Heck reaction catalysed by pyridyl-imine palladium(0) and palladium(II) complexes

Abstract

Abstract Three different palladium(II) complexes Pd(NN′)Cl 2 and two different palladium(0) complexes Pd(NN′)(dmfu) (dmfu = dimethylfumarate) containing NN′ pyridyl-imine ligands, have been prepared and used as pre-catalysts for the coupling between iodobenzene and methyl acrylate (Meac) (Heck reaction). All the reactions have been conducted in DMF at 80 °C, using Et 3 N as base and a Pd:PhI:Meac:Et 3 N = 1:1000:1100:1000 molar ratio. In all cases the complete conversion of iodobenzene into trans -methylcynnamate has been observed within 2.5 h of reaction, with the highest TOF value (1253) obtained with a palladium(II) complex. At the end of the catalytic reactions the dimeric species [Pd 2 I 6 ][Et 3 NH] 2 has been recovered, pointing out a partial palladium loss from the starting complex species. A poisoning Hg(0) test has shown a heterogeneous nature of the catalytic reactions and this indicates that the Pd(NN′) complexes act as metallic palladium reservoirs. The dimer [Pd 2 I 6 ][Et 3 NH] 2 probably forms from soluble palladium(0) particles arising from the complex decomposition once the substrates have been totally consumed. Interestingly, the dimer [Pd 2 I 6 ][DMFH] 2 has been obtained by reacting PdCl 2 with PhI and Meac in DMF at 80 °C.