Ruthenium-Catalyzed Oxidative Homocoupling of Arylboronic Acids in Water: Ligand Tuned Reactivity and Mechanistic Study

Molecular
catalysts based on water-soluble arene-Ru­(II) complexes ([Ru]-1–[Ru]-5) containing aniline (L1),
2-methylaniline (L2), 2,6-dimethylaniline (L3), 4-methylaniline (L4), and 4-chloroaniline (L5) were designed for the homocoupling of arylboronic acids
in water. These complexes were fully characterized by 1H, 13C NMR, mass spectrometry, and elemental analyses.
Structural geometry for two of the representative arene-Ru­(II) complexes [Ru]-3 and [Ru]-4 was established by single-crystal
X-ray diffraction studies. Our studies showed that the selectivity
toward biaryls products is influenced by the position and the electronic
behavior of various substituents of aniline ligand coordinated to
ruthenium. Extensive investigations using 1H NMR, 19F NMR, and mass spectral studies provided insights into the
mechanistic pathway of homocoupling of arylboronic acids, where the
identification of important organometallic intermediates, such as
σ-aryl/di­(σ-aryl) coordinated arene-Ru­(II) species, suggested
that the reaction proceeds through the formation of crucial di­(σ-aryl)-Ru
intermediates by the interaction of arylboronic acid with Ru-catalyst
to yield biaryl products.