Nova Ainara, Erhardt Stefan, Jasim Naseralla A, Perutz Robin N, Macgregor Stuart A, McGrady John E, Whitwood Adrian C
Department of Chemistry, University of York, Heslington, York YO10 5DD, United Kingdom.
J Am Chem Soc. 2008 Nov 19;130(46):15499-511. doi: 10.1021/ja8046238. Epub 2008 Oct 25.
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)}(P iPr3)2] shows planar coordination at Pt with r(F-Pt) = 2.029(3) A and P(1)-Pt-P(2) = 109.10(3) degrees. 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) A.
对模型零价双膦配合物[Pt(PH3)(PH2Me)]使五氟吡啶4位的C-F键活化的计算机制进行的一项研究揭示了三条截然不同的通向平面正方形Pt(II)产物的途径。直接氧化加成通过传统的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机制竞争，其中C-F活化涉及通过新型四中心过渡态将氟转移到磷中心。两种膦辅助过程中较容易发生的一种涉及烷基从磷协同转移到金属上，生成铂(烷基)(氟膦)，反式-[Pt(Me)(4-C5NF4)(PH3)(PH2F)]，其实验类似物已被观察到。第二种膦辅助途径是氟转移到其中一个膦配体上，形成一种亚稳的金属磷叶立德中间体，从中烷基或氟都有可能转移到金属上。因此，通过这条途径可以得到Pt-氟化物和Pt(烷基)(氟膦)产物。我们的计算突出了金属磷叶立德物种作为中间体或过渡态在芳族C-F键活化中的核心作用。此外，所有三个过程的计算势垒相似，这表明Pt-氟化物物种应该是可以得到的。[Pt(PR3)2]物种（R = 异丙基(iPr)、环己基(Cy)和环戊基(Cyp)）与2,3,5-三氟-4-(三氟甲基)吡啶反应生成顺式-[Pt(F){2-C5NHF2(CF3)}(PR3)2]，实验证实了这一点。这些物种随后通过热或光化学转化为反式异构体。顺式-[Pt(F){2-C5NHF2(CF3)}(P iPr3)2]的晶体结构显示Pt处为平面配位，r(F-Pt) = 2.029(3) Å，P(1)-Pt-P(2) = 109.10(3)°。反式-[Pt(F){2-C5NHF2(CF3)}(PCyp3)2]的晶体结构显示Pt处为标准的平面正方形配位，r(F-Pt) = 2.040(19) Å。
