Understanding the Use of Phosphine-(EWO) Ligands in Negishi Cross-Coupling: Experimental and Density Functional Theory Mechanistic Study

The easily prepared hemilabile ligand 1-(PPh2),2-(trans-CHCHCOPh)–C6F4 (PhPEWO-F) and other PEWO ligands are well-known promoters of C–C reductive eliminations and very effective in Negishi couplings. As an example, the efficient Negishi coupling of (C6F5)–I and Zn­(C6F5)2 is reported. The thorough experimental study of the steps involved in the catalytic cycle uncovers the potential weakness of this ligand that could frustrate at some points the desired cycle and provide some simple precautions to keep the catalytic cycle working efficiently. Density functional theory (DFT) calculations complete the experimental study and provide insight into nonobservable transition states and intermediates, comparing the potential conflict between reductive elimination and olefin insertion. Our results showcase the importance the transmetalation step, facilitated by the strong trans effect of the electron-withdrawing ligand, and the choice of organozinc nucleophiles, critical to ensure fast group exchange and a positive outcome of the catalytic reactions.