Solid state [2 + 2] photocycloaddition for constructing dimers of N,N′-diacyl-1,4-dihydropyrazines based on thiourea-induced assembly

The solid state [2 + 2] photocycloaddition of N,N′-diacyl-1,4-dihydropyrazines has been investigated using thiourea as a template to preorient the reactants in the crystalline state in a manner that is beneficial for the [2 + 2] photocycloaddition. The structures of the photoadducts, syn-dimers and cage-dimers, were determined using NMR and single-crystal X-ray diffraction. According to the topochemical principles used to assess the feasibility of solid state photochemical [2 + 2] cycloaddition, topological analysis of cocrystals consisting of the N,N′-diacyl-1,4-dihydropyrazines and thiourea, as well as the syn-dimers, was performed to investigate the influence of packing modes and non-covalent interactions on their solid state photoreactivity. The results showed that the solid state [2 + 2] reactivities of both the N,N′-diacyl-1,4-dihydropyrazines assembled using thiourea and the syn-dimers were controlled by the topochemical orientation of the potentially reactive double bonds and the H-bonding interactions within the adjacent reactive molecules.