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.