Felix D. Bobbink

Metal‐Free Catalyst for the Chemoselective Methylation of Amines Using Carbon Dioxide as a Carbon Source

N-methylation of amines is an important step in the synthesis of many pharmaceuticals and has been widely applied in the preparation of other key intermediates and chemicals. Therefore, the development of efficient methylation methods has attracted considerable attention. In this respect, carbon dioxide is an attractive C1 building block because it is an abundant, renewable, and nontoxic carbon source. Consequently, we developed a highly chemoselective, metal-free catalytic system that operates under ambient conditions for the N-methylation of amines.

Cycloaddition of CO2 to epoxides catalyzed by imidazolium-based polymeric ionic liquids

A series of cross-linked ionic polymers based on styrene-functionalized imidazolium salts with chloride, hexafluorophosphate, or tetrafluoroborate counter anions have been prepared and characterized using a range of analytical and spectroscopic techniques and electron microscopy. The polymer with the chloride anion is an efficient catalyst for the cycloaddition of carbon dioxide with epoxides to afford cyclic carbonates. The cross-linked polymer is insoluble in organic solvents and is highly stable and therefore can be easily recycled and reused.

Conversion of chitin derived N-acetyl-D-glucosamine (NAG) into polyols over transition metal catalysts and hydrogen in water

N-Acetyl-D-glucosamine (NAG), the monomer of the worlds second most abundant biopolymer chitin, has been for the first time converted to its corresponding amide/amino substituted sugar alcohols, smaller C2–4 polyols and N-acetylmonoethanolamine (NMEA), over noble metal catalysts in the presence of hydrogen in water. Four commercialized catalysts were investigated, and Ru/C exhibited the best performance—achieving 8.7% NMEA, 6.1% C4 polyols, and 71.9% C6 polyols (N-containing) under 180 °C, 40 bar H2, 1 h with 1 mol% loading. Kinetic studies were conducted, which revealed four major reaction pathways that lead to various products. In particular, retro-aldol reaction-hydrogenation was confirmed to be the pathway forming NMEA. The effects of additives (NaOH and WO3) on the reaction were also tested.

Carbon Dioxide Based N-Formylation of Amines Catalyzed by Fluoride and Hydroxide Anions

We described herein a simple approach for N‐formylation with CO2 and hydrosilane reducing agents. Fluoride and hydroxide salts efficiently catalyzed the reaction, principally through activation of the hydrosilanes, which led to hydrosilane reactivities comparable to those of NaBH4/LiAlH4. Consequently, the N‐formylation of amines with CO2 could be achieved at room temperature and atmospheric pressure. The mechanism of these anionic catalysts contrasts that of the currently reported systems, for which activation of CO2 is the key mechanistic step. Using tetrabutylammonium fluoride as a simple ammonium salt catalyst, the N‐formylated products of both aliphatic and aromatic amines could be obtained in excellent yields with high selectivities.

Synthesis of cyclic carbonates from diols and CO2 catalyzed by carbenes

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.

Polyimidazolium Salts: Robust Catalysts for the Cycloaddition of Carbon Dioxide into Carbonates in Solvent‐Free Conditions

There is a growing interest in sustainable heterogeneous catalysts based on organic polymers. Here, we describe a series of polyimidazolium salt catalysts, prepared from the direct reaction of arene-bridged bis- and tris-alkyl halides with trimethylsilylimidazole. The polyimidazolium salts were characterized by spectroscopic and analytical techniques and it was found that their morphology and porosity could be controlled by adjusting the steric parameters of the spacer in the alkyl-halide starting materials. Moreover, the polymers are excellent heterogeneous organocatalysts for the cycloaddition of CO2 to epoxides to afford cyclic carbonates at atmospheric pressure under solvent-free conditions. The polymer catalysts exhibit long-term stability and may be recycled and reused at least 10 times.

Influence of Elemental Iodine on Imidazolium-Based Ionic Liquids: Solution and Solid-State Effects.

Ionic liquids doped with I2, usually resulting in the formation of polyiodide anions, are extensively used as electrolytes and in iodination reactions. Herein, NMR spectroscopy and single-crystal X-ray diffraction were used to rationalize the structures of imidazolium-based polyiodide ionic liquids in the liquid and solid states. Combined, these studies show that extensive interactions between the imidazolium cation and the resulting polyiodide anion are present, which have the net effect of lengthening, polarizing, and weakening the I-I bonds in the anion. This bond weakening rationalizes the high conductivity and reactivity of ionic liquids doped with I2.

Soft Approaches to CO2 Activation

The utilization of CO(2) as a C1 synthon is becoming increasingly important as a feedstock derived from carbon capture and storage technologies. Herein, we describe some of our recent research on carbon dioxide valorization, notably, using organocatalysts to convert CO(2) into carboxylic acid, ester, formyl and methyl groups on various organic molecules. We describe these studies within the broader context of CO(2) capture and valorization and suggest approaches for future research.

N-formylation and N-methylation of amines using metal-free N-heterocyclic carbene catalysts and CO2 as carbon source

N-formylation and N-methylation of amines are important reactions that are used to produce a wide range of key intermediates and compounds. This protocol describes the environmentally benign N-formylation and N-methylation of primary and secondary amines using carbon dioxide (CO2) as the carbon source, hydrosilanes as reductants and N-heterocyclic carbenes (NHCs) as catalysts. Using CO2 as a reagent has the advantage of low cost and negligible toxicity. However, the catalyst is air-sensitive and must be generated fresh before use; consequently, the techniques used to prepare and manipulate the catalyst are described. The synthetic approach described in this protocol does not use any toxic reagents; using the appropriate catalyst, N-formylated or N-methylated products can be obtained with high selectivity. The overall time for catalyst preparation and for conducting several catalytic reactions in parallel is 15–48 h, depending on the nature of the substrates.

Ionic liquid containing electron-rich, porous polyphosphazene nanoreactors catalyze the transformation of CO2 to carbonates

We show that ionic liquids (ILs) interact with electron-rich, porous polyphosphazene (PPZ), to form hybrid PPZ-IL nanoreactors able to simultaneously capture and transform CO2 into carbonates. The PPZ nanospheres swell in organic solvents and effectively absorb IL cations by virtue of the electron-rich sites, while leaving the anions exposed and increasing their nucleophilicity. This leads to considerably higher catalytic activity compared to the IL alone in the cycloaddition reaction of CO2 to epoxides. The cation shielding effect is dependent on the structure of the IL cation and, hence, the catalytic activity can be tuned by varying the structure of the cation in the IL and DFT calculations were used to rationalize the experimentally observed differences in catalytic activity. These studies indicate that PPZ nanospheres could find widespread uses in catalysis, acting as active nanosupports for homogeneous catalysts, not only for the transformations of CO2, but also for other substrates.