Aryl-F Bond Cleavage vs. C-E Reductive Elimination: Competitive Pathways of Metal-Ligand-Cooperation-Based E-H Bond Activation (E = N, S)

An 8-fluoroquinoline-based dearomatized PNF-pincer (Ar)Pd^II complex 1 reacts with N-methylaniline to give an aromatized PNN_H-pincer complex, the product of the aromatic nucleophilic substitution of the PNF ligand fluorine atom. In contrast, the reaction between 1 and thiophenol leads exclusively to the Ar-S coupling product. Experimental and theoretical (DFT) studies suggest that both reactions proceed via substrate coordination and substrate-to-PNF hydrogen atom transfer through a metal-ligand cooperation mechanism to produce a Pd^II amide or sulfide intermediate. In the case of the amide, intramolecular nucleophilic substitution of the fluorine by the amide has a lower activation barrier than Ar-N coupling, whereas in the case of the phenylsulfide intermediate, Ar-S elimination is favored kinetically. Interestingly, for EtSH, both reaction pathways have similar activation energies, consistent with the experimentally observed formation of mixtures of products of both reactions. Overall, the DFT calculations support the feasibility of the Ar-E (E = N, S) coupling reactions via a metal-ligand cooperation mechanism.