Competing C-F activation pathways in the reaction of Pt(0) with fluoropyridines: Phosphine-assistance versus oxidative addition

Ainara Nova, Stefan Erhardt, Naseralla A. Jasim, Robin N. Perutz, Stuart A. Macgregor, John E. McGrady, Adrian C. Whitwood

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83 Citations (Scopus)

Abstract

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-C5NF 4)(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(PR 3)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-C 5NHF2(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 frans-[Pt(F){2-C5NHF 2(CF3)}(PCyp3)2] shows standard square-planar coordination at Pt with r(F-Pt) = 2.040(19) Å. © 2008 American Chemical Society.

Original languageEnglish
Pages (from-to)15499-15511
Number of pages13
JournalJournal of the American Chemical Society
Volume130
Issue number46
DOIs
Publication statusPublished - 19 Nov 2008

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