Regio- and stereoselective 1,3-dipolar cycloaddition of cyclic azomethine imines to platinum(IV)-bound nitriles giving Δ2-1,2,4-triazoline species,

Abstract

The complex trans-[PtCl4(EtCN)2] (14) reacts smoothly at RT 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 mono-cycloadducts 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 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 dppe. Platinum(II) complexes 21–26 were characterized by elemental analyses (C, H, N), high-resolution ESI-MS, and IR and 1H and 13C{1H} NMR spectroscopies and single crystal X-ray diffraction in 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 XRD 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 DFT 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.