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Issue 38, 2012
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Synthetic and computational studies of the palladium(IV) system Pd(alkyl)(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C–C reductive elimination

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Abstract

Synthetic routes to methyl(aryl)alkynylpalladium(IV) motifs are presented, together with studies of selectivity in carbon–carbon coupling by reductive elimination from PdIV centres. The iodonium reagents IPh(C[triple bond, length as m-dash]CR)(OTf) (R = SiMe3, But, OTf = O3SCF3) oxidise PdIIMe(p-Tol)(L2) (1–3) [L2 = 1,2-bis(dimethylphosphino)ethane (dmpe) (1), 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3)] in acetone-d6 or toluene-d9 at −80 °C to form complexes PdIV(OTf)Me(p-Tol)(C[triple bond, length as m-dash]CR)(L2) [R = SiMe3, L2 = dmpe (4), bpy (5), phen (6); R = But, L2 = dmpe (7), bpy (8), phen (9)] which reductively eliminate predominantly (>90%) p-Tol-C[triple bond, length as m-dash]CR above ∼−50 °C. NMR spectra show that isomeric mixtures are present for the PdIV complexes: three for dmpe complexes (4, 7), and two for bpy and phen complexes (5, 6, 8, 9), with reversible reduction in the number of isomers to two occurring between −80 °C and −60 °C observed for the dmpe complex 4 in toluene-d8. Kinetic data for reductive elimination from PdIV(OTf)Me(p-Tol)(C[triple bond, length as m-dash]CSiMe3)(dmpe) (4) yield similar activation parameters in acetone-d6 (66 ± 2 kJ mol−1, ΔH 64 ± 2 kJ mol−1, ΔS −67 ± 2 J K−1 mol−1) and toluene-d8 (Ea 68 ± 3 kJ mol−1, ΔH 66 ± 3 kJ mol−1, ΔS −74 ± 3 J K−1 mol−1). The reaction rate in acetone-d6 is unaffected by addition of sodium triflate, indicative of reductive elimination without prior dissociation of triflate. DFT computational studies at the B97-D level show that the energy difference between the three isomers of 4 is small (12.6 kJ mol−1), and is similar to the energy difference encompassing the six potential transition state structures from these isomers leading to three feasible C–C coupling products (13.0 kJ mol−1). The calculations are supportive of reductive elimination occurring directly from two of the three NMR observed isomers of 4, involving lower activation energies to form p-TolC[triple bond, length as m-dash]CSiMe3 and earlier transition states than for other products, and involving coupling of carbon atoms with higher s character of σ-bonds (sp2 for p-Tol, sp for C[triple bond, length as m-dash]C–SiMe3) to form the product with the strongest C–C bond energy of the potential coupling products. Reductive elimination occurs predominantly from the isomer with Me3SiC[triple bond, length as m-dash]C trans to OTf. Crystal structure analyses are presented for PdIIMe(p-Tol)(dmpe) (1), PdIIMe(p-Tol)(bpy) (2), and the acetonyl complex PdIIMe(CH2COMe)(bpy) (11).

Graphical abstract: Synthetic and computational studies of the palladium(iv) system Pd(alkyl)(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C–C reductive elimination

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Publication details

The article was received on 21 May 2012, accepted on 07 Aug 2012 and first published on 08 Aug 2012


Article type: Paper
DOI: 10.1039/C2DT31086D
Citation: Dalton Trans., 2012,41, 11820-11828
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    Synthetic and computational studies of the palladium(IV) system Pd(alkyl)(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C–C reductive elimination

    M. Sharma, A. Ariafard, A. J. Canty, B. F. Yates, M. G. Gardiner and R. C. Jones, Dalton Trans., 2012, 41, 11820
    DOI: 10.1039/C2DT31086D

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