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Despite the ubiquity of singlet carbenes in chemistry, their utility as true monomeric building blocks for the synthesis of functional organic polymers has been underexplored. In this work, we exploit the capability of purposely designed mono- and bis-acyclic amino(aryl)carbenes to selectively dimerize as a general strategy to access diaminoalkenes and hitherto unknown amino-containing poly(p-phenylene vinylene)s (N-PPV's). The unique selectivity of the dimerization of singlet amino(aryl)carbenes, relative to putative C-H insertion pathways, is rationalized by DFT calculations. Of particular interest, unlike classical PPV's, the presence of amino groups in α-position of C=C double bonds in N-PPV's allows their physico-chemical properties to be manipulated in different ways by a simple protonation reaction. Hence, depending on the nature of the amino group (iPr2N vs. piperidine), either a complete loss of conjugation or a blue-shift of the maximum of absorption is observed, as a result of the protonation at different sites (nitrogen vs. carbon). Overall, this study highlights that singlet bis-amino(aryl)carbenes hold great promise to access functional polymeric materials with switchable properties, through a proper selection of their substitution pattern. Despite the ubiquity of singlet carbenes in chemistry, their utility as true monomeric building blocks for the synthesis of functional organic polymers has been underexplored. Here the authors show mono- and bis-acyclic amino(aryl)carbenes selectively dimerize to form diaminoalkenes and hitherto unknown amino-containing poly(p-phenylene vinylene)s. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The activation behavior of two N-heterocyclic carbenes (NHCs), namely, 1,3-bis(isopropyl)imidazol-2-ylidene(NHC iPr) and 1,3-bis( tert-butyl) imidazol-2-ylidene (NHC tBu), as organic nucleophiles in the reaction with methyl methacrylate (MMA) is described. NHC tBu allows the polymerization of MMA in DMF at room temperature and in toluene at 50 °C, whereas NHC iPr reacts with two molecules of MMA, forming an unprecedented imidazolium-enolate cyclodimer (NHC iPr/MMA=1:2). It is proposed that the reaction mechanism occurs by initial 1,4-nucleophilic addition of NHC iPr to MMA, generating a zwitterionic enolate 2, followed by addition of 2 to a second MMA molecule, forming a linear imidazolium-enolate 3 (NHC iPr/MMA=1:2). Proton transfer, generating intermediate 5, followed by cyclization and release of methanol yielded the aforementioned zwitterionic cyclodimer 1:2 adduct 7, the molecular structure of which has been established by NMR spectroscopy, X-ray diffraction, and mass spectrometry. This unexpected difference between NHC tBu and NHC iPr in the reaction with MMA (polymerization and cyclodimerization, respectively) can be rationalized by using DFT calculations. In particular, the nature of the NHC strongly influences the cyclodimerization pathway, the cyclization of 5 and the release of methanol are the discriminating step and limiting step, respectively. In the case of NHC tBu, both steps are strongly disfavoured compared with that of NHC iPr (energetic difference of around 14 and 9 kcal mol−1, respectively), preventing the cyclization mechanism from a kinetic viewpoint. Moreover, addition of a third molecule of MMA in the polymerization pathway results in a lower activation barrier than that of the limiting step in the cyclodimerization pathway (difference of around 14 kcal mol−1), in agreement with the formation of polymethyl methacrylate (PMMA) by using NHC tBu as nucleophile. [ABSTRACT FROM AUTHOR]

ABSTRACT Synthesis of novel poly(ionic liquid)s, namely, poly(1-vinyl-3-alkylimidazolium hydrogen carbonate)s, denoted as poly([NHC(H)][HCO3])s or PVRImHCO3, where R is an alkyl group (R = ethyl, butyl, phenylethyl, dodecyl), is described. Two distinct synthetic routes were explored. The first method is based on the free-radical polymerization (FRP) of 1-vinyl-3-alkylimidazolium monomers featuring a hydrogen carbonate counter anion (HCO3−), denoted as VRImHCO3. The latter monomers were readily synthesized by alkylation of 1-vinylimidazole (VIm), followed by direct anion exchange of 1-vinyl-3-alkylimidazolium bromide monomers (VRImBr), using potassium hydrogen carbonate (KHCO3) in methanol at room temperature. Alternatively, the same anion exchange method could be applied onto FRP-derived poly(1-vinyl-3-alkylimidazolium bromide) precursors (PVRImBr). All PVRImHCO3 salts proved air stable and could be manipulated without any particular precautions. They could serve as polymer-supported precatalysts to generate polymer-supported N-heterocyclic carbenes, referred to as poly(NHC)s, formally by a loss of 'H2CO3' (H2O +CO2) in solution. This was demonstrated through selected organocatalyzed reactions of molecular chemistry, known as being efficiently mediated by molecular NHC catalysts, including benzoin condensation, transesterification and cyanosilylation of aldehyde. Of particular interest, recycling of the polymer-supported precatalysts was possible by re-carboxylation of in situ generated poly(NHC)s. Organocatalyzed reactions could be performed with excellent yields, even after five catalytic cycles. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4530-4540 [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A metal-free ion-exchange strategy is implemented to achieve physically crosslinked imidazolium-based copoly(ionic liquid)s. • The gel-type structure serves as heterogeneous thermally latent polymer-supported N -heterocyclic carbenes (NHCs). • Organocatalyzed molecular reactions can be performed in presence of the physically crosslinked gel-supported NHCs. • The crosslinked gel can be recycled and reused without any loss of efficiency after 5 catalytic cycles. • This work introduces an innovative alternative for supporting NHCs toward their facile manipulation for organocatalysis. Abstract Owing to their broad modularity, polymeric versions of ionic liquids, referred to as poly(ionic liquids) (PILs), have attracted increasing attention as recyclable polymer-supported (pre)catalysts for various chemical transformations. Herein, an imidazolium-based statistical copolymer based on PIL (=coPIL) is specifically designed by free-radical copolymerization of styrene and 4-vinylbenzylethyl(benz)imidazolium chloride. A selective ion-exchange reaction can be subsequently achieved to incorporate bio-sourced difunctional sebacate-type counter-anions, causing the physical crosslinking of the coPIL precursor via electrostatic interactions between pendant imidazolium moieties and sebacate dianions. The as-obtained gel-type precursor exhibits a thermally latent behavior in THF, proving advantageous for a facile manipulation and practical use for organocatalysis. Upon heating, typically at 80 °C, interaction between the sebacate dianion and the proton in C2-position of the imidazolium moieties generates polymer-supported N -heterocyclic carbene units that act as catalytic active species towards NHC-organocatalyzed reactions, namely, benzoin condensation, transesterification and cyanosilylation. The PIL-based gel precursors can be restored, recycled and reused by simply cooling down, i.e. with no need of an external chemical reagent, due to the shift of the intramolecular equilibrium towards the formation of imidazolium sebacate-type units. Overall, this novel gel-type copolymeric platform shows a thermo-responsive behaviour, and proves particularly versatile for heterogeneous organocatalysis. [ABSTRACT FROM AUTHOR]

