Activation of CH bonds of imines by transition metal compounds

Abstract

In this work, we describe the synthesis of some metallacycles by reaction between aromatic imines and palladium acetate. In this process, a carbon-palladium bond is formed by the activation of an ortho CH bond (a cyclometallation reaction). The reaction between the imines ClC6H4CHNCH2C6H5 or C6H5CHNCH2C6H5 and palladium acetate afforded the cyclometallated compounds [Pd(μ-AcO)(o-ClC6H3CHNCH2C6H5)]2 and [Pd(μ-AcO)(C6H4CHNCH2C6H5)]2 which contains an acetate bridged ligand. In a second reaction, the halogen bridged metallacycle, [Pd(μ-Cl)(o-ClC6H3CHNCH2C6H5)]2, was obtained by reaction between the acetate bridged compound [Pd(μ-AcO)(o-ClC6H3CHNCH2C6H5)]2 and LiCl. The monomer [PdCl(o-ClC6H3CHNCH2C6H5)(PPh3)] was obtained by the reaction between [Pd(μ-Cl)(o-ClC6H3CHNCH2C6H5)]2 and PPh3. Monomers [PdCl(C6H4CHNCH2C6H5)(PPh3)] and [PdBr(C6H4CHNCH2C6H5)(PPh3)] were obtained through a one pot reaction, by reaction between the corresponding acetate bridged compound and LiCl (or LiBr) and PPh3, in order to increase the yield of the process. Finally, the monomer [Pd(C7H9N)(o-RC6H3CHNCH2C6H5)(PPh3)], in which the halogen ligand is substituted by the 2,4-lutidine, has been tried to obtain, without success, by different reactions and conditions. The products have been characterized by IR, 1H NMR and 31P NMR and the following remarkable features have been found: i) the compounds with a chloro in ortho position present an interaction between this atom and the imine proton; ii) the acetate bridged compounds present an “open book” structure; iii) the P atom of the PPh3 is coupled with the HC=N proton showing the trans disposition between the phosphine and the methinic proton.