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-he teroaromatic 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) r esults 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 t o high yields. A wide range of olefins can be utilized in the carbonylation reaction, and a variety of functional groups are compatible under the reac tion conditions. Other five-membered N-heteroaromatic compounds, such as py razoles, oxazoles, and thiazoles, can also be used for the carbonylation re action, and in all cases, carbonylation takes place exclusively at a C-H bo nd alpha to the sp(2) nitrogen. The reactivity of the five-membered heteroc ycles 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 coordinatio n of the substrates to the ruthenium center in the catalyst complex is a ne cessary prerequisite for the carbonylation to proceed.