Accessing a regiospecific isomer and a metastable polymorph through crystal engineering and solid-state reaction

Abstract

Methods to produce molecules regioselectively are extremely important in organic synthesis. It is vital to limit the number of possible products by making one orientation more favoured using synthesis conditions or by locking the reagents in a particular orientation. One way to do this is by actively using interactions between molecules in crystals to create a suitable alignment between the reacting molecules. Here, a thermally reactive co-crystal was designed by exploiting electron donor and acceptor interactions as well as C–H⋯O hydrogen bonding involving (N-cyclododecyl, N′-p-nitrophenylimino)-1,4-dithiin and anthracene. These intermolecular interactions facilitate the 1 : 1 stacking of alternating reactants and enable regioselectivity in the formation of a [4 + 2] Diels–Alder product. The crystals were found to undergo a single-crystal-to-single-crystal (SCSC) transformation, studied via single crystal X-ray diffraction (SCXRD), yielding the product completely after 6 days of heating at 50 °C though a faster reaction is possible at 80 °C. This reaction not only yields a regiospecific product but also results in a metastable crystal form found to be inaccessible through conventional means. Reaction in solution results in two products in an 80 : 20 ratio.