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

Summary

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.

Abstract

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.

Introduction

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 reactions^3,4. 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 ....

Protocol

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. Preparation of the solution of sodium tetraphenylborate (NaBPh₄) in ethanol.
      1. Add 1.35 g (3.92 mmol) of sodium tetrapheny....

Results

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.......

Discussion

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.......

Materials

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