Crystal engineering of molecules with through-space α-effect hydrogen bonds: 3a,6 : 7,9a-diepoxybenzo[de]isoquinolines possessing a free amino group

This manuscript reports the synthesis, X-ray characterization and DFT study of a series of all-cis 3a,6 : 4,5 : 7,9a-triepoxy and 3a,6 : 7,9a-diepoxybenzo[de]isoquinolines obtained by the tandem intramolecular [4 + 2]/[4 + 2] cycloaddition in bis-furans (the IMDAF reaction) between difurfurylamine derivatives and dimethyl acetylenedicarboxylate (DMAD) as a dienophile. In one compound instead of DMAD, 1-phenyl-1H-pyrrole-2,5-dione (N-phenylmaleimide) was used as a dienophile. The existence of a “through-space” alpha-effect is analyzed in these compounds, since in three of them N-H⋯O2 H-bonds are observed in the solid state promoting the formation of self-assembled dimers. That is, the close O⋯O distance between the O-bridge atoms due to the rigidity of these molecules causes a lone pair⋯lone pair (LP⋯LP) interaction that enhances the H-bond acceptor ability of the bridging atoms (through-space α-effect). This fact has been studied using DFT calculations, molecular electrostatic potential (MEP) surfaces, electron localization function (ELF), quantum theory of atoms-in-molecules (QTAIM) and noncovalent interaction plot (NCIplot) index computational tools. This is the third study of the literature where “α-effect” hydrogen bonds (AEHBs) are described. The novelty of the present work resides in the utilization of an amine linker between the furan rings that allows the incorporation of a good H-bond donor in the model system. As a result, NH⋯O2 instead of CH⋯O2 AEHBs described in previous works are established. The X-ray analyses of the synthesized di- and tri-epoxides reveal that the AEHBs are predominant in the X-ray architecture of these compounds. © 2022 The Royal Society of Chemistry.