Aerobic Palladium(II)/Copper(II)-Catalyzed Oxidation of Olefins under Chloride-Free Nonacidic Conditions
Aerobic Palladium(II)/Copper(II)-Catalyzed Oxidation of Olefins under Chloride-Free Nonacidic Conditions
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DOI：
10.1021/om8011213
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发表时间：
2009-06-08
期刊：
影响因子：
2.8
通讯作者：
Gusevskaya, Elena V.
中科院分区：
文献类型：
--
作者：
Speziali, Marcelo G.;Costa, Vinicius V.;Gusevskaya, Elena V.
无氯化物的 Pd(OAc)(2)/Cu(OAc)(2) 体系在非酸性甲醇溶液中催化分子氧对有机底物的氧化。末端烯烃，即1-己烯、1-辛烯、苯乙烯和2-乙烯基萘，产生相应的甲基酮，而长链底物没有明显的双键异构化。该催化剂还促进无环多烯中空间阻碍的三取代内部双键的异常烯丙基氧化，能够与钯形成 eta(2)-eta(2)-螯合物。芳樟醇和橙花叔醇，单萜和倍半萜烯丙醇，通过内部双键的排他氧化产生相应的烯丙醚。由于烯丙基羟基的失活作用，两种底物中的单取代末端双键保持完整。二氢月桂烯是一种非官能化的单萜二烯，通过三取代和末端双键的氧化分别产生烯丙醚和甲基酮。另一方面，没有末端烯键的底物（例如香茅醇、香茅醛、橙花醇和柠檬醛）中的内部双键在类似条件下不会发生氧化。同一钯原子上的末端双键和内部双键螯合的空间可能性似乎是决定这些底物中内部双键氧化反应性的关键因素。这些螯合物中第二个 eta(2)-配位双键的存在有利于烯烃的烯丙基 C-H 键的活化，从而产生 eta(3)-烯丙基中间体，然后产生烯丙醚。橙花醇是一种伯烯丙醇，可被氧化为 (Z)-柠檬醛，尽管该过程由于柠檬醛的进一步非选择性转化而变得复杂。本工作中获得的所有含氧萜烯衍生物都具有令人愉悦的花香或水果香，可用作合成香料的组分。
A chloride-free Pd(OAc)(2)/Cu(OAc)(2) system catalyzes the oxidation of organic substrates by molecular oxygen in nonacidic methanol solutions. Terminal olefins, i.e., 1-hexene, 1-octene, styrene, and 2-vinylnaphthalene, give the corresponding methyl ketones without a significant double-bond isomerization of long-chain substrates. The catalyst also promotes an unusual allylic oxidation of sterically encumbered trisubstituted internal double bonds in acyclic polyenes able to form eta(2)-eta(2)-chelates with palladium. Linalool and nerolidol, mono- and sesquiterpenic allylic alcohols, give corresponding allylic ethers arising from the exclusive oxidation of the internal double bonds. Monosubstituted terminal double bonds in both substrates remain intact due to the deactivating effect of the allylic hydroxyl group. Dihydromyrcene, a nonfunctionalized monoterpenic diene, gives allylic ether and methyl ketone arising from the oxidation of both trisubstituted and terminal double bonds, respectively. On the other hand, internal double bonds in the substrates without a terminal olefinic bond, such as citronellol, citronellal, nerol, and citral, do not undergo oxidation under similar conditions. A steric possibility for chelation of the terminal and internal double bonds on the same palladium atom seems to be a crucial factor to determine the reactivity of the internal double bonds in these substrates toward oxidation. The presence of the second eta(2)-coordinated double bond in these chelates favors the activation of the allylic C-H bond of the olefin, which results in eta(3)-allyl intermediates and, then, in allylic ethers. Nerol, a primary allylic alcohol, is oxidized to (Z)-citral, although the process is complicated by further nonselective transformations of citral. All oxygenated terpenic derivatives obtained in the present work have a pleasant scent with a flower or fruit tinge and are potentially useful as components of synthetic perfumes.