Cleavage of Ether, Ester, and Tosylate C(sp<sup>3</sup>)–O Bonds by an Iridium Complex, Initiated by Oxidative Addition of C–H Bonds. Experimental and Computational Studies
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A pincer-ligated iridium complex, (PCP)Ir (PCP = κ3-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp3)–O bonds of methyl esters (CH3–O2CR′), methyl tosylate (CH3–OTs), and certain electron-poor methyl aryl ethers (CH3–OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C–H bonds followed by oxygenate migration, rather than by direct C–O addition. Thus, methyl aryl ethers react via addition of the methoxy C–H bond, followed by α-aryloxide migration to give cis-(PCP)Ir(H)(CH2)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH3)(OAr). Methyl acetate undergoes C–H bond addition at the carbomethoxy group to give (PCP)Ir(H)[κ2-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(κ2-O2CMe) (6-Me) in which the methoxy C–O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP Cipso–Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(κ2-O2CR), the net product of methoxy group C–O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO2CMe or ROTs, where R possesses β-C–H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O2CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C–O bond oxidative additions, these reactions also proceed via initial activation of a C–H bond; in this case, C–H addition at the β-position is followed by β-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the β-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C–O bond is cleaved with no direct participation by the iridium center.
本研究发现，一种钳形配体配位的铱配合物(PCP)Ir（其中PCP为κ3-C6H3-2,6-[CH2P(t-Bu)2]2）可发生甲酯（CH3–O2CR′）、对甲苯磺酸甲酯（CH3–OTs）以及部分缺电子甲基芳基醚（CH3–OAr）的碳(sp3)-氧键氧化加成反应。密度泛函理论(DFT)计算与机理研究表明，该类反应并非通过直接C-O键加成进行，而是先经历C-H键氧化加成，再发生含氧基团迁移。以甲基芳基醚为例，反应首先发生甲氧基C-H键加成，经α-芳氧基迁移生成顺式-(PCP)Ir(H)(CH2)(OAr)，随后发生铱到亚甲基的氢迁移，最终得到(PCP)Ir(CH3)(OAr)。乙酸甲酯则在甲氧羰基位点发生C-H键加成，生成(PCP)Ir(H)[κ2-CH2OC(O)Me]，该中间体进一步转化为(PCP-CH2)Ir(H)(κ2-O2CMe)（6-Me），此过程中甲氧基的C-O键发生断裂，且源自甲氧基的亚甲基迁移至PCP配体的ipso碳-铱键之间。对6-Me进行热解最终得到(PCP)Ir(CH3)(κ2-O2CR)，即甲氧基C-O键氧化加成的净产物。当(PCP)Ir与ROAr、RO2CMe或ROTs类底物（其中R含有β位C-H键，如R为乙基或异丙基）反应时，分别生成(PCP)Ir(H)(OAr)、(PCP)Ir(H)(O2CMe)或(PCP)Ir(H)(OTs)，同时得到相应的烯烃或(PCP)Ir(烯烃)配合物。与C-O键氧化加成反应类似，该类反应同样首先通过C-H键活化启动：β位C-H键加成后，发生芳氧基、羧酸根或对甲苯磺酸根基团的β迁移。计算结果表明，羧酸根基团的β迁移经由一种特殊的六元环过渡态进行，此过程中烷氧基C-O键发生断裂，且铱中心未直接参与该过程。
