Experiment and Theory of Bimetallic Pd-Catalyzed α-Arylation and Annulation for Naphthalene Synthesis

Abstract

We report the synthesis of bimetallic Pd­(I) and Pd­(II) complexes with bidentate 2-phosphinoimidazole ligands and their catalytic activity to generate substituted naphthalenes. This process involves the coupling of an aryl iodide and 2 equiv of a ketone via sequential ketone α-arylation and then annulation to generate disubstituted and tetrasubstituted naphthalenes in a regioselective manner. Excellent substrate scope for both aryl iodide and ketone partners is demonstrated, including that for heteroaryl iodides. Bimetallic Pd complexes are much more reactive than monometallic Pd catalysts in this transformation. Density functional theory calculations, isotope effect experiments, and substrate competition experiments were used to examine bimetallic mechanisms, reactivity, and selectivity.

Effect of Precatalyst Oxidation State in C–N Cross-Couplings with 2-Phosphinoimidazole-Derived Bimetallic Pd(I) and Pd(II) Complexes

“…We previously reported the catalytic activity of bimetallic complexes 1 and 2 in a newly discovered naphthalene synthesis reaction that occurs via tandem ketone arylation and cyclization . In that study, we found that Pd­(II) complex 2 had slightly higher reactivity than the Pd­(I) complex.…”

“…We recently reported the synthesis of new bimetallic palladium complexes scaffolded on a 2-phosphinoimidazole ligand that can be isolated at either the Pd­(I) or Pd­(II) oxidation states (Figure b) . Mechanistic experiments and DFT calculations confirmed the thermodynamic stability of the dimeric Pd complex with respect to the monomeric form and suggested that the Pd complex stays intact during catalysis.…”

Changes in ligand coordination mode induce bimetallic C–C coupling pathways

“…In the context of C–C coupling catalysis, only a few examples with a postulated bimetallic mechanism are known, and little mechanistic detail has been revealed. 19–22 Recently, bimetallic Pd I and Pd II complexes supported by bridging 2-phosphinoimidazole ligands were identified as active catalysts for α-arylation to give naphthalene derivatives. 22 Computations supported a bimetallic mechanism with redox cycling between Pd II and Pd III or Pd IV and Ar–I oxidative addition occurred at a single Pd atom within the bimetallic framework.…”

“…19–22 Recently, bimetallic Pd I and Pd II complexes supported by bridging 2-phosphinoimidazole ligands were identified as active catalysts for α-arylation to give naphthalene derivatives. 22 Computations supported a bimetallic mechanism with redox cycling between Pd II and Pd III or Pd IV and Ar–I oxidative addition occurred at a single Pd atom within the bimetallic framework. Of note, similar higher valent Pd II /Pd III or Pd II /Pd IV bimetallic catalytic routes are established for C–X 23 or C–C 24,25 bond formations.…”

“…37–41 Indeed, one of the viable catalytic pathways for bimetallic α-arylation, noted above, involves a switch between κ 2 -P,N and κ 1 -P 2-phosphinoimidazole coordination. 22…”