Unexpected Substituent Effects in Aryl-Aryl Negishi Cross-Coupling Reactions Rationalized by Density Functional Theory and Natural Charges

While numerous studies consider substitution effects at the aromatic system in the arylzinc reactant during the transmetalation step in palladium-catalyzed Negishi cross-coupling reactions, the effects caused by varying substituents at the arylpalladium complex have not been the center of attention so far. Herein, we present results from competition experiments providing the counterintuitive observation that electron-withdrawing (donating) groups in the para position to the palladium center inhibit (accelerate) the transmetalation step in aryl-aryl Negishi cross-coupling reactions. Quantum-chemical calculations based on density functional theory provide mechanistic insights and show that an associative transmetalation mechanism is favored over a dissociative one. Foster-Boys localized orbitals and Mayer bond orders aid in rationalizing molecular and electronic structural features. Natural population analysis reveals that charge accumulation at the palladium center causes a destabilization of the transition state, which clearly correlates with the electronic properties of the substituents represented by their Hammett parameters.

Automated summary

Studies show that electron-withdrawing (donating) groups in the para position to the palladium center inhibit (accelerate) the transmetalation step in aryl-aryl Negishi cross-coupling reactions. Quantum-chemical calculations suggest that an associative transmetalation mechanism is preferred. Molecular and electronic structural features can be rationalized using Foster-Boys localized orbitals and Mayer bond orders.