Tuning the Regioselectivity of Topochemical Polymerization through Cocrystallization of the Monomer with an Inert Isostere

Abstract

Regiochemistry of topochemical reactions depends on the crystal packing and biasing the regiochemistry necessitates precise crystal engineering. The pristine crystals of monomer 1 upon topochemical azidealkyne cycloaddition (TAAC) reaction give a 1 : 1 blend of 1,4-and 1,5-triazole-linked polymers due to the presence of two self-sorted reactive conformers in the crystal. We designed a binary isomorphous cocrystal of monomer 1 and a structurally similar dummy molecule 2 to limit the number of reactive conformers of 1 to one and thus to get one type of polymer. Equimolar solution of 1 and 2 in chloroform-acetone mixture gave two 1 : 1 cocrystals Co-I and Co-II. The Co-II, a chloroform adduct, on heating undergoes desolvation and polymorphic transition to Co-I. Co-I is isomorphic to 1 and 2 and possess self-sorted arrays of 1 and 2. Heating Co-I results in the TAAC polymerization giving 1,4-triazolyllinked polymer of 1 selectively, showing the power of crystal engineering in regiocontrol. Chemical reactions governed by the molecular arrangementin crystals-topochemical reactions-are attractive in terms of greener, catalyst-free and solvent-free reaction conditions, nonnecessity of chromatographic purification, and their ability to form unique products that are not accessible by the conventional solution-phase methods. [1][2][3][4][5][6] However unlike solution-state reactions that are well-explored, factors that influence topochemical reactions are still poorly understood. [7][8][9][10] The solution-state reactivity of molecules could be easily regulated by altering the reaction conditions such as solvent, reagents, catalyst, etc. However in the case