Computational study of C(sp³)-O bond formation at a Pd^IV centre

This report describes a computational study of C(sp³)-OR bond formation from Pd^IV complexes of general structure Pd^IV(CH₂CMe₂-o-C₆H₄-C,C')(F)(OR)(bpy-N,N') (bpy = 2,2'-bipyridine). Dissociation of ^-OR from the different octahedral Pd^IV starting materials results in a common square-pyramidal Pd^IV cation. An S_N2-type attack by ^-OR (^-OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp³)-OR bond formation. In contrast, when ^-OR = triflate, concerted C(sp³)-C(sp²) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and S_N2 steps with different ^-OR follow opposing trends. The S_N2 transition states exhibit "PdCO" angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally.