Acylation of five-membered N-heteroaromatic compounds by ruthenium carbonyl-catalyzed direct carbonylation at a C-H bond

The ruthenium-catalyzed carbonylation at the C-H bond of five-membered N-heteroaromatic compounds is described. The reaction of imidazoles with CO and olefins in toluene in the presence of a catalytic amount of Ru-3(CO)(12) results in carbonylation of the C-H bond at the 4-position (adjacent to the sp(2)-nitrogen) of the imidazole ring to give acylated imidazoles in good to high yields. A wide range of olefins can be utilized in the carbonylation reaction, and a variety of functional groups are compatible under the reaction conditions. Other five-membered N-heteroaromatic compounds, such as pyrazoles, oxazoles, and thiazoles, can also be used for the carbonylation reaction, and in all cases, carbonylation takes place exclusively at a C-H bond alpha to the sp(2) nitrogen. The reactivity of the five-membered heterocycles corresponds to the pK(a) of the conjugate acid:of these heterocycles. The higher the pK(a) of the substrate, the higher is the reactivity. This indicates that the pK(a) values are related to the ability of the nitrogen atom in the substrates to coordinate to a ruthenium center. The coordination of the substrates to the ruthenium center in the catalyst complex is a necessary prerequisite for the carbonylation to proceed.