Arene Activation at Iridium Facilitates C–O Bond Cleavage of Aryl Ethers

An
arene activation strategy for the selective disassembly of aryl
ethers is reported. A variety of aryl ethers readily bind an electrophilic
pentamethylcyclopentadienyl iridium center by η6-arene
coordination, generating complexes that are activated toward hydrolysis
and cleavage of the Ar–OR bond (R = Me, Et, Ph). Hydrolysis
occurs rapidly at room temperature in aqueous pH 7 phosphate buffer
(or upon modest heating under acidic conditions), releasing alcohol
while forming cyclohexadienyl-one products. Under strongly acidic
conditions, protonation of the dienyl-one followed by substitution
with starting aryl ether completes a hydrolysis cycle. Mechanistic
studies suggest that the key hydrolysis step proceeds via nucleophilic
attack at the ipso position of the arene (SNAr mechanism).
The observed mechanism is considered in the context of lignocellulosic
biomass conversion.

本文报道了一种用于芳基醚选择性拆解的芳烃活化策略。多种芳基醚可通过η⁶-芳烃配位（η⁶-arene coordination）与亲电五甲基环戊二烯基铱（pentamethylcyclopentadienyl iridium）中心结合，生成可发生水解并断裂Ar–OR键（R为甲基、乙基、苯基）的活化配合物。该反应可在室温下的pH 7磷酸缓冲水溶液中快速进行，或在酸性条件下适度加热以加速反应，过程中会释放醇类并生成环己二烯酮类产物。在强酸性条件下，烯酮中间体发生质子化后与起始芳基醚发生取代反应，从而完成整个水解循环。机理研究表明，关键水解步骤通过亲核试剂进攻芳烃的ipso位（ipso position），遵循SNAr机理实现。本研究所揭示的反应机理可为木质纤维素生物质转化研究提供参考。