Published: November 29th, 2018
We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.
We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the protected form of NHC is employed. IMesH+BPh4- can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesH+BPh4-, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.
In chemistry, N-heterocyclic carbenes (NHCs) species fulfill the twofold role of ligand and organocatalyst1. In the former case, the introduction of NHCs has resulted in the design of metal transition catalysts with improved activity and stability2. In the latter case, NHCs have proved to be superior catalysts for manifold organic reactions3,4. Despite this versatility, handling bare NHCs is still a significant challenge5, and producing these highly reactive compounds so they are released in situ and "on demand" is a very ....
1. NHC Photogenerating System: Synthesis and Reactivity
Step 1.1 describes the efficient anion metathesis between 1,3-dimesitylimidazolium chloride (IMesH+Cl-) and sodium tetraphenylborate (NaBPh4) to yield 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-). The desired photolatent NHC is obtained in excellent yield (98%). Figure 1 shows 1H and 13C NMR spectra, both testifying that a pure product exhibiting the correct st.......
Reported here is an easy and versatile protocol for the in-situ generation of NHC upon UV-irradiation at 365 nm. The anion exchange reaction between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate provides straightforward access to the NHC protected from IMesH+BPh4- in quantitative yield. Nevertheless, if using another starting imidazolium salt, the solvent employed to perform the metathesis reaction should be chosen with care so that it allows the solubilization of both st.......
Financial support by the French National Research Agency (ANR program: DS0304 2016, contract number: ANR-16-CE07-0016) and the French Ministry of Research (doctoral grant of Emeline Placet) are gratefully acknowledged.....
|Dimesitylimidazolium chloride, 97%
|Sodium tetraphenylborate, 99%
|Dichloro(p-cymene) ruthenium dimer, 98%
|Carbon disulfide, 99.9%
|Brij S 100
|Phenol red, 98%
|Rayonet photochemical reactor
|Southern New England Ultraviolet Company
|UV lamps for photochemical reactor
|Southern New England Ultraviolet Company
|1H and 13C NMR spectrometer
|Avance III HD spectrometer
|LED Lamp and Photo-cabinet
|Dynamic Light Scattering
|zetasizer Nano ZS
|365 nm UV-LED light source coupled with a flexible light-guide
|Hg- Xe lamp with filter centred at 365 nm
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