Polyaziridines (PAz) were synthesized for the first time by the 1,3-bis(isopropyl)-4,5(dimethyl)imidazol-2-ylidene-organocatalyzed ring-opening polymerization (OROP) of 2-alkyl- N - p -toluenesulfonyl aziridine (alkyl = methyl or phenyl), in the presence of both functional activated amine and non-functional non-activated amine initiators. Thus, not only an allyl-functionalized N -sulfonyl amine could serve as initiator, but also trimethylsilyl azide allowed introducing an allyl and an azido functionality in α-position of PAz chains, respectively. A non-activated and commercially available secondary amine, such as di- n -butylamine, also effectively initiated the OROP of N -tosylaziridines. Excellent control over molar masses, high chain-end fidelity and narrow dispersities ( Ð ≤ 1.20) were achieved, as attested by NMR spectroscopy, size exclusion chromatography and MALDI ToF mass spectrometry. PAz precursors consisting of the alkene or the azido functionality could further be derivatized, highlighting the accessibility of those functional groups. Overall, this N -heterocyclic carbene-OROP methodology offers a metal-free route to well-defined α-functionalized PAz. [ABSTRACT FROM AUTHOR]

Imidazolium-2-carboxylates (NHC-CO2 adducts, 3) and (benz)imidazolium hydrogen carbonates ([NHC(H)][HCO3], 4) were independently employed as organic precatalysts for various molecular N-heterocyclic carbene (NHC) catalyzed reactions. NHC--CO2 adducts were obtained by carboxylation in THF of related free NHCs (2), while the synthesis of [NHC(H)][HCO3] precursors was directly achieved by anion metathesis of imidazolium halides (1) using potassium hydrogen carbonate (KHCO3) in methanolic solution, without the need for the prior preparation of free carbenes. Thermogravimetric analysis (TGA) and TGA coupled with mass spectrometry (TGA-MS) or most [NHC(H)[HCO3] precursors 4 showed a degradation pronle in stages, with either a concomitant or a stepwise release of H2O and CO2, between 108 and 280 °C, depending on the nature of the azolium and substituents. In solution, NHC generation from both [NHC(H)] [HCO3] salts and NHC--CO2 adducts could be achieved at room temperature, most likely by a simple solvation effect Both types of precursors proved efficient for organocatalyzed molecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficiencies of NHC--CO2 adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)] [HCO3] counterparts 4. [ABSTRACT FROM AUTHOR]

Anion metathesis of imidazol(in)ium chlorides with KHCO3 afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO3]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO2 adducts. The [NHC(H)][HCO3] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO3] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO3] precursors occurred via the formal loss of H2CO3via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations. [ABSTRACT FROM AUTHOR]

An N-heterocyclic carbene (NHC), namely, 1 ,3-bis-(diisopropyl)imidazol-2-ylidene (1), was demonstrated to bring about the metal-free ring-opening polymerization of ethylene oxide at 50°C in dimethyl sulfoxide, in absence of any other reagents. PoIy(ethylene oxide) (PEO) of polydispersities <1.2 and molar masses perfectly matching the [monomer]/[(1)] ratio could thus be obtained in quantitative yields, attesting to the controlled/living character of such carbene-initiated polymerizations. It is argued that (1) adds to ethylene oxide to form a zwitterionic species, namely 1,3 -bis-(diisopropyl)imidazol-2-ylidinium alkoxide, that further propagates by a zwitterionic ring-opening polymerization (ZROP) mechanism. Through an appropriate choice of terminating agent NuH or NuSiMe3 at the completion of the polymerization, a variety of end-functionalized PEO chains could be generated. In particular, α,ω-bis(hydroxy)-telechelic PEO, α-benzyl,ω-hydroxy, and α-azido,ω-hydroxy-difunctionalized PEOs were synthesized by NHC (1)-initiated ZROP, using H2O, PhCH2OH, and N3SiMe3 as terminating agent, respectively. Characterization of these α,ω-difunctionalized PEOs by techniques such as ¹H NMR spectroscopy, MALDI-TOF spectrometry, and size exclusion chromatography confirmed the quantitative introduction of functional groups at both a and w positions of the PEO chains and the formation of very narrow molar mass polymers. Finally, the synthesis of a poly(ethylene oxide)-b-poly(ϵ-caprolactone) diblock copolymer by sequential ZROP of the corresponding monomers was successfully achieved using (1) as organic initiator without isolation of the PEO block intermediate. [ABSTRACT FROM AUTHOR]