Fluorinated 1,3λ4δ2,2,4-benzodithiadiazines - A synthetic, structural and theoretical study

Article abstract of DOI:10.1002/ejic.200390016

The series of title compounds has been prepared through both electrophilic (C₆H_nF_5-n-N=S=N-SiMe₃ + SCl₂) and nucleophilic (C₆HF₄_S-N=S=N-SiMe₃ + CsF) intramolecular ortho-cyclisation reactions, and the former route seems to be the more effective. High regioselectivity of the ring-closing procedures is observed in both cases. The compounds were characterised by X-ray crystallography and multinuclear (¹H, ¹³C, ¹⁵N and ¹⁹F) NMR spectroscopy. In accordance with GIAO calculations, ¹⁵N{¹H} experiments and the effects observed on complete substitution of hydrogen by fluorine, the high-field signal in the ¹⁵N NMR spectra can be assigned to N-4 and the low-field signal to N-2. In the crystal, 5,6,7-trifluoro-(5) and 5,6,8-trifluoro-1,3λ⁴δ²2,4- benzodithiadiazine (6) are planar, whereas the 6,8-difluoro derivative 3 is bent along the S1...N4 line by 8.3°. According to NICS calculations the heterocycle moiety in this compound class is antiaromatic while the carbocycle is aromatic. The fluorine substituents increase the aromaticity - and in some cases (especially when a fluorine atom is present in the 8-position) the antiaromaticity - of the corresponding rings. The ortho-fluoro-containing starting material C₆H_nF_5-n-N=S=N-SiMe₃ (n = 2: 10) cyclises to the fluorinated 2,1,3-benzothiadiazole 27 upon treatment with CsF instead of SCl₂. For starting compound 6-HC₆F₄-S-N=S=N-SiMe₃ (14) the planar (Z,E) configuration features a short intramolecular H...N contact, as evidenced by X-ray diffraction. Both the reaction pathways mentioned are also discussed. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2003.