Homogeneous and heterogenised masked N-heterocyclic carbenes for bio-based cyclic carbonate synthesis

Stewart, Joseph; Drexel, Roland; Arstad, Bjørnar; Reubsaet, Erik; Weckhuysen, Bert M.; Bruijnincx, Pieter C. A.

doi:10.1039/c5gc02046h

Cited by 42 publications

(37 citation statements)

References 67 publications

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“…24,25 Interestingly, these intermediates have been previously reported to catalyze the synthesis of cyclic carbonates from diols employing DMC as the carbonyl source rather than CO 2 . 26 Herein, we show the utility of carbene catalysts for the synthesis of cyclic carbonates from diols and CO 2 and, based on key experiments, propose plausible mechanisms for this transformation.…”

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confidence: 94%

Synthesis of cyclic carbonates from diols and CO₂ catalyzed by carbenes

et al. 2016

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The synthesis of cyclic carbonates from epoxides and CO2 is a well-established reaction, whereas the synthesis of cyclic carbonates from diols and CO2 is considerably more challenging, and few efficient catalysts are available. Here, we describe heterocyclic carbene catalysts, including one derived from a cheap and efficient thiazolium salt, for this latter reaction. The reaction proceeds at atmospheric pressure in the presence of an alkyl halide and Cs2CO3. Reaction mechanisms for the transformations involved are also proposed.

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confidence: 94%

Synthesis of cyclic carbonates from diols and CO₂ catalyzed by carbenes

et al. 2016

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“…In numerous transformations N ‐heterocyclic carbenes (NHC) have been utilized extensively as organocatalysts . More specifically, 1,3‐dimethylimidazolium‐2‐carboxylate (DMI‐CO 2 ) is an N ‐heterocyclic carbene precursor (a “masked carbene”) that has been employed as a catalyst in the ring‐opening polymerization and copolymerization of propylene oxide, and in the synthesis of cyclic carbonates from bio‐based diols, including glycerol . DMI‐CO 2 is attractive, because it can be obtained easily by heating DMC with 1‐methylimidazole (Scheme ), which is itself a less expensive heterocycle than DBU…”

Section: Methodsmentioning

confidence: 99%

“…The free NHC catalyst, 3 , which is generated from the decarboxylation of DMI‐CO 2 , attacks the carbonyl group of dialkyl carbonate at elevated temperature to generate 2‐alkoxycarbonyl‐1,3‐dimethylimidazolium alkoxide 4 . In the studies by Bruijnincx and co‐workers, 2‐methylcarbonyl‐1,3‐dimethylimidazolium methoxide could be generated at relatively low temperature (74 °C) from 3 and dimethyl carbonate. The cation of 4 is rationalized as being a more activated alkylating agent than dialkyl carbonate, owing to its positive charge.…”

Section: Methodsmentioning

confidence: 99%

Masked N‐Heterocyclic Carbene‐Catalyzed Alkylation of Phenols with Organic Carbonates

et al. 2016

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An easily prepared masked N-heterocyclic carbene, 1,3-dimethylimidazolium-2-carboxylate (DMI-CO2 ), was investigated as a "green" and inexpensive organocatalyst for the alkylation of phenols. The process made use of various low-toxicity and renewable alkylating agents, such as dimethyl- and diethyl carbonate, in a focused microwave reactor. DMI-CO2 was found to be a very active catalyst and excellent yields of a range of aryl alkyl ethers were obtained under relatively benign conditions. The observed difference in the conversion behavior of phenol methylation, in the presence of either the carbene or 1,8-diazabicycloundec-7-ene (DBU) catalyst, was rationalized on the basis of mechanistic investigations. The primary mode of action for the N-heterocyclic carbene is nucleophilic catalysis. Activation of the dialkyl carbonate electrophile results in concomitant evolution of an organo-soluble alkoxide, which deprotonates the phenolic starting material. In contrast, DBU is initially protonated by the phenol and thus consumed. Subsequent regeneration and participation in nucleophilic catalysis only becomes significant after some phenolate alkylation occurs.

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“…These NHC-CO 2 adducts are also useful NHC masked organocatalysts for the carboxylation of organic compounds (see as examples: Tommasi and Sorrentino, 2006; Desens and Werner, 2016; Stewart et al, 2016) and NHC-transfer agent in the synthesis of NHC-metal complexes (see as examples: Voutchkova et al, 2007; Voutchkova and Crabtree, 2010; Li et al, 2011). …”

Section: The Presence Of Co2mentioning

confidence: 99%

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

et al. 2018

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Ionic liquids (ILs) are considered in the majority of cases green solvents, due to their virtually null vapor pressure and to the easiness in recycling them. In particular, imidazolium ILs are widely used in many fields of Chemistry, as solvents or precursors of N-heterocyclic carbenes (NHCs). The latter are easily obtained by deprotonation of the C2-H, usually using strong bases or cathodic reduction. Nevertheless, it is known that weaker bases (e.g., triethylamine) are able to promote C2-H/D exchange. From this perspective, the possibility of deprotonating C2-H group of an imidazolium cation by means of a basic counter-ion was seriously considered and led to the synthesis of imidazolium ILs spontaneously containing NHCs. The most famous of this class of ILs are N,N'-disubstituted imidazolium acetates. Due to the particular reactivity of this kind of ILs, they were appointed as “organocatalytic ionic liquids” or “proto-carbenes.” Many papers report the use of these imidazolium acetates in organocatalytic reactions (i. e., catalyzed by NHC) or in stoichiometric NHC reactions (e.g., with elemental sulfur to yield the corresponding imidazole-2-thiones). Nevertheless, the actual presence of NHC in N,N'-disubstituted imidazolium acetate is still controversial. Moreover, theoretical studies seem to rule out the presence of NHC in such a polar environment as an IL. Aim of this Mini Review is to give the reader an up-to-date overview on the actual or potential presence of NHC in such an “organocatalytic ionic liquid,” both from the experimental and theoretical point of view, without the intent to be exhaustive on N,N'-disubstituted imidazolium acetate applications.

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Homogeneous and heterogenised masked N-heterocyclic carbenes for bio-based cyclic carbonate synthesis

Abstract: Substrate scope (including crude glycerol), influence of organocatalyst structure, mechanistic aspects and catalyst heterogenisation are reported.

Cited by 42 publications

References 67 publications

Synthesis of cyclic carbonates from diols and CO₂ catalyzed by carbenes

Synthesis of cyclic carbonates from diols and CO₂ catalyzed by carbenes

Masked N‐Heterocyclic Carbene‐Catalyzed Alkylation of Phenols with Organic Carbonates

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

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Homogeneous and heterogenised masked N-heterocyclic carbenes for bio-based cyclic carbonate synthesis

Abstract: Substrate scope (including crude glycerol), influence of organocatalyst structure, mechanistic aspects and catalyst heterogenisation are reported.

Cited by 42 publications

References 67 publications

Synthesis of cyclic carbonates from diols and CO2 catalyzed by carbenes

Synthesis of cyclic carbonates from diols and CO2 catalyzed by carbenes

Masked N‐Heterocyclic Carbene‐Catalyzed Alkylation of Phenols with Organic Carbonates

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

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Synthesis of cyclic carbonates from diols and CO₂ catalyzed by carbenes

Synthesis of cyclic carbonates from diols and CO₂ catalyzed by carbenes