Topochemical Ene–Azide Cycloaddition Reaction

Abstract

Topochemical reactions, high‐yielding solid‐state reactions arising from the proximal alignment of reacting partners in the crystal lattice, do not require solvents, catalysts, and additives, are of high demand in the context of green processes and environmental safety. However, the bottleneck is the limited number of reactions that can be done in the crystal medium. We present the topochemical ene–azide cycloaddition (TEAC) reaction, wherein alkene and azide groups undergo lattice‐controlled cycloaddition reaction giving triazoline in crystals. A designed monomer that arranges in a head‐to‐tail manner in its crystals pre‐organizing the reacting groups of adjacent molecules in proximity undergoes spontaneous cycloaddition reaction in a single‐crystal‐to‐single‐crystal fashion, yielding the triazoline‐linked polymer. A unique advantage of this reaction is that the triazoline can be converted to aziridine by simple heating, which we exploited for the otherwise challenging post‐synthetic backbone modification of the polymer. This reaction may revolutionize the field of polymer science.

Topochemical Cycloaddition Reaction between an Azide and an Internal Alkyne

“…These reactions gained much attention recently not only due to the urge for promoting environmentally benign, sustainable and green protocols but also in view of their use in synthesizing complex and higher order polymers [9] . We have exploited topochemical alkyne‐azide cycloaddition (TAAC) reaction [10] and topochemical ene‐azide cycloaddition (TEAC) reaction [11] for the synthesis of various polymers. Especially, the TAAC reaction of monomers containing an azide and a terminal alkyne group provided access to a variety of structurally diverse disubstituted‐triazole‐linked polymers having interesting properties [12] .…”

Circularly Recyclable Polymers Featuring Topochemically Weakened Carbon–Carbon Bonds

“…For this reason, in conventional semicrystalline hydrocarbon-based polymers, such as polyethene (PE), polystyrene (PS), and polymethylmethacrylate (PMMA), all C–C bonds exhibit standard lengths of 1.53–1.54 Å between the repeating units (Figure c). Alternatively, this limitation can be avoided in solid-state topochemical polymerization, where the condensed-phase interactions of the crystal lattice favorably position monomer molecules to react. − In this way, adding extra steric hindrance to fine-tune the bond length is possible without inhibiting polymerization. Another advantage is that topochemical polymerization normally produces high-quality stereo-regular and ultra-high-molecular-weight crystalline polymers. − Thus, this strategy potentially enables polymer systems that perform similarly to traditional hydrocarbon-based polymers while also easily depolymerizing to monomers through thermal cleavage of the relatively weak C–C bond.…”

Recent progress in single-crystal structures of organic polymers

“…Besides controlling the crystallization process, topochemical polymerization has also been confirmed as a promising way to prepare single crystals of conjugated polymers. 3,25–49 Topochemical polymerization is a transformation process in the crystal state, where the critical factor is the suitable spatial arrangement of functional groups that are able to form new covalent bonds under the stimulation of light/heat/pressure. 17,25,50–52 Thus, minimizing the impact of newly formed covalent bonds on the structural arrangement while simultaneously maintaining the crystallinity (single crystals) is highly desirable but extremely challenging.…”