Elucidation of Substituent‐Responsive Reactivities via Hierarchical and Asymmetric Assemblies in Crystalline p‐Quinodimethane Derivatives

Abstract

AbstractWe propose a mechanism for substituent‐responsive reactivities of p‐quinodimethane derivatives with four ester groups through their hierarchical and asymmetric assembly modes. Four asymmetric 7,8,8‐tris(methoxycarbonyl)‐p‐quinodimethanes with a 7‐positioned ethoxycarbonyl (2 a(H)), 2’‐fluoroethoxycarbonyl (2 b(F)), 2’‐chloroethoxycarbonyl (2 c(Cl)), or 2’‐bromoethoxycarbonyl (2 d(Br)) were synthesized and crystallized. 2 a(H), 2 b(F) and 2 d(Br) afforded only one shape crystal, while 2 c(Cl) did two polymorphic 2 c(Cl)‐α and 2 c(Cl)‐β. UV‐irradiation induced topochemical polymerization for 2 a(H), no reactions for 2 b(F) and 2 c(Cl)‐α, and [6+6] photocycloaddition dimerization for 2 c(Cl)‐β and 2 d(Br). Such substituent‐responsive reactivities and crystal structures were compared with those of the known symmetric 7,7,8,8‐tetrakis(alkoxycarbonyl)‐p‐quinodimethanes such as 7,7,8,8‐tetrakis(methoxycarbonyl)‐ (1 a(Me)‐α and 1 a(Me)‐β), 7,7,8,8‐tetrakis(ethoxycarbonyl)‐ (1 b(Et)), and 7,7,8,8‐tetrakis(bromoethoxycarbonyl)‐ (1 c(BrEt)). The comparative study clarified that the reactivities and crystal structures are classified into four types that link to each other. This linkage is understandable when we analyze the crystal structures through the following hierarchical and asymmetric assemblies; conformers, dimers, one dimensional (1D)‐columns, two dimensional (2D)‐sheets, and three dimensional (3D)‐stacked sheets (3D‐crystals). This supramolecular viewpoint is supported by intermolecular interaction energies among neighbored molecules with the density functional theory (DFT) calculation. Such research enables us to elucidate the substituent‐responsive reactivities of the crystals, and reminds us of the selection of the right path in a so‐called “maze game”.