| 18950168 |
Nova A, Erhardt S, Jasim NA, Perutz RN, Macgregor SA, McGrady JE, Whitwood AC: Competing C-F activation pathways in the reaction of Pt (0) with fluoropyridines: phosphine-assistance versus oxidative addition. J Am Chem Soc. 2008 Nov 19;130(46):15499-511. 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. |
1(0,0,0,1) |