Cocrystals for photochemical solid-state reactions: An account on crystal engineering perspective

Syntheses of functional cyclobutanes, i.e., strained four-membered ring compounds are mainly achieved via photochemical [2 + 2] cycloaddition reaction. The key is to bring two reacting olefins (C=C bonds) in close proximity. Conducting such reactions in the solution-phase leads to the formation of a mixture of products, including trans–cis isomerization of the olefin monomer. On the other hand, in the solid-state, a particular product is obtained exclusively. However, the challenges remain to bring a pair of reacting olefin molecules in parallel stacking in its crystalline state, within the range of appropriate distance (Schimdt criteria). In crystal engineering, by exploiting weak intermolecular interactions, a wide variety of crystalline solids, including cocrystals, organic salts, host–guest adduct, coordination complexes and polymers have been designed for synthesizing cyclobutane compounds exclusively in the solid-state. In this review article, we emphasize the importance of cocrystals, underlying crystal engineering principles, and the recent developments on fascinating cocrystals for template-controlled synthesis of functional cyclobutane compounds. Utility of such compounds as ligands in the construction of network solids (cocrystals and coordination polymers) has also been addressed. However, we have restricted our discussion only on the neutral cocrystals, and hence organic salts or ionic cocrystals are not included in this article.