Synthesis of (1,2,4-oxadiazole)palladium(II) complexes by [2+3] cycloaddition of nitrile oxides to organonitriles in the presence of PdCl2

The reaction between the nitrile oxides 2,4,6-R'3C6H2CNO (R' = Me, OMe) and trans-[PdCl2(RCN)(2)], or RCN (R = Me, Et, CH2CN, NMe2, Ph) in the presence of PdCl2, proceeded smoothly under mild conditions and allowed the isolation of the 1,2,4-oxadiazole complexes trans- [PdCl2{N-a=C(R)-ON=C-b(C6H2R'(3))(N-a-C-b)}(2)] (1-8) in 40-85% yields. In CH2Cl2, the reaction between 2,4,6-R'3C6H2CNO and [PdCl2(MeCN)(2)] furnishes [PdCl2(ONCC6H2R'(3))(2)] (9 and 10), which are the first representatives of metal compounds where nitrile oxides act as ligands. The 1,2,4-oxadiazole complexes 1-8 were characterized by elemental analysis, FAB mass spectrometry, and IR, H-1 and C-13{H-1} NMR spectroscopy, while 2, 3, 7, and 8 were additionally characterized by X-ray crystallography. The liberation of the heterocyclic species from 1-8 was successfully performed by substitution reaction either with 1, 2-bis(diphenylphosphanyl)ethane or with an excess amount of Na2S center dot 7H(2)O in MeOH; the liberated 1,2,4-oxadiazoles (11-18) were characterized by positive-ion FAB mass spectrometry and H-1 and C-13{H-1} NMR spectroscopy. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).