Current Green Chemistry - Volume 2, Issue 1, 2015

Carbon dioxide (CO2) is not only an easily available, renewable carbon resource, but also an environmentally friendly chemical reagent. Fixation of CO2 with epoxides into cyclic carbonates (CC) is one of the most promising methodologies in this area. Recently, increasing attentions have paid to the utilization of environmentally benign hydrogen bond donors as the co-catalysts, and it is clear that hydrogen bond donors remarkably promote the reaction. In this review, we highlight the progress that have been made, and summarize the hydrogen bond donors (HBD) as co-catalysts for fixation of CO2 with epoxides into CC, and the results have been tabulated to summarize reaction conditions and catalytic activities for various typical epoxides conversions. Furthermore, the HBD-promoted reaction mechanism will also be summarized and discussed in details.

Carbon dioxide is generally known as one of the main greenhouse gasses which have caused a series of environmental problems. On the other hand, CO2 can also be regard as an abundant, nontoxic, renewable and economical C1-synthon in organic synthesis. Chemical utilization of CO2 has attracted more and more attentions all over the world and has been studied extensively and intensely for decades. Meanwhile, acrylic acid is a valuable chemical which has been widely used in industry. From the point of sustainable development, the direct installation of CO2 into organic compounds to afford carboxylic acid derivatives is one of the most concise and promising ways for CO2 utilization. In this article, advances in the metal-promoted carboxylation reaction of alkynes and allenes with carbon dioxide are highlighted with mechanistic understanding on a molecular level.

Various amino acids were covalently grafted onto DVB cross-linked polystyrene resin (Merrifield resin, PS) and functionalized with alkyl halides of different alkyl chain lengths and counter-anions. The catalysts were characterized by elemental analysis (EA), FT-IR, TGA, XPS, and SEM. The catalysts exhibited good reactivity in the cycloaddition of alkyl glycidyl ether (AGE) and CO2. The effects of the alkyl-halide structure, reaction temperature and CO2 pressure on the reactivity of the catalysts were also investigated. The catalyst could be reused for up to three consecutive runs without a severe reduction in the initial activity.

Bi-functional Zn-EDTA complexes that are non-toxic and stable can be conveniently synthesized using water as solvent. The complexes are efficient heterogeneous catalysts for the synthesis of cyclic carbonates from CO2 and epoxides without the need of a co-catalyst or solvent. Among the complexes, ZnBr2- EDTA-2Na is the most effective, showing a turnover frequency of 96.9 h-1 (cat./PO = 0.25 mol%, 150 °C, 2.25 MPa, 4 h). The catalyst can be easily recovered and reused without significant loss of activity across a test of five runs. We studied the effects of reaction temperature, pressure, and time as well as catalyst amount on the reaction. It was found that the synergetic actions of metal centers (Lewis acidic) and halide anions (nucleophilic) promote the cycloaddition reaction. A possible mechanism is proposed.

The use of carbon dioxide as a raw profitable material involves its integration in a chemical process using as mild reaction conditions as possible, which necessarily requires the use of a catalytic process. The incorporation of CO2 in cyclic carbonates by condensation with epoxides is a suitable process for that purpose. If a large scale process is the pursued objective, homogeneous catalysts proposed must be transformed in heterogeneous ones with high activity in order to simplify reaction conditions and, mainly, product isolation processes. The proposed catalysts, ranging from simple tetraalkylammonium salts or ionic liquids to salen or porphyrin metal complexes, can be supported on polymeric or inorganic matrices via covalent grafting. Due to the salt nature of some catalysts, simple adsorption on supports is also possible. Each catalytic system has specific reaction characteristics, needing or not a co-catalyst or a solvent. The support plays an additional role on the catalytic process; the existence of porosity in some cases as well as the presence of functional groups can create synergies with catalysts, and therefore improve catalysis enhancing the efficiency of the process compared with non-supported catalysis. In this review, recent progresses for the synthesis of cyclic carbonates from CO2 and epoxides will be described, particularly the aspects related to the reaction conditions and the efficiency of the system. Also, the same kind of parameters is analyzed in the use of supported catalysts for the preparation of oxazolidinones using CO2 and aziridines as reagents.

The indole ring is one of the most abundant heterocyclic scaffolds found in nature. It is built into the essential amino acid tryptophan in which the arene component of the indole facilitates π-π interactions, and the N-H group serving as a hydrogen bond donor to proximal proteinogenic species. Accordingly, the indole sub-structure forms an essential component of the framework of higher order protein structures. Although myriad synthetic routes exist to access the indole nucleus including the venerable Fisher indole synthesis and other named reactions, new methods and modifications to the existing methods remain a key area of research. Drivers include improving regioselectivity for functionalizing the core, and the need for greener approaches. Microwave and flow mediated methods have been an area of significant interest to address these issues, and this mini review will survey recent advances in the field.

Today, the need to control carbon dioxide production and emission has become an important target for both scientific and industrial world. To this purpose, the use of carbon dioxide as raw material and building block for the synthesis of complex molecules represents an efficient tool to its recycle and re-use. A great effort has been made in order to reconvert carbon dioxide to valuable organic compounds and utilize it as a source of carbon in organic synthesis. However, carbon dioxide is considered as an inert molecule and the electrochemical methodologies represent a valuable tool for its "activation" either in a direct or indirect way. In this review, the main electrochemical methodologies using carbon dioxide for the carboxylation of a large number of organic compounds appeared in the literature in the last decade, will be reported. In particular, the incorporation reaction of carbon dioxide molecule in alkenes, carbonyl compounds, alcohols, epoxides, amines and halides yielding carboxylic acids, linear and cyclic carbamates and carbonates will be discussed. The electrochemical methods that utilize CO2 in carboxylation reactions will be considered in detail, to provide a detailed picture of the state of the art.

Biomass can efficiently replace petroleum in the production of fuels for the transportation sector. One effective strategy for the processing of complex biomass feedstocks involves previous conversion into simpler compounds (platform molecules) which are more easily transformed in subsequent upgrading reactions. This contribution is aimed to provide a short overview on biomass characteristics and processing chemistries illustrated by some examples from the group related to relevant catalytic strategies for biorefineries.