Ru3(CO)12-Catalyzed Reaction of Pyridylbenzenes with Carbon Monoxide and Olefins. Carbonylation at a C−H Bond in the Benzene Ring

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Abstract
A ruthenium-catalyzed carbonylation at a C−H bond in a benzene ring is described. The reaction of pyridylbenzenes with CO (20 atm) and ethylene in toluene at 160 °C in the presence of a catalytic amount of Ru3(CO)12 results in propionylation at an ortho C−H bond in the benzene ring. Carbonylation does not occur at the pyridine ring, although this is necessary as a directing group to promote the reaction. Olefins such as trimethylvinylsilane and tert-butylethylene in place of ethylene can also be used in this reaction, however 1-hexene, cyclohexene, allyltrimethylsilane, styrene, methyl methacrylate, vinyl acetate, triethoxyvinylsilane, and isopropenyltrimethylsilane do not afford the coupling products. Transition metal complexes, other than ruthenium carbonyl, examined thus far, do not show catalytic activity. In the reaction of meta-substituted pyridylbenzenes, such as those having Me, OMe, CF3, and COOMe group at the meta position in the benzene ring, carbonylation takes place at the less hindered C−H bond exclusively, irrespective of the electronic nature of the substituents. It is apparent that steric factors are more important for the control of regioselectivity. The reaction is also applicable to naphthyl and thienyl rings. Six-membered heterocycles, such as 2-pyrimidine and 4-pyrimidine, are also effective directing groups for carbonylation at a C−H bond in the benzene ring. The present reaction represents the first, effective catalytic carbonylation reaction involving cleavage of the benzene C−H bond.