A RuH₂(CO)(PPh₃)₃-Catalyzed Regioselective Arylation of Aromatic Ketones with Arylboronates via Carbon−Hydrogen Bond Cleavage

When the reaction of aromatic ketones with arylboronates (arylboronic acid esters) using RuH₂(CO)(PPh₃)₃ (3) as a catalyst was conducted in toluene, the corresponding arylation product was obtained in moderate yields. In this case, a nearly equivalent amount of a benzyl alcohol derived from a reduction of an aromatic ketone was also formed. The use of aliphatic ketones, such as pinacolone and acetone, as an additive or a solvent dramatically suppressed the reduction of the aromatic ketones and, as a result, ortho-arylation products were obtained in high yield based on the aromatic ketones. In these reactions, the aliphatic ketone functioned as a scavenger of ortho-hydrogens of the aromatic ketones and the B(OR)₂ moiety of the arylboron compound (HB species). A variety of aromatic ketones, such as acetophenones, acetonaphthones, tetralones, and benzosuberone, could also be used in this coupling reaction. Several arylboronates containing electron-donating (NMe₂, OMe, and Me) and -withdrawing (CF₃ and F) groups were also applicable to this coupling reaction. Intermolecular competitive reaction using pivalophenone-d₀ and -d₅ and intramolecular competitive reaction using pivalophenone-d₁ were carried out using 3 as a catalyst. The k_H/k_D value for the intermolecular competitive reaction was substantially different, compared with intramolecular competitive reaction. This strongly suggests the production of an intermediate where the ketone carbonyl is coordinated to the ruthenium involved in this catalytic reaction. ¹H and ¹¹B NMR studies using 2‘-methylacetophenone, phenylboronate (2), and pinacolone (6) indicate that 6 functions effectively as a scavenger of the HB species.