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

Synthetic routes to methyl(aryl)alkynylpalladium(iv) motifs are presented, together with studies of selectivity in carbon-carbon coupling by reductive elimination from Pd^IV centres. The iodonium reagents IPh(CCR)(OTf) (R = SiMe₃, Bu^t, OTf = O₃SCF₃) oxidise Pd^IIMe(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-d ₉ at -80 °C to form complexes Pd^IV(OTf)Me(p-Tol)(CCR) (L₂) [R = SiMe₃, L₂ = dmpe (4), bpy (5), phen (6); R = Bu^t, L₂ = dmpe (7), bpy (8), phen (9)] which reductively eliminate predominantly (>90%) p-Tol-CCR above ∼-50 °C. NMR spectra show that isomeric mixtures are present for the Pd^IV 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 Pd ^IV(OTf)Me(p-Tol)(CCSiMe₃)(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 (E_a 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-TolCCSiMe ₃ 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 CC-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 Me₃SiCC trans to OTf. Crystal structure analyses are presented for Pd^IIMe(p-Tol) (dmpe) (1), Pd^IIMe(p-Tol)(bpy) (2), and the acetonyl complex Pd ^IIMe(CH₂COMe)(bpy) (11).