Proton-Transfer Polymerization (HTP): Converting Methacrylates to Polyesters by an N-Heterocyclic Carbene

Reactions of Diethylazo-Dicarboxylate with Frustrated Lewis Pairs

Reactions of PAr₃ /B(C₆ F₅ )₃ (Ar=o-Tol, Mes, Ph) FLPs with diethyl azodicarboxylate (DEAD) afford the corresponding FLP addition products 1-3 in which P-N and B-O linkages are formed. In contrast, the reaction of BPh₃ , PPh₃ and DEAD gave product 4 where P-N and N-B linkages were confirmed. In all cases, other binding modes were computed to be both higher in energy and readily distinguishable by ³¹ P and ¹¹ B NMR parameters. These data illustrate the influence of steric demands and electronic structures on the nature of the products of FLP reactions with DEAD.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Insights into the N-Heterocyclic Carbene (NHC)-Catalyzed Intramolecular Cyclization of Aldimines: General Mechanism and Role of Catalyst

One of the most challenging questions in the Lewis base organocatalyst field is how to predict the most electrophilic carbon for the complexation of N-heterocyclic carbene (NHC) and reactant. This study provides a valuable case for this issue. Multiple mechanisms (A, B, C, D, and E) for the intramolecular cyclization of aldimine catalyzed by NHC were investigated by using density functional theory (DFT). The computed results reveal that the NHC energetically prefers attacking the iminyl carbon (AIC mode, which is associated with mechanisms A and C) rather than attacking the olefin carbon (AOC mode, which is associated with mechanisms B and D) or attacking the carbonyl carbon (ACC mode, which is associated with mechanism E) of aldimine. The calculated results based on the different reaction models indicate that mechanism A (AIC mode), which is associated with the formation of the aza-Breslow intermediate, is the most favorable pathway. For mechanism A, there are five steps: (1) nucleophilic addition of NHC to the iminyl carbon of aldimine; (2) [1,2]-proton transfer to form an aza-Breslow intermediate; (3) intramolecular cyclization; (4) the other [1,2]-proton transfer; and (5) regeneration of NHC. The analyses of reactivity indexes have been applied to explain the chemoselectivity, and the general principles regarding the possible mechanisms would be useful for the rational design of NHC-catalyzed chemoselective reactions.

Sulfenate anions as organocatalysts for benzylic chloromethyl coupling polymerization via C=C bond formation

Organocatalytic polymerization reactions have a number of advantages over their metal-catalyzed counterparts, including environmental friendliness, ease of catalyst synthesis and storage, and alternative reaction pathways. Here we introduce an organocatalytic polymerization method called benzylic chloromethyl-coupling polymerization (BCCP). BCCP is catalyzed by organocatalysts not previously employed in polymerization processes (sulfenate anions), which are generated from bench-stable sulfoxide precatalysts. The sulfenate anion promotes an umpolung polycondensation via step-growth propagation cycles involving sulfoxide intermediates. BCCP represents an example of an organocatalyst that links monomers by C=C double bond formation and offers transition metal-free access to a wide variety of polymers that cannot be synthesized by traditional precursor routes.

Polymerization reactions are often catalysed by metal compounds and hence there are concerns surrounding toxicity, cost and environmental friendliness. Here the authors show sulfenate anions as organocatalysts for benzylic chloromethyl-coupling polymerization reactions to form poly(stilbene)s.

Polycondensation of methacrylates: auto-tandem organocatalysis using N-heterocyclic carbenes

N-Heterocyclic carbenes catalyzed the self-polycondensation of hydroxy-functionalized methacrylates and the direct polycondensation of n-butyl methacrylate with diols to produce new unsaturated polyesters.

Chemoselective Lewis pair polymerization of renewable multivinyl-functionalized γ-butyrolactones

Multivinyl-functionalized γ-butyrolactones, γ-vinyl-γ-methyl-α-methylene-γ-butyrolactone (γVMMBL) and γ-allyl-γ-methyl-α-methylene-γ-butyrolactone (γAMMBL), have been synthesized from biorenewable ethyl levulinate and effectively polymerized by Lewis pairs consisting of an organic N-heterocyclic carbene Lewis base and a strong organo-Lewis acid E(C6F5)3 (E = Al, B). This Lewis pair polymerization is quantitatively chemoselective, proceeds exclusively via polyaddition across the conjugated α-methylene double bond without participation of the γ-vinyl or γ-allyl double bond, and produces high-molecular-weight functionalized polymers with unimodal molecular-weight distributions. The Al-based Lewis pair produces a polymer with approximately 5.5 times higher molecular weight than that produced by the B-based Lewis pair. The resulting vinyl-functionalized polymers are soluble in common organic solvents and stable at room temperature, and can be thermally cured into crosslinked materials.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Phosphine catalysed (5 + 1) annulation of ynone/cinnamates with primary amines

The (5 + 1) annulation of ynone/cinnamates with protected primary amines gives rise to various heterocycles in good to excellent yields under phosphine catalysis.

The organocatalytic ring-opening polymerization of N-tosyl aziridines by an N-heterocyclic carbene

The ring-opening polymerization of N-tosyl aziridines, in the presence of 1,3-bis(isopropyl)-4,5(dimethyl)imidazol-2-ylidene as an organocatalyst and an N-tosyl secondary amine as initiator mimicking the growing chain, provides the first metal-free route to well defined poly(aziridine)s (PAz) and related PAz-based block copolymers.

