Diels-Alder topochemistry via charge-transfer crystals: Novel (Thermal) single-crystal-to-single-crystal transformations

The solid-state [4+2] cycloaddition of anthracene to bis(N-ethylimino)-1,4- dithiin occurs via a unique single-phase topochemical reaction in the inter molecular (1:1) charge-transfer crystal. The thermal heteromolecular solid- state condensation involves the entire crystal, and this rare crystalline e vent follows topochemical control during the entire cycloaddition. As a res ult, a new crystalline madification of the Diels-Alder product is formed wi th a crystal-packing similar to that of the starting charge-transfer crysta l but very different from that of the (thermodynamically favored) product m odification obtained from solution-phase crystallization. Such a single-pha se transformation is readily monitored by X-ray crystallography at various conversion stages, and the temporal changes in crystallographic parameters are correlated with temperature-dependent (solid-state) kinetic data that a re obtained by H-1 NMR spectroscopy at various reaction times. Thus, an acc eleration of the solid-state reaction over time is found which results from a progressive lowering of the activation barrier for cycloaddition in a si ngle crystal as it slowly and homogeneously converts from the reactant to t he product lattice.