Competing C−F Activation Pathways in the Reaction of Pt(0) with Fluoropyridines: Phosphine-Assistance versus Oxidative Addition

A survey of computed mechanisms for C−F bond activation at the 4-position of pentafluoropyridine by the model zero-valent bis-phosphine complex, [Pt(PH3)(PH2Me)], reveals three quite distinct pathways leading to square-planar Pt(II) products. Direct oxidative addition leads to cis-[Pt(F)(4-C5NF4)(PH3)(PH2Me)] via a conventional 3-center transition state. This process competes with two different phosphine-assisted mechanisms in which C−F activation involves fluorine transfer to a phosphorus center via novel 4-center transition states. The more accessible of the two phosphine-assisted processes involves concerted transfer of an alkyl group from phosphorus to the metal to give a platinum(alkyl)(fluorophosphine), trans-[Pt(Me)(4-C5NF4)(PH3)(PH2F)], analogues of which have been observed experimentally. The second phosphine-assisted pathway sees fluorine transfer to one of the phosphine ligands with formation of a metastable metallophosphorane intermediate from which either alkyl or fluorine transfer to the metal is possible. Both Pt−fluoride and Pt(alkyl)(fluorophosphine) products are therefore accessible via this route. Our calculations highlight the central role of metallophosphorane species, either as intermediates or transition states, in aromatic C−F bond activation. In addition, the similar computed barriers for all three processes suggest that Pt−fluoride species should be accessible. This is confirmed experimentally by the reaction of [Pt(PR3)2] species (R = isopropyl (iPr), cyclohexyl (Cy), and cyclopentyl (Cyp)) with 2,3,5-trifluoro-4-(trifluoromethyl)pyridine to give cis-[Pt(F){2-C5NHF2(CF3)}(PR3)2]. These species subsequently convert to the trans-isomers, either thermally or photochemically. The crystal structure of cis-[Pt(F){2-C5NHF2(CF3)}(PiPr3)2] shows planar coordination at Pt with r(F−Pt) = 2.029(3) Å and P(1)−Pt−P(2) = 109.10(3)°. The crystal structure of trans-[Pt(F){2-C5NHF2(CF3)}(PCyp3)2] shows standard square-planar coordination at Pt with r(F−Pt) = 2.040(19) Å.

本综述针对模型零价双膦配合物[Pt(PH3)(PH2Me)]活化五氟吡啶4位碳氟键的理论计算反应机理展开调研，共发现三条截然不同的反应路径，均可生成平面正方形构型的Pt(II)产物。直接氧化加成过程经由常规三中心过渡态生成顺式-[Pt(F)(4-C5NF4)(PH3)(PH2Me)]。该过程与两种不同的膦辅助活化机理存在竞争：两条膦辅助路径中的碳氟键活化均通过新颖的四中心过渡态实现氟原子向磷中心的转移。两种膦辅助路径中更具可及性的过程为烷基从磷原子向金属中心的协同转移，最终生成铂(烷基)(氟膦)类产物反式-[Pt(Me)(4-C5NF4)(PH3)(PH2F)]，该类配合物的类似物已有实验观测报道。第二条膦辅助路径则为氟原子向其中一个膦配体转移，形成亚稳态金属膦烷中间体，该中间体可进一步发生烷基或氟原子向金属中心的转移。因此经由该路径可同时得到Pt-氟化物与Pt(烷基)(氟膦)两类产物。本次理论计算凸显了金属膦烷物种（作为中间体或过渡态）在芳香族碳氟键活化过程中的核心作用。此外，三条反应路径的计算能垒相近，表明Pt-氟化物物种具备可及性。[Pt(PR3)2]类配合物（R为异丙基(iPr)、环己基(Cy)与环戊基(Cyp)）与2,3,5-三氟-4-(三氟甲基)吡啶的反应证实了这一结论，产物为顺式-[Pt(F){2-C5NHF2(CF3)}(PR3)2]。该类配合物后续可通过热或光化学途径转化为反式异构体。顺式-[Pt(F){2-C5NHF2(CF3)}(PiPr3)2]的晶体结构显示Pt中心呈平面配位，F-Pt键长为2.029(3) Å，P(1)-Pt-P(2)键角为109.10(3)°。反式-[Pt(F){2-C5NHF2(CF3)}(PCyp3)2]的晶体结构显示Pt中心为标准平面正方形配位，F-Pt键长为2.040(19) Å。