Reversible cyclodimerization of cyclic (alkyl)(amino)carbene–carbon disulfide adduct
Gayeong Lim, Jaegeum Cha, Youngsuk Kim*
Cite this article as: Chem. Commun. 2025, 61, 18384.
DOI: https://doi.org/10.1039/D5CC04726A
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Short Summary
This study introduces a novel, reagent-free switchable covalent system where a CAAC-CS₂ adduct thermally dimerizes into a tetrathiane ring and quantitatively dissociates back to the monomer under LED light, enabling highly controllable and reversible S–S bond formation.
Key Findings
Thermal Dimerization (Forward Reaction): Heating the CAAC–CS₂ monomer (1) at 77 °C for 24 hours induces the formation of two S–S bonds, yielding a 1,2,4,5-tetrathiane dimer (2) in an 80% isolated yield.
Photochemical Dissociation (Reverse Reaction): Irradiating the dimer with 390 nm (UV) or 440 nm (Blue) LED light perfectly cleaves the S–S bonds within 70 minutes, recovering the original monomer in a 93% yield without any external chemical reagents.
Mechanistic Insight via TD-DFT: Natural transition orbital (NTO) analysis reveals that light absorption at 391 nm triggers an intramolecular electron transfer into the σ*(S–S) antibonding orbital, driving a fast, stepwise photochemical cleavage of the disulfide bonds.
The Necessity of CAAC: The choice of carbene is strictly critical. While the highly π-accepting CAAC allows nearly thermoneutral dimerization (ΔG° = –2.2 kcal/mol), the less accepting NHC–CS₂ adduct fails to dimerize due to a massive thermodynamic barrier (ΔG° = +24.9 kcal/mol) driven by the loss of aromatic stabilization.
Abstract
The cyclic (alkyl)(amino)carbene–carbon disulfide adduct undergoes thermally induced cyclodimerization to a 1,2,4,5-tetrathiane and quantitatively dissociates back to the monomer under light irradiation. This stimulus-driven, reagent-free, and reversible bond formation and cleavage contrasts with the conventional dynamic behavior of S–S bonds.
The Limits of Dynamic Disulfide Bonds
Disulfide (S–S) bonds are the most common dynamic covalent linkers used in self-healing polymers and biological systems. However, traditional dynamic disulfide exchange reactions merely create transient thiyl radicals that instantly recombine, making it nearly impossible to "lock" the molecule in a completely cleaved state. Furthermore, deliberately switching between bonded and non-bonded states typically requires the addition of harsh chemical oxidants or reductants, which limits their long-term applicability.
A Truly Reagent-Free Covalent Switch
Our research fundamentally solves this challenge by leveraging the unique electronic properties of cyclic (alkyl)(amino)carbenes (CAACs). By utilizing the CAAC–CS₂ adduct, we created a system where both the bonded (dimer) and cleaved (monomer) states are thermodynamically stable and can be isolated. More importantly, the toggle between these two states is exclusively controlled by clean, external stimuli: heat directly forms the bonds, and light precisely cuts them.
Implications for Smart Materials
This reagent-free, orthogonally switchable covalent linkage provides a groundbreaking design principle for advanced materials. Because the reaction requires no chemical inputs to switch states, it offers a clean, sustainable, and highly controllable mechanism for developing the next generation of light-responsive adaptive networks, smart adhesives, and dynamic polymers with on-demand self-healing properties.
Author's Manuscript (PDF)
2025-cc-CAACCS2-dimerization.pdf
Room 405, Chem. Bldg., Pusan National University, Geumjeong-gu, Busan, Republic of Korea
TEL : 051.510.2291 / E-mail : youngsuk.kim@pusan.ac.kr부산대학교 화학과 김영석 교수 연구실
TEL : 051.510.2291 / E-mail : youngsuk.kim@pusan.ac.kr부산대학교 화학과 김영석 교수 연구실
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