Charge-transfer forces in the self-assembly of heteromolecular reactive solids: Successful design of unique (single-crystal-to-single-crystal) Diels-Alder cycloadditions

Electron donor/acceptor (EDA) interactions are found to be a versatile meth odology for the engineering of reactive heteromolecular crystals. In this w ay, a series of the charge-transfer pi -complexes between bis(alkylimino)-1 ,4-dithiin accepters and anthracene donors are shown to form heteromolecula r(l:l)crystalline solids that spontaneously undergo stereoselective [2 + 4] Diels-Alder cycloadditions. The flexible nature of the 1.4-dithiin moiety allows this homogeneous topochemical transformation to proceed with minimal distortion of the crystal lattice As a result, a unique (single) crystal p hase of the Dials-Alder adduct can be produced anti-thermodynamically with a molecular arrangement very different from that in solvent-grown crystals. Such a topochemical reaction between bis(methylimino)- 1,4-dithiin and ant hracene proceeds thermally and homogeneously up to very high conversions wi thout disintegration of the single crystal. This ideal case of the mono-pha se topochemical conversion can be continuously monitored structurally (X-ra y crystallography) and kinetically (NMR spectroscopy) throughout the entire range of the crystalline transformation. The resultant "artificial" crysta l of the Diels-Alder adduct is surprisingly stable despite its different sy mmetry and packing mode compared to the naturally grown (thermodynamic) cry stal.