Direct Carbonylation at a C−H Bond in the Benzene Ring of 2-Phenyloxazolines Catalyzed by Ru3(CO)12. Scope, Limitations, and Mechanistic Aspects

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Abstract
The ruthenium-catalyzed carbonylation at a C−H bond in the benzene ring of a 2-phenyloxazoline is described. The reaction of 2-phenyloxazolines with CO and ethylene in toluene in the presence of a catalytic amount of Ru3(CO)12 resulted in propionylation at an ortho C−H bond in the benzene ring. The presence of the oxazoline ring on the benzene ring is essential for the carbonylation to proceed. Other heterocycles, such as oxazine, oxazole, and thiazoline rings, also served as acceptable directing groups as did the oxazoline ring. A wide functional group compatibility was observed. The site selectivity of the carbonylation was examined using meta-substituted phenyloxazolines. It was found that the carbonylation took place exclusively at the less-hindered C−H bond, irrespective of the nature of substituents, indicating that the site selectivity was determined by steric factors. The reaction was also applicable, not only to a benzene ring, but also to naphthyl and thiophenyl rings. Olefins such as propene and trimethylvinylsilane in place of ethylene could also be used in the carbonylation reaction, while other olefins, such as 1-hexene, tert-butylethylene, vinylcyclohexane, isoprene, 1,5-hexadiene, cyclohexene, 1,5-cyclooctadiene, styrene, methyl acrylate, vinyl acetate, allyltrimethylsilane, and triethoxyvinylsilane did not afford the coupling products. An equilibrium between 2-phenyloxazolines, carbon monoxide, and olefins exists on one hand and the corresponding ketones on the other hand, and product composition is governed by the equilibrium thermodynamics of the system. The results of deuterium labeling experiments suggest that the catalysis involves a reversible C−H bond cleavage and that the rate-determining step is not the cleavage of a C−H bond. The results of kinetic study of the effects of CO pressure show that the reaction rate accelerates with decreasing CO pressure.