Researchers at the State Key Laboratory for Oxo Synthesis and Selective Oxidation of the Lanzhou Institute of Chemical Physics and National University of Singapore have found mixed carbene/co-ligand Pd(II) complexes of the type [PdBr2(′Pr2-bimy)L] (C2–C13) effective in the carbonylative annulation of 2-iodophenol with phenyl acetylene to give the respective flavone.
Through a facile preparation and test of their catalytic activity, it was found that complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chloride giving the desirable 2-substituted 4H-3,1-benzoxazin- 4-ones (4) in good yields.
Additionally, this Pd–N-heterocyclic carbene ligands (NHC) complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure.
These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)–NHC complex in different types of carbonylations of aryl iodides under mild conditions.
The work has received support from the National Natural Science Foundation of China. The detailed report of the work has been published in Dalton Trans. (Dalton Trans., 2011, 40, 7632–7638).
Screening of different trans-[PdBr2(′Pr2-bimy)L] catalysts in the carbonylative annulation of 2-iodophenol with phenylacetylene.