Palladium mediated C–H bond activation of thiosemicarbazones: Catalytic application of organopalladium complexes in C–C and C–N coupling reactions

Upon reaction with Pd(PPh3)2Cl2 benzaldehyde thiosemicarbazone undergoes a geometrical change around the imine bond and affords a mixed-ligand complex 1, incorporating an NS-coordinated thiosemicarbazone, a triphenylphosphine and a chloride. Similar reactions with benzophenone thiosemicarbazone and acetophenone thiosemicarbazone proceed without any geometrical change to afford complexes 2 and 3 respectively, containing a CNS-coordinated thiosemicarbazone and a triphenylphosphine. Crystal structures of complexes 1, 2 and 3 have been determined. All the complexes display intense absorptions in the visible region. Catalytic activity of complexes 1, 2 and 3 toward Suzuki type C–C coupling reactions has been examined, and all three are found to efficiently activate all types of C–X bonds (X = I, Br, Cl and even F) and bring about the C–C bond formation with high yield. These complexes are also found to catalyze C–N coupling reactions between aryl halides and primary or secondary amines. While all three complexes are found to be efficient catalysts for both C–C and C–N coupling reactions, complex 2 is found to be a better catalyst than the other two complexes.

Graphical abstractBenzaldehyde thiosemicarbazone undergoes a geometrical change around the imine bond during its reaction with Pd(PPh3)2Cl2 and affords complex 1. Substitution of the azomethine proton by phenyl or methyl group prevents the geometrical change and affords complexes 2 and 3 respectively. All the complexes efficiently catalyze C–C and C–N coupling reactions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Benzaldehyde thiosemicarbazone undergoes geometrical change upon reaction with Pd. ► Replacement of azomethine proton by Me or Ph prevents the geometrical change. ► Pd mediated C–H activation takes place with the modified thiosemicarbazones. ► The Pd complexes efficiently catalyze both C–C and C–N coupling reactions.