Stereoselective Formation of (Aminoalkyl)platinum Complexes from Imines

Reaction of [PtMe₂(μ-SMe₂)]₂ with cis-1,2-C₆H₁₀(NCH-2-C₅H₄N)₂ (1) and cis-1,2-C₆H₁₀(NCH-2-C₉H₆N)₂ (2) gives the complexes [PtMe₂{cis-1,2-C₆H₁₀(NCH-2-C₅H₄N)₂}] (3) and [PtMe₂{cis-1,2-C₆H₁₀(NCH-2-C₉H₆N)₂}] (4), respectively (C₅H₄N = pyridyl; C₉H₆N = quinolyl), in which the ligand is bidentate such that each complex is chiral and contains a free imine substituent. Complexes 3 and 4 react with protic acids, HCl and CF₃CO₂H, to form (aminoalkyl)platinum(IV) products that contain a new asymmetric carbon center, often with very high stereoselectivity. In these products the complex ligand may act as a tridentate N,N,C donor or, in complexes containing an azaplatinacyclobutane ring, as a tetradentate N,N,N,C donor. The absolute configurations of five (aminoalkyl)platinum(IV) products were determined by X-ray crystal structure determinations and gave a benchmark such that the stereochemistries of other complexes could be assigned by NMR. It is suggested that the reactions occur by protonation at the free imine nitrogen to give an iminium ion, and the carbocation then adds to the adjacent platinum(II) center. The initially formed products may rearrange by mechanisms involving reversible ligand dissociation from platinum(IV) or deprotonation of the aminoalkyl group.