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 Ru3(CO)12 results in carbonylation of the C-H bond at the 4-position (adjacent to the sp2-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 α to the sp2 nitrogen. The reactivity of the five-membered heterocycles corresponds to the pKa of the conjugate acid of these heterocycles. The higher the pKa of the substrate, the higher is the reactivity. This indicates that the pKa 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.
ASJC Scopus subject areas
- Organic Chemistry