Imidazolium‐Based Poly(Ionic Liquid)s Featuring Acetate Counter Anions: Thermally Latent and Recyclable Precursors of Polymer‐Supported N‐Heterocyclic Carbenes for Organocatalysis

Abstract

Statistical copoly(ionic liquid)s (coPILs), namely, poly(styrene)-co-poly(4-vinylbenzylethylimidazolium acetate) are synthesized by free-radical copolymerization in methanolic solution. These coPILs serve to in situ generate polymer-supported N-heterocyclic carbenes (NHCs), referred to as polyNHCs, due to the noninnocent role of the weakly basic acetate counter-anion interacting with the proton in C2-position of pendant imidazolium rings. Formation of polyNHCs is first evidenced through the quantitative formation of NHC-CS2 units by chemical postmodification of acetate-containing coPILs, in the presence of CS2 as electrophilic reagent (= stoichiometric functionalization of polyNHCs). The same coPILs are also employed as masked precursors of polyNHCs in organocatalyzed reactions, including a one-pot two-step sequential reaction involving benzoin condensation followed by addition of methyl acrylate, cyanosilylation, and transesterification reactions. The catalytic activity can be switched on and off successively upon thermal activation, thanks to the deprotonation/reprotonation equilibrium in C2-position. NHC species are thus in situ released upon heating at 80 °C (deprotonation), while regeneration of the coPIL precursor occurs at room temperature (reprotonation), triggering its precipitation in tetrahydrofuran. This also allows recycling the coPIL precatalyst by simple filtration, and reusing it for further catalytic cycles. The different organocatalyzed reactions tested can thus be performed with excellent yields after several cycles.

Reversible ionically-crosslinked single chain nanoparticles as bioinspired and recyclable nanoreactors forN-heterocyclic carbene organocatalysis

“…In the previous work from our group, SCNP-supported NHCs were generated upon heating to a temperature of 80°C involving a post-functionalization approach to confer the thermally latent behavior to NHC precatalyst moieties. 40,41 As a more facile and scalable alternative, we apply here an anionexchange method enabling the triggering of the self-folding of the RAFT-derived copolymer precursor. This provides, on the one hand, a three-dimensional robust SCNP structure and, on the other hand, a basic counter anion for the benzimidazolium moieties, hence enabling their utility as a latent NHC source upon thermal triggering.…”

“…generated upon heating SCNP 5, an NHC-CS 2 adduct forming reaction was carried out (copolymer 6; Scheme 2). 40 Upon the addition of CS 2 irreversible and selective "trapping" of the generated active NHCs in the SCNPs was expected. This was confirmed by identifying the characteristic signals of the NHC-CS 2 betaine (223.9 ppm, corresponding to the dithiolate group in the 13 C NMR spectrum; Fig.…”

“…36,37 These masked NHCs have been widely implemented in both molecular and supported macromolecular structures using a variety of different imidazolium precatalysts. Interestingly, supported imidazolium-based masked NHCs were previously decorated in our group with different basic counter anions in a wide array of different polymeric structures such as with HCO 3 − (in linear polymers), 38,39 acetate (in both linear polymers and folded SCNPs), 40,41 or sebacate (in gels), 42 by simple anion exchange. Inspired by such a concept, supporting on the one hand the imidazolium cationic moieties (NHC source) and on the other hand the basic counter anions (thermally latent trigger) on the same single polymeric chain should enable the creation of a confined SCNP pocket, based on electrostatic interactions.…”

A mechanistic insight into the organocatalytic properties of imidazolium-based ionic liquids and a positive co-solvent effect on cellulose modification reactions in an ionic liquid

“…Such a rational optimization based on a detailed mechanistic understanding could help to realize, e.g., greener conditions for cellulose modication reactions. The catalytic activities of Im-ILs in a range of different reactions were proposed to be effected via (1) the spontaneous generation of N-heterocyclic carbenes (NHCs) 21,22 from parental ILs [23][24][25] or (2) cooperative molecular recognition either by anions or by both the cations and anions of the ILs. [26][27][28] Another possibility that should be considered is the nucleophilic assistance by anions of the ILs.…”

NHC in Imidazolium Acetate Ionic Liquids: Actual or Potential Presence?

“…The demonstration of the potential presence of NHC in imidazolium acetates was obtained by many groups, carrying out successfully catalyzed or stoichiometric reactions (Lambert et al, 2016; Baumruck et al, 2017; Binks et al, 2018; Pandolfi et al, 2018), but in no case any attempt to evidence the actual presence of NHC was done. This last topic was faced in a few papers.…”