (1, 0.5 M) and benzil (0.1 M) with a Hanovia 450-W medium-pressure mercury lamp at 0 for 15 min, oxygen being bubbled through during the irradiation, gave allylic hydroperoxide (2a) and epoxide (3) in 58 and 12% yield, respectively, together with 30% of re-covered olefin. Benzil was recovered almostquantitatively. Table Ishows the contrast between these results and either the dye-sensitized oxidation5^ 7 or the autoxidation reaction. 7, 8 Important differencesbetween dye and benzil sensitizations appear also in the oxidation of aromaticolefins. Benzil-sensi-tized oxidation of five aromatic olefins gave epoxides as shown in Table II, whereas it is known that these aromatic olefins are converted very slowly to ketones by long exposure to singlet