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 <i>trans-</i>[PtCl₄(EtCN)₂] (<b>14</b>) reacts smoothly at 25 °C with the stable cyclic azomethine imines R¹CHN^<i>a</i>NC­(O)­CH­(NHC­(O)­C₆H₄R³)­C^<i>b</i>H­(C₆H₄R²)^{(<i>a</i>−<i>b</i>)} [R¹/R²/R³ = <i>p-</i>Me/H/H (<b>8</b>); <i>p-</i>Me/<i>p-</i>Me/H (<b>9</b>); <i>p-</i>Me/<i>p-</i>MeO/H (<b>10</b>); <i>p-</i>Me/<i>p-</i>Cl/<i>p-</i>Cl (<b>11</b>); <i>p-</i>MeO/<i>p-</i>Me/H (<b>12</b>); <i>p-</i>MeO/<i>p-</i>Cl/<i>m-</i>Me (<b>13</b>)], and the reaction proceeds as stereoselective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the <i>mono</i>cycloadducts <i>trans-</i>[PtCl₄(EtCN)­{N^<i>a</i>C­(Et)­<i>N</i>^<i>b</i>C­(O)­CH­(NHC­(O)­C₆H₄R³)­CH­(C₆H₄R²)­<i>N</i>^<i>c</i>C^<i>d</i>HR¹}])^{(<i>a–d;b–c</i>)} [R¹/R²/R³ = <i>p-</i>Me/H/H (<b>15</b>); <i>p-</i>Me/<i>p-</i>Me/H (<b>16</b>); <i>p-</i>Me/<i>p-</i>MeO/H (<b>17</b>); <i>p-</i>Me/<i>p-</i>Cl/<i>p-</i>Cl (<b>18</b>); <i>p-</i>MeO/<i>p-</i>Me/H (<b>19</b>); <i>p-</i>MeO/<i>p-</i>Cl/<i>m-</i>Me (<b>20</b>)]. Inspection of the obtained and literature data indicate that the cycloaddition of the azomethine imines to the C≡N bonds of HCN and of Pt^IV-bound EtCN has different regioselectivity leading to Δ²-1,2,3-triazolines and Δ²-1,2,4-triazolines, respectively. Platinum­(II) species <i>trans-</i>[PtCl₂(EtCN)­{N^<i>a</i>C­(Et)­<i>N</i>^<i>b</i>C­(O)­CH­(NHC­(O)­C₆H₄R³)­CH­(C₆H₄R²)­<i>N</i>^<i>c</i>C^<i>d</i>HR¹}]^{(<i>a–d;b–c</i>)} [R¹/R²/R³ = <i>p-</i>Me/H/H (<b>21</b>); <i>p-</i>Me/<i>p-</i>Me/H (<b>22</b>); <i>p-</i>Me/<i>p-</i>MeO/H (<b>23</b>); <i>p-</i>Me/<i>p-</i>Cl/<i>p-</i>Cl (<b>24</b>); <i>p-</i>MeO/<i>p-</i>Me/H (<b>25</b>); <i>p-</i>MeO/<i>p-</i>Cl/<i>m-</i>Me (<b>26</b>)] were obtained by a one-pot procedure from <b>14</b> and <b>8</b>–<b>13</b> followed by addition of the phosphorus ylide Ph₃PCHCO₂Me. Δ²-1,2,4-Triazolines N^<i>a</i>C­(Et)<i>­N</i>^<i>b</i>C­(O)­CH­(NHC­(O)­C₆H₄R³)­CH­(C₆H₄R²)­<i>N</i>^<i>c</i>C^<i>d</i>HR^{1(<i>a–d;b–c</i>)} [R¹/R²/R³ = <i>p-</i>Me/H/H (<b>27</b>); <i>p-</i>Me/<i>p-</i>Me/H (<b>28</b>); <i>p-</i>Me/<i>p-</i>MeO/H (<b>29</b>); <i>p-</i>Me/<i>p-</i>Cl/<i>p-</i>Cl (<b>30</b>); <i>p-</i>MeO/<i>p-</i>Me/H (<b>31</b>); <i>p-</i>MeO/<i>p-</i>Cl/<i>m-</i>Me (<b>32</b>)] were liberated from <b>21</b>–<b>26</b> by the treatment with bis­(diphenylphosphyno)­ethane (dppe). Platinum­(II) complexes <b>21</b>–<b>26</b> 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 <b>25</b>·CH₃OH, <b>26</b>·(CHCl₃)_0.84. The structure of <b>26</b> 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 <b>15</b>–<b>20</b> were characterized by ¹H NMR spectroscopy. Metal-free 6,7-dihydropyrazolo­[1,2-<i>a</i>]­[1,2,4]­triazoles (<b>27</b>–<b>32</b>) were characterized by high-resolution ESI-MS and IR and ¹H and ¹³C­{¹H} NMR spectroscopies and single crystal X-ray diffraction for <b>29</b>·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