Ruthenium-Catalyzed Addition of Carbon-Hydrogen Bonds in Aromatic Ketones to Olefins. The Effect of Various Substituents at the Aromatic Ring

To obtain further insight into the new ruthenium-catalyzed reaction of carbon-hydrogen bonds in aromatic ketones with olefins, the effect of various substituents at the aromatic ring is examined. Reaction of o-methylacetophenone with triethoxyvinylsilane (2) in the presence of [Ru(H)₂(CO)(PPh₃)₃] (3)as the catalyst gave the 1 : 1 addition product in quantitative yield. Similarly, the ketone having an o-CF₃ group gave the coupling product 9 in 92% yield. However, ortho substituents such as OMe, F, and CN, seem to react with and kill the catalyst so that no efficient reaction was attained. In the cases of the reactions of p-methoxy- and p-fluoroacetophenones with 2, the corresponding 1 : 2 addition products were obtained as the major product. In the cases of m-substituted acetophenones, two different C-H bonds at the ortho positions are available. The C-C bond formation preferentially occurred at the sterically less congested positions. Exceptions are the reactions of m-methoxy- and m-fluoroacetophenones, in which the C-C bond formation took place preferentially at the more congested position. This may be due to the coordination of oxygen or fluorine atom to ruthenium. The factors controlling these selectivities are discussed.