Interplay between nitrones and (nitrile)PdII complexes:: Cycloaddition vs. complexation followed by cyclopalladation and deoxygenation reactions

The reaction between the nitrone p-MeC6H4CH=N(Me)O and trans-[PdCl2(RCN)(2)] (R = Ph, Me) in the corresponding RCN (or of the nitrone in neat RCN in the presence of PdCl2) proceeds at 45 degrees C (R = Ph) or reflux (R = Me) for 1 d and allows the isolation of the Delta(4)-1,2,4-oxadiazoline complexes [PdCl2{N-a=C(R)ON(Me)(CH)-H-b(C6H4Me-p)}(N-a-C-b)] (R = Ph 1; R = Me 2) in ca. 50 and ca. 15 % yields, respectively. The reaction time can be drastically reduced by focused microwave irradiation of the reaction mixture. In CH2Cl2 or acetone, this reaction proceeds in another direction to achieve the unstable nitrone complex [PdCl2{ON(Me)=CH(C6H4Me-p)}(2)] (3), which was characterized by electrospray mass spectrometry, IR and H-1 NMR spectroscopy. Complex 3 is the first representative of (nitrone)Pd-II compounds and is the key intermediate in at least two further reactions, i.e. cyclopalladation to give the dimeric complex [Pd-2(mu-Cl)(2){ON(Me)=C(H) C6H3Me-p}(2)] (5; 30 % isolated yield) and deoxygenation of the nitrone to furnish the imine compound [PdCl2{N(Me)=CH(C6H4Me-p)}(2)] (4). The palladium complexes 1, 2, 4 and 5 were characterized by C, H, and N analyses, FAB-MS, IR, H-1 and C-13{H-1} spectroscopy, while 4 and 5 additionally by X-ray crystallography. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.