Oxadiazoline and ketoimine palladium(II) complexes as highly efficient catalysts for Suzuki-Miyaura cross-coupling reactions in supercritical carbon dioxide

Abstract

The [2+3] cycloaddition of nitriles (RCN) with 2,2-dimethyl-3,4-dihydro-2H- pyrrole 1-oxide, in the presence of palladium dichloride (PdCl2) gives the corresponding 2,3-dihydro[1.2.4]oxadiazole (Δ4-1,2,4- oxadiazoline) palladium(II) complexes 1-4 in good yields. However, the Pd(II)-assisted reaction of pentafluorobenzonitrile with the same pyrroline N-oxide gives a mixture of oxadiazoline 5, ketoimine 6 and pyrrolylbenzamide- ketoimine 7 Pd(II) complexes, which affords upon heating in refluxing acetone the unprecedented fused tricyclic ketoimine complex 8 as the exclusive product. Under heating, compounds 5 and 7 transform to 6, the latter undergoing intramolecular cyclization by nucleophilic attack of the amino moiety to the ortho carbon of the pentafluorophenyl ring leading ultimately to 8. The compounds were characterized by IR, 1H and 13CNMR, ESI+-MS, elemental analyses and, in the cases of 3, 6, 7 and 8, also by X-ray diffraction analyses. The catalytic properties of the Pd complexes were evaluated in Suzuki-Miyaura cross-coupling reactions, using supercritical carbon dioxide (scCO2) as a green solvent. Cross-couplings of aryl halides with phenylboronic acid give the desired biaryl products in quantitative yields, in a short reaction time, for substrate-to-catalyst molar ratios as high as 4.0·104. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.