Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009, Zaragoza, Spain.
Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193, Barcelona, Spain.
Chemistry. 2019 Nov 18;25(64):14546-14554. doi: 10.1002/chem.201903068. Epub 2019 Oct 1.
Alkoxylation and hydroxylation reactions of 1,5-cyclooctadiene (cod) in an iridium complex with alcohols and water promoted by the reduction of oxygen to hydrogen peroxide are described. The exo configuration of the OH/OR groups in the products agrees with nucleophilic attack at the external face of the olefin as the key step. The reactions also require the presence of a coordinating protic acid (such as picolinic acid (Hpic)) and involve the participation of a cationic diolefin iridium(III) complex, [Ir(cod)(pic) ] , which has been isolated. Independently, this cation is also involved in easy alkoxy group exchange reactions, which are very unusual for organic ethers. DFT studies on the mechanism of olefin alkoxylation mediated by oxygen show a low-energy proton-coupled electron-transfer step connecting a superoxide-iridium(II) complex with hydroperoxide-iridium(III) intermediates, rather than peroxide complexes. Accordingly, a more complex reaction, with up to four different products, occurred upon reacting the diolefin-peroxide iridium(III) complex with Hpic. Moreover, such hydroperoxide intermediates are the origin of the regio- and stereoselectivity of the hydroxylation/alkoxylation reactions. If this protocol is applied to the diolefin-rhodium(I) complex [Rh(pic)(cod)], free alkyl ethers ORC H (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l-proline, is used instead of Hpic.
描述了醇和水在铱配合物中促进 1,5-环辛二烯（cod）的烷氧基化和羟化反应，其中氧气被还原为过氧化氢。产物中 OH/OR 基团的反式构型与烯烃外部面的亲核攻击一致，这是关键步骤。这些反应还需要一个配位质子酸（如吡啶甲酸（Hpic））的存在，并涉及阳离子二烯烃铱（III）配合物[Ir(cod)(pic)]的参与，该配合物已被分离。独立地，这种阳离子也参与容易的烷氧基交换反应，这对于有机醚来说非常不寻常。通过氧介导的烯烃烷氧基化反应机理的 DFT 研究表明，连接超氧化物-铱（II）配合物与过氧化物-铱（III）中间体的质子耦合电子转移步骤能量较低，而不是过氧化物配合物。因此，当二烯烃-过氧化物铱（III）配合物与 Hpic 反应时，会发生更复杂的反应，产生多达四种不同的产物。此外，这些过氧化物中间体是羟化/烷氧基化反应的区域和立体选择性的来源。如果将该方案应用于二烯烃-铑（I）配合物[Rh(pic)(cod)]，则会得到游离的烷基醚 ORC H（R=Me，Et），如果使用手性氨基酸（如 l-脯氨酸）代替 Hpic，则反应是对映选择性的。
