Synthesis and structure of 7-thiabicyclo[2.2.1]heptenes: functional complementarity of the sulfur atom and the ethenyl fragment

Abstract

The manuscript reports on the synthesis of diastereomeric exo- and endo-adducts of the Diels–Alder reaction between thiophene and N-phenylmaleimide. It has been shown that the [4 + 2] cycloaddition can be carried out under catalytic (AlCl₃, normal pressure, r.t.) or hyperbaric (8–10 kbar, 100 °C) conditions. The simultaneous effect of both factors on the reaction was also investigated; the synergistic effect of high pressure and LA catalysis allowed the preparation of the target DA adducts under 10 kbar at r.t. Single crystals of exo- and endo-substituted 7-thiabicyclo[2.2.1]heptenes as well as their crystal mixtures were obtained and characterized by XRD. In these studies, the interchange of the S atom and the ethene (CHCH) fragment was evidenced by the observation of orientational disorder in the X-ray crystal structures of two compounds (mixed crystals 3 and 4). The formation of conglomerates is attributed to the interchangeability of the CC double bond with a chalcogen atom. To our knowledge, this is the first study to report the interchangeability of sulfur and ethenyl substituents in crystal structures, which may have future applications in crystal engineering. In addition, the non-covalent interactions revealed by XRD analysis were characterized using computational approaches such as QTAIM and NCI plot, and rationalized using MEP surface analysis. The latter is useful to reveal the qualitative and quantitative values of the electrostatic potential on the surface and corresponds to the interacting sites of non-covalent interactions. It is also important to emphasize that the energetic analysis reveals that both H-bonds and π-stacking interactions are equally important in dictating the X-ray packing of the obtained mixed crystals.