Role of alkali halides in the synthesis of nitrogen containing heterocycles by reductive carbonylation of aromatic nitro-derivatives catalysed by Ru3(CO)12

Abstract

The presence of alkali halides as co-catalysts in the reductive carbonylation of o-nitrobiphenyl 1 catalysed by Ru₃(CO)₁₂ strongly increases the rate of reactions and selectivity towards heterocyclization products [carbazole 2 and 5H-phenanthridin-6-one 3]. With sodium halides, depending on the anion, the selectivity for carbazole is in the order Cl^– > Br^–≈ F^– I^–; whereas the selectivity for 5H-phenanthridin-6-one is the reverse I^– > Br^–≈ F^– > Cl^–. The influence of the cation can be explained with the polarization of a CO ligand of the ruthenium cluster, due to the interaction of the alkali cation with the oxygen lone pair. In fact high yields (>80%) of heterocyclization products are obtained when the alkali cation is free to coordinate the CO. When this interaction is reduced, by sequestering the cation with crown-ethers, or better with cryptands, the yield of heterocyclic products is 14% and 0% respectively. The role of the anion and cation in the catalytic mechanism are discussed. The attempted extension of this reaction to other nitro-derivatives for the potential synthesis of aromatic heterocycles with larger rings are also reported.