Synthetic and computational studies of the palladium(IV) system Pd(alkyl)-(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C-C reductive elimination
Sharma, M and Ariafard, A and Canty, AJ and Yates, BF and Gardiner, MG and Jones, RC, Synthetic and computational studies of the palladium(IV) system Pd(alkyl)-(aryl)(alkynyl)(bidentate)(triflate) exhibiting selectivity in C-C reductive elimination, Dalton Transactions, 41, (38) pp. 11820-11828. ISSN 1477-9226 (2012) [Refereed Article]
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(CCR)(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)(CCR)(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-CCR 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)(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 (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-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 PdIIMe(p-Tol)(dmpe) (1), PdIIMe(p-Tol)(bpy) (2), and the acetonyl complex PdIIMe(CH2COMe)(bpy) (11).