Organocatalytic and Chemoselective Polymerization of Multivinyl-Functionalized γ-Butyrolactones

Achieving complete chemoselectivity in the polymerization of multivinyl polar monomers is an important yet challenging task, currently achievable only by metal- or metalloid-mediated polymerization processes but in a noncatalytic fashion. Now this work shows that organic N-heterocyclic carbene (NHC) catalysts effect rapid, chemoselective, and catalytic polymerization of multivinyl-functionalized γ-butyrolactones, particularly γ-vinyl-α-methylene-γ-butyrolactone (VMBL). Thus, the NHC-catalyzed polymerization of VMBL not only is quantitatively chemoselective, proceeding exclusively via polyaddition across the conjugated α-methylene double bond while leaving the γ-vinyl double bond intact, but also requires only an exceptionally low catalyst loading of 50 ppm, thus, exhibiting a remarkably high catalyst turnover frequency of 80000 h^-1 and producing on average 33.6 polymer chains of M_n = 73.8 kg/mol per NHC molecule. The resulting PVMBL can be either thermally cured into cross-linked materials or postfunctionalized with the thiol-ene "click" reaction to achieve complete conversion of the pendant vinyl group on every repeat unit into the corresponding thioether.

Proton-Transfer Polymerization by N-Heterocyclic Carbenes: Monomer and Catalyst Scopes and Mechanism for Converting Dimethacrylates into Unsaturated Polyesters

This contribution presents a full account of experimental and theoretical/computational investigations into the N-heterocyclic carbene (NHC)-catalyzed proton-transfer polymerization (HTP) that converts common dimethacrylates (DMAs) containing no protic groups into unsaturated polyesters. This new HTP proceeds through the step-growth propagation cycles via enamine intermediates, consisting of the proposed conjugate addition-proton transfer-NHC release fundamental steps. This study examines the monomer and catalyst scopes as well as the fundamental steps involved in the overall HTP mechanism. DMAs having six different types of linkages connecting the two methacrylates have been polymerized into the corresponding unsaturated polyesters. The most intriguing unsaturated polyester of the series is that based on the biomass-derived furfuryl dimethacrylate, which showed a unique self-curing ability. Four MeO- and Cl-substituted TPT (1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene) derivatives as methanol insertion products, (Rx)TPT(MeO/H) (R = MeO, Cl; x = 2, 3), and two free carbenes (catalysts), (OMe2)TPT and (OMe3)TPT, have been synthesized, while (OMe2)TPT(MeO/H) and (OMe2)TPT have also been structurally characterized. The structure/reactivity relationship study revealed that (OMe2)TPT, being both a strong nucleophile and a good leaving group, exhibits the highest HTP activity and also produced the polyester with the highest Mn, while the Cl-substituted TPT derivatives are least active and efficient. Computational studies have provided mechanistic insights into the tail-to-tail dimerization coupling step as a suitable model for the propagation cycle of the HTP. The extensive energy profile was mapped out, and the experimentally observed unicity of the TPT-based catalysts was satisfactorily explained with the thermodynamic formation of key spirocyclic species.

Convenient preparation of high molecular weight poly(dimethylsiloxane) using thermally latent NHC-catalysis: a structure-activity correlation

The polymerization of octamethylcyclotetrasiloxane (D4) is investigated using several five-, six- and seven-membered N-heterocyclic carbenes (NHCs). The catalysts are delivered in situ from thermally susceptible CO2 adducts. It is demonstrated that the polymerization can be triggered from a latent state by mild heating, using the highly nucleophilic 1,3,4,5-tetramethylimidazol-2-ylidene as organocatalyst. This way, high molecular weight PDMS is prepared (up to >400 000 g/mol, 1.6 < Ð M < 2.5) in yields >95%, using low catalyst loadings (0.2-0.1 mol %). Furthermore, the results suggest that a nucleophilic, zwitterionic mechanism is in operation, in preference to purely anionic polymerization.

From the N-Heterocyclic Carbene-Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

Among various N-heterocyclic carbenes (NHCs) tested, only 1,3-bis(tert-butyl)imidazol-2-ylidene (NHC(tBu) ) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC-catalyzed 1,4-addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well-defined α-alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]0/[ROH]0 molar ratio, were ultimately obtained in N,N-dimethylformamide at 25 °C. A hydroxyl-terminated poly(ethylene oxide) (PEO-OH) macro-initiator was also employed to directly access PEO-b-PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC(tBu) .

Reaction of N-heterocyclic carbenes with chalcones leading to the synthesis of deoxy-Breslow intermediates in their oxidized form

Presented herein is the reaction of N-heterocyclic carbenes (NHCs) with chalcones resulting in the isolation of the deoxy-Breslow intermediate in the oxidized form. In addition, the tetrahedral intermediate formed by the initial 1,4-addition of NHC to chalcones has also been isolated.

Organopolymerization of naturally occurring Tulipalin B: a hydroxyl-functionalized methylene butyrolactone

Naturally occurring, OH-containing, tri-functional Tulipalin B has been successfully polymerized by N-heterocyclic carbene and phosphazene superbase catalysts into polymers with M_n up to 13.2 kg mol⁻¹.

Oxa-Michael addition polymerization of acrylates catalyzed by N-heterocyclic carbenes

N-heterocyclic carbenes (NHCs) smoothly catalyze the oxa-Michael addition polymerization of hydroxyl functionalized acrylate monomers at room temperature via a zwitterionic intermediate.

N-Heterocyclic carbenes as organocatalysts for polymerizations: trends and frontiers

This review spotlights current areas of high interest for NHC-based polymerization research.