Interaction of 1,3,2,4-Benzodithiadiazines and Their 1-Se Congeners with Ph₃P and Some Properties of the Iminophosphorane Products

Interaction between Ph3P and 1,3,2,4-benzodithiadiazine (1); its 6,7-difluoro (2), 5,6,8-trifluoro (3) and 5,6,7,8-tetrafluoro (4) derivatives; and 5,6,8-trifluoro-3,1,2,4-benzothiaselenadiazine (5) proceeded via a 1:1 condensation to give Ph3PN−R iminophosphoranes (1a−5a, R = corresponding 1,2,3-benzodichalcogenazol-2-yls), which are inaccessible by general approaches based on the Staudinger and Kirsanov reactions. In contrast, neither Ph3As nor Ph3Sb reacted with 1 and 4. Molecular structures of 1a−5a and 5 were confirmed by X-ray diffraction (XRD). The crystals formed by chiral molecules of 2a−5a were racemic, whereas the crystal of 1a was formed by a single enantiomer. In all of the Ph3PN−R derivatives, one of the Ph rings is oriented face-to-face to the hetero ring, R. Upon heating to ∼120 °C in squalane (1a, 3a, 4a) or dissolving in chloroform at ambient temperatures (1a, 2a, 4a), the Ph3PN−R derivatives generated the 1,2,3-benzodithiazolyls (1b−4b, respectively) whose identity was confirmed by electron paramagnetic resonance (EPR). 2,1,3-Benzothiaselenazolyls 5b and 6b were detected by EPR as the main paramagnetic products of solution thermolysis of 5 and its 5,6,7,8-tetrafluoro congener (6), respectively. Passing a chloroform solution of 4a through silica column unexpectedly gave 5-6-6-6 tetracyclic (9) and 6-10-6 tricyclic (10) sulfur−nitrogen compounds, which were characterized by XRD.