Ruthenium- and Rhodium-Catalyzed Direct Carbonylation of the Ortho C−H Bond in the Benzene Ring of N-Arylpyrazoles

The direct carbonylation of C−H bonds in the benzene ring of N-phenylpyrazoles via catalysis by ruthenium or rhodium complexes is described. The reaction of N-phenylpyrazoles with carbon monoxide and ethylene in the presence of Ru3(CO)12 or Rh4(CO)12 resulted in the site-selective carbonylation of the ortho C−H bonds in the benzene ring to give the corresponding ethyl ketones. A variety of functional groups on the benzene ring can be tolerated. N-Phenylpyrazoles have higher reactivities than would be expected, based on the pKa values of the conjugate acid of pyrazole. The choice of solvent for this reaction is significant, and N, N-dimethylacetamide (DMA) gives the best result.