Ru₃(CO)₁₂- and Rh₄(CO)₁₂-Catalyzed Reactions of Pyridylolefins or N-(2-Pyridyl)enamines with CO and Olefins. Carbonylation at Olefinic C-H Bonds

This paper describes a study of the Ru₃(CO)₁₂-catalyzed carbonylation at an olefinic C-H bond. The reaction of pyridylolefins with CO and ethylene in the presence of a catalytic amount of Ru₃-(CO)₁₂ in toluene results in propionylation at an olefinic C-H bond in pyridylolefins. The carbonylation occurs regioselectively at a position γ to the pyridine nitrogen. Transition-metal complexes other than Ru₃(CO)₁₂, that have thus far been examined exhibit no catalytic activity, and ethylene serves as the only olefin. A similar tendency has been noted in the previously reported carbonylation at a C-H bond in the benzene ring of pyridylbenzenes. This reaction can be also applied to N-(2-pyridyl)enamines, in which an olefin unit is separated from the pyridine ring by an sp³-nitrogen atom. The reaction of N-(2-pyridyl)enamines with CO and ethylene gives the corresponding ethyl ketones as the coupling products. Interestingly, Rh₄(CO)₁₂ also shows high catalytic activity in the case of N-(2-pyridyl)enamines. In addition, olefins such as propene, 1-hexene, 3,3-dimethyl-1-butene, styrene, cyclopentene, acryl acid methyl ester, ethyl vinyl ether, and trimethylvinylsilane can also be used. This is in sharp contrast to the case of the carbonylation at a C-H bond in pyridylbenzenes reported previously and to the results of pyridylolefins as mentioned above, where Ru₃(CO)₁₂ is the only active catalyst and hexene cannot substitute for ethylene.