Synthetic Inorganic Chemistry Lab - 2025-eurjoc-Mechanochem

Mechanochemical Dissociation of Singlet Carbene–CS₂ Adducts

Gayeong Lim, Subin Park, Minji Lee, Dongmin Kang, Youngsuk Kim*

Cite this article as: Eur. J. Org. Chem. 2025, 28, e202500004.
DOI: https://doi.org/10.1002/ejoc.202500004

Short Summary

This study demonstrates that the mechanochemical release of CS₂ from singlet carbene–CS₂ adducts is strictly governed by the carbene's HOMO energy, discovering that the highly electrophilic diamidocarbene (DAC) enables the fastest and cleanest base-free generation of free carbenes.

Key Findings

Distinct Mechanochemical Reactivities: Under ball-milling conditions (30 Hz) with a sulfur trapping agent, DAC–CS₂ achieved complete dissociation to free carbene within just 20 minutes, whereas NHC–CS₂ required over 60 minutes, and CAAC–CS₂ rapidly degraded into a complex mixture.
Activation Energy Correlation: Density functional theory (DFT) calculations revealed that the activation energy for CS₂ release strictly correlates with the reaction rates: DAC–CS₂ (22.9 kcal/mol) < NHC–CS₂ (25.3 kcal/mol) ≪ CAAC–CS₂ (32.1 kcal/mol).
HOMO-Driven σ-Donation: The strength of the C–C bond in the adducts is governed by the nucleophilicity of the carbene. CAAC, possessing the highest HOMO energy, forms the strongest bond via intensive σ-donation to CS₂, preventing dissociation. Conversely, DAC's low HOMO energy weakens this bond, facilitating easy release.
Triplet State Decomposition: Computations indicate that the high singlet–triplet energy gap in CAAC–CS₂ forces the mechanically excited adduct into a low-lying triplet state decomposition pathway rather than a clean CS₂ dissociation.

Abstract

Owing to the extensive application of singlet carbenes, their generation through the release of small molecules from stable adducts is attracting increasing research interest. This study explores the mechanochemical release of CS2 from Carbene–CS2 adducts formed by three distinct singlet carbenes: cyclic (alkyl)(amino)carbene (CAAC); N-heterocyclic carbene (NHC); and N,N’-diamidocarbene (DAC). Under ball-milling conditions, these adducts exhibit notably different reactivities; DAC–CS2 releases CS2 completely within 20 min, NHC–CS2 has a slower release rate, while CAAC–CS2 decomposes into a complex mixture. DFT calculations reveal that the release of CS2 from CAAC–CS2 requires the highest activation energy (32 kcal/mol), suggesting that it is inaccessible under the reaction conditions. This is likely because of the higher HOMO energy of CAAC, which correlates with a stronger bond with CS2, thereby limiting its dissociation. This study highlights how the electronic structures of carbenes influence their interactions with small molecules, paving the way for controlling the reactivity of carbene–small-molecule adducts.

The Quest for Base-Free Carbene Generation

Stable singlet carbenes are incredibly powerful tools in synthetic chemistry and catalysis. However, traditionally generating them requires treating protonated precursors with strong bases, which heavily limits their use in base-sensitive reactions. Recently, releasing small molecules (like CO₂ or CS₂) from bench-stable carbene adducts using thermal or mechanochemical energy has emerged as a promising, base-free alternative.

Electronic Tuning of Mechanical Release

A critical question in this emerging field is: How can we control the ease of this small-molecule release? To answer this, we systematically compared three representative singlet carbenes—cyclic (alkyl)(amino)carbene (CAAC), N-heterocyclic carbene (NHC), and N,N’-diamidocarbene (DAC)—which span a wide range of frontier molecular orbital energies.

Nucleophilicity Dictates Dissociation

We discovered a direct, intuitive principle: the harder the carbene pushes electrons toward CS₂ (σ-donation), the harder it is to break them apart mechanically. Because back-donation from CS₂ is negligible, the carbene's HOMO energy dictates the entire bond strength. This fundamental insight into carbene–small-molecule interactions provides a clear predictive model for chemists. By carefully selecting or designing carbenes based on their electronic properties (specifically HOMO levels), researchers can now precisely engineer pathways to generate valuable carbene compounds under mild, mechanochemical conditions without using any strong bases or reductants.

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