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 methodology for the engineering of reactive heteromolecular crystals. In this way, a series of the charge-transfer pi-complexes between bis(alkylimino)-1,4-dithiin acceptors and anthracene donors are shown to form heteromolecular (1:1) 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 phase of the Diels--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 anthracene proceeds thermally and homogeneously up to very high conversions without disintegration of the single crystal. This ideal case of the mono-phase topochemical conversion can be continuously monitored structurally (X-ray crystallography) and kinetically (NMR spectroscopy) throughout the entire range of the crystalline transformation. The resultant "artificial" crystal of the Diels--Alder adduct is surprisingly stable despite its different symmetry and packing mode compared to the naturally grown (thermodynamic) crystal.