Pyridinium amidate ligands for efficient Pd-catalyzed reductive nitroarene cyclization using formate as CO surrogate.

The field of reductive cyclization of substituted nitroarenes by CO is dominated by the use of palladium complexes with phenanthroline ligands due to their, up to now, unrivalled combination of resistance to oxidation by the nitroarene and ability to stabilize both Pd(II) and Pd(0) complexes. Recently developed pyridinium amidate and pyridinium amide (PYA) ligands are shown here to play the same role. We tested several N,N-bidentate PYAs in the reductive cyclization of methyl o-nitrocinnamate to 2- carbomethoxyindole using either phenyl formate or the formic acid/acetic anhydride combination as a suitable CO surrogate. This approach avoids high-pressure setups for CO handling, allowing the reactions to be performed in standard pressure tubes. Two of the tested ligands, viz. a quinoline-substituted parapyridinium amide and a pyridine-functionalized ortho-pyridinium amidate gave excellent results that were comparable to those achieved using phenanthroline, even at a low catalyst loading (0.25 mol%). However, the results obtained with the pyridyl-PYA ligand strongly depend on the acidity of the reaction mixture and the reason was identified as its protonation in an acidic environment. Lower activity observed with other ligands was attributed to either an ortho-palladation of the pyridinium N-Me moiety or to an insufficient stabilization of key palladium(0) intermediates.