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

Abstract

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-d₉ 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-TolCCSiMe₃ 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).