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Chemistry

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Published: November 29th, 2018

DOI:

10.3791/58539

1ICGM, Université de Montpellier, CNRS, ENSCM, 2Institut de Science des Matériaux de Mulhouse (IS2M UMR 7361 CNRS), Université de Haute-Alsace, 3Université de Strasbourg, 4Laboratoire de Chimie des Polymères Organiques (LCPO UMR 5629 ENSCBP), Université de Bordeaux

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 ....

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1. NHC Photogenerating System: Synthesis and Reactivity

  1. Synthesis of 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-)
    1. Preparation of the solution of 1,3-dimesitylimidazolium chloride (IMesH+Cl-) in ethanol.
      1. Add 1.00 g (2.93 mmol) of 1,3-dimesitylimidazolium chloride to a 50 mL round bottom flask equipped with a stir bar.
      2. Dissolve the 1,3-dimesitylimidazolium chloride in 30 mL of ethanol.
    2. .......

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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.......

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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.......

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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.

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Name Company Catalog Number Comments
Material
Dimesitylimidazolium chloride, 97% ABCR AB130859
Sodium tetraphenylborate, 99% ABCR AB118843
Dichloro(p-cymene) ruthenium dimer, 98% ABCR AB113524
Norbornene, 99% ABCR AB171849
Isopropythioxanthone, 97% Sigma Aldrich 406317
Carbon disulfide, 99.9% Sigma Aldrich 335266
Dichloromethane Sigma Aldrich 270997
Ethanol VWR 20821.31
Deuterated DMSO Eurisotop D010FE
Deuterated THF Eurisotop D149CB
1,2-Dichloroethane Sigma Aldrich 284505
Brij S 100 Sigma Aldrich 466387
Hexadecane Sigma Aldrich H6703
Phenol red, 98% Sigma Aldrich P4633
Acetonitrile VWR 83639.290
1,3-Bis(mesityl)imidazol-2-ylidene, 97% Sigma Aldrich 696188
Name Company Catalog Number Comments
Equipment
Rayonet photochemical reactor Southern New England Ultraviolet Company RPR-200
UV lamps for photochemical reactor Southern New England Ultraviolet Company RPR-3500A
1H and 13C NMR spectrometer Bruker Avance III HD spectrometer
Sonication probe BioBlock Vibra-cell
Gas chromatography Varian GC3900
LED Lamp and Photo-cabinet Peschl ultraviolet novaLIGHT TLED100-365
Dynamic Light Scattering Malvern zetasizer Nano ZS
365 nm UV-LED light source coupled with a flexible light-guide Hamamastu LC-L1V3
UV/vis spectrometer Perkin Elmer Lambda 35
Hg- Xe lamp with filter centred at 365 nm Hamamastu LC-9588/01A
Radiometer Ocean Optics USB4000

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