Regio- and Stereoselective 1,3-Dipolar Cycloaddition of Cyclic Azomethine Imines to Platinum(IV)-Bound Nitriles Giving Δ2‑1,2,4-Triazoline Species

The complex trans-[PtCl4(EtCN)2] (14) reacts smoothly at 25 °C with the stable cyclic azomethine imines R1CHNaNC­(O)­CH­(NHC­(O)­C6H4R3)­CbH­(C6H4R2)(a−b) [R1/R2/R3 = p-Me/H/H (8); p-Me/p-Me/H (9); p-Me/p-MeO/H (10); p-Me/p-Cl/p-Cl (11); p-MeO/p-Me/H (12); p-MeO/p-Cl/m-Me (13)], and the reaction proceeds as stereoselective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the monocycloadducts trans-[PtCl4(EtCN)­{NaC­(Et)­NbC­(O)­CH­(NHC­(O)­C6H4R3)­CH­(C6H4R2)­NcCdHR1}])(a–d;b–c) [R1/R2/R3 = p-Me/H/H (15); p-Me/p-Me/H (16); p-Me/p-MeO/H (17); p-Me/p-Cl/p-Cl (18); p-MeO/p-Me/H (19); p-MeO/p-Cl/m-Me (20)]. Inspection of the obtained and literature data indicate that the cycloaddition of the azomethine imines to the C≡N bonds of HCN and of PtIV-bound EtCN has different regioselectivity leading to Δ2-1,2,3-triazolines and Δ2-1,2,4-triazolines, respectively. Platinum­(II) species trans-[PtCl2(EtCN)­{NaC­(Et)­NbC­(O)­CH­(NHC­(O)­C6H4R3)­CH­(C6H4R2)­NcCdHR1}](a–d;b–c) [R1/R2/R3 = p-Me/H/H (21); p-Me/p-Me/H (22); p-Me/p-MeO/H (23); p-Me/p-Cl/p-Cl (24); p-MeO/p-Me/H (25); p-MeO/p-Cl/m-Me (26)] were obtained by a one-pot procedure from 14 and 8–13 followed by addition of the phosphorus ylide Ph3PCHCO2Me. Δ2-1,2,4-Triazolines NaC­(Et)­NbC­(O)­CH­(NHC­(O)­C6H4R3)­CH­(C6H4R2)­NcCdHR1(a–d;b–c) [R1/R2/R3 = p-Me/H/H (27); p-Me/p-Me/H (28); p-Me/p-MeO/H (29); p-Me/p-Cl/p-Cl (30); p-MeO/p-Me/H (31); p-MeO/p-Cl/m-Me (32)] were liberated from 21–26 by the treatment with bis­(diphenylphosphyno)­ethane (dppe). Platinum­(II) complexes 21–26 were characterized by elemental analyses (C, H, N), high-resolution electrospray ionization mass spectrometry (ESI-MS), and IR and 1H and 13C­{1H} NMR spectroscopies and single crystal X-ray diffraction in the solid state for 25·CH3OH, 26·(CHCl3)0.84. The structure of 26 was also determined by COSY-90 and NOESY NMR methods in solution. Quantitative evaluation of several pairs of interproton distances obtained by NMR and X-ray diffraction agrees well with each other and with those obtained by the MM+ calculation method. Platinum­(IV) complexes 15–20 were characterized by 1H NMR spectroscopy. Metal-free 6,7-dihydropyrazolo­[1,2-a]­[1,2,4]­triazoles (27–32) were characterized by high-resolution ESI-MS and IR and 1H and 13C­{1H} NMR spectroscopies and single crystal X-ray diffraction for 29·CDCl3. Theoretical density functional theory calculations were carried out for the investigation of the reaction mechanism, interpretation of the reactivity of Pt-bound and free nitriles toward azomethine imines and analysis of the regio- and stereoselectivity origin.