Pd−X Bond Homolysis Dissociation Free Energy (BDFE) Scales of [(tmeda)Pd(4‐F−C6H4)X] (X=OR, NHAr) in DMSO

Abstract

AbstractTransition metal intermediates bearing M−X σ‐bonds are ubiquitous in metal‐mediated C−X bond transformations. Thermodynamic knowledge of M−X bond cleavage is crucial to explore relevant reactions; but little was accumulated till present due to lack of suitable determination methods. We here report the first systematic study of the Pd−X bond homolysis dissociation free energies [BDFE(Pd−X)] of representative [(tmeda)Pd(4‐F−C6H4)X] (tmeda=N,N,N′,N′‐tetramethylethylenediamine, X=OR or NHAr) in DMSO on the basis of reliable measurement of their bond heterolysis energies (ΔGhet(Pd−X)). Despite ΔGhet(Pd−O)s of palladium‐phenoxides are generally found about 8 kcal/mol smaller than ΔGhet(Pd−N)s of palladium‐amidos, their BDFE(Pd−X)s are observed comparable. The structure‐property relationship was investigated to disclose an enhancement effect of electron‐withdrawing groups on BDFE(Pd−X)s. Linear free energy relationship analysis revealed that Pd−X bonds are more sensitive than X−H bonds to structural variation. The energetic propensity of reductive elimination from arylpalladium complexes was evaluated by combinatorial use of BDFE(Pd−X)s and BDFE(C−X)s, indicating an overall thermodynamic bias to C−N bond formation.