Palladium-catalyzed intermolecular fluoroesterification of styrenes: exploration and mechanistic insight
作者:Haihui Peng、Zheliang Yuan、Hao-yang Wang、Yin-long Guo、Guosheng Liu
DOI:10.1039/c3sc50690h
日期:——
A novel palladium-catalyzed intermolecular oxidative fluoroesterification of vinylarenes has been developed using NFSI, one of the mildest electrophilic fluorinating reagents. The reaction presents an efficient synthetic pathway to afford a series of α-monofluoromethylbenzyl carboxylates in good to excellent yields. Rather than following an electrophilic fluorination pathway, the reaction is initiated through oxidation of Pd(0) to a Pd(II) fluoride complex by NFSI, followed by fluoropalladation of a styrene to generate an α-monofluoromethylbenzyl–Pd intermediate. Generally, reductive elimination of benzyl–PdII complexes is favored with relatively strong oxy-nucleophiles to afford C–O bonds. This reaction, however, exhibited the opposite reactivity: strong acids with weak nucleophilicity, such as CF3CO2H and CCl3CO2H, were prone to afford the fluoroesterification product, while weak acids with strong nucleophilicity, such as HOAc and BzOH, did not deliver the C–O bond product. Further mechanistic studies determined that Csp3–Pd(O2CR), a key intermediate, was generated through ionic ligand exchange between benzyl–Pd(NZ2) and CF3CO2H, and the final C–O bond was possibly formed through reductive elimination of a high-valent Csp3–Pd(O2CR) complex via an SN2-type nucleophilic attack pathway.
一种新型钯催化的乙烯基芳烃分子间氧化性氟酯化反应已被开发,使用的试剂是NFSI,即一种最温和的亲电氟化试剂。该反应提供了一条高效的合成途径,能够以良好至优异的产率获得一系列α-单氟甲基苄基羧酸酯。反应并非遵循亲电氟化途径,而是通过NFSI将Pd(0)氧化为Pd(II)氟化物配合物,随后进行氟钯化作用于苯乙烯生成α-单氟甲基苄基-Pd中间体。通常情况下,与相对较强的氧亲核试剂反应有利于实现苄基-PdII配合物的还原消除,从而形成C-O键。然而,在此反应中表现出了相反的反应性:易与强酸(弱亲核性)如CF3CO2H和CCI3CO2H形成氟酯化产物,而与强亲核性弱酸如HOAc和BzOH则无法形成C-O键产物。进一步的机理研究表明,关键中间体Csp3-Pd(O2CR)是通过苄基-Pd(NZ2)与CF3CO2H之间的离子配体交换生成的,最终的C-O键很可能是通过高氧化态Csp3-Pd(O2CR)配合物的还原消除形成的,该过程可能涉及SN2型亲核攻击路径。