Interplay between Nitrones and (Nitrile)Pd II Complexes: Cycloaddition vs. Complexation Followed by Cyclopalladation and Deoxygenation Reactions

The reaction between the nitrone p ‐MeC 6 H 4 CH=N(Me)O and trans ‐[PdCl 2 (RCN) 2 ] (R = Ph, Me) in the corresponding RCN (or of the nitrone in neat RCN in the presence of PdCl 2 ) proceeds at 45 °C (R = Ph) or reflux (R = Me) for 1 d and allows the isolation of the Δ 4 ‐1,2,4‐oxadiazoline complexes [PdCl 2 {N a =C(R)ON(Me)C b H(C 6 H 4 Me‐ p )} 2 (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 CH 2 Cl 2 or acetone, this reaction proceeds in another direction to achieve the unstable nitrone complex [PdCl 2 { O N(Me)=CH(C 6 H 4 Me‐ p )} 2 ] ( 3 ), which was characterized by electrospray mass spectrometry, IR and 1 H 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 (μ‐Cl) 2 { O N(Me)=C(H) C 6 H 3 Me‐ p } 2 ] ( 5 ; 30 % isolated yield) and deoxygenation of the nitrone to furnish the imine compound [PdCl 2 {N(Me)=CH(C 6 H 4 Me‐ p )} 2 ] ( 4 ). The palladium complexes 1 , 2 , 4 and 5 were characterized by C, H, and N analyses, FAB‐MS, IR, 1 H and 13 C{ 1 H} spectroscopy, while 4 and 5 additionally by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)