Green Chemistry Perspective Research Articles

Green chemistry oriented organic synthesis in water

The use of water as solvent features many benefits such as improving reactivities and selectivities, simplifying the workup procedures, enabling the recycling of the catalyst and allowing mild reaction conditions and protecting-group free synthesis in addition to being benign itself. In addition, exploring organic chemistry in water can lead to uncommon reactivities and selectivities complementing the organic chemists' synthetic toolbox in organic solvents. Studying chemistry in water also allows insight to be gained into Nature's way of chemical synthesis. However, using water as solvent is not always green. This tutorial review briefly discusses organic synthesis in water with a Green Chemistry perspective.

Direct Asymmetric Aldol Reactions Inspired by Two Types of Natural Aldolases: Water-Compatible Organocatalysts and ZnIIComplexes

In this article the utility of water-compatible amino-acid-based catalysts was explored in the development of diastereo- and enantioselective direct aldol reactions of a broad range of substrates. Chiral C(2)-symmetrical proline- and valine-based amides and their Zn(II) complexes were designed for use as efficient and flexible chiral catalysts for enantioselective aldol reactions in water, on water, and in the presence of water. The presence of 5 mol % of the prolinamide-based catalyst affords asymmetric intermolecular aldol reactions between unmodified ketones and various aldehydes to give anti products with excellent enantioselectivities. We also demonstrate aldol reactions of more demanding substrates with high affinity to water (i.e., acetone and formaldehyde). Newly designed serine-based organocatalyst promoted aldol reaction of hydroxyacetone leading to syn-diols. For presented catalytic systems organic solvent-free conditions are also acceptable, making the elaborated methodology interesting from a green chemistry perspectives.

Olefin epoxidation in solventless conditions and apolar media catalysed by specialised oxodiperoxomolybdenum complexes

The epoxidation of olefin substrates, in both apolar organic media and under solventless conditions, with aqueous hydrogen peroxide and catalysed by molybdenum complexes has been investigated. The catalysts compounds employed were the oxodiperoxomolybdenum complexes of several pyridine, 2,2′-bipyridine and pyrazole ligands with apolar functions (alkyl chains, alkyl-trimethylsilyl groups and polydimethylsiloxanyl polymer), which showed enhanced solubility in relatively apolar organic media. Both the isolated complexes and in situ preparations were catalytically active. The solubility of the new catalyst complexes appears to facilitate the catalytic activity in these systems, since activity was not observed for the analogous, insoluble complexes of unfunctionalised ligands. In these systems, the oxidant, aqueous hydrogen peroxide, forms a separate phase and the catalyst resides in the organic phase. From a green chemistry and economic perspective the elimination of organic solvents and co-catalysts from a reaction system would present advantages and, consequently, the epoxidation reaction was also investigated under solventless conditions. The 3-hexyl-5-methylpyrazole and 3-hexyl-5-heptylpyrazole complexes were found to show heightened activities, the latter being particularly efficient in these conditions, whilst bipyridines apparently inhibit the epoxidation. In addition, the mechanism of the epoxidation reaction was studied through DFT calculations for the model olefin substrate ethylene with the oxodiperoxomolybdenum complex of 3-hexyl-5-heptylpyrazole. The oxo-transfer reaction occurred by interaction of the ethylene with the peroxo ligand via the spirocyclic transition state proposed by Sharpless.

Radiolytic degradation mechanism of gallic acid and its end‐products

Gallic acid is one of the most representative biorecalcitrant phenolic compounds present in cork processing wastewater. In this communication, chemical oxidation of gallic acid was studied by gamma irradiation as an advanced oxidation process. This technology turns out to be an advantageous tool for the degradation of gallic acid. The results obtained by UV-Vis and electrospray ionization mass spectrometry (ESI-MS) techniques are in agreement concerning the suitability of this technique to degrade gallic acid. ESI-MS and ESI-MS(2) monitoring of the non-irradiated and irradiated gallic acid solutions leads to the identification of the main intermediate products. Based on the overall results obtained a consistent mechanism of radiolytic degradation of gallic acid is proposed. The application of radiation as a tool to increase the biodegradability of wastewaters is an important issue from the perspective of Green Chemistry.

A greener synthetic protocol for the preparation of carbodiimide

A new and facile preparation of symmetrical and unsymmetrical 1,3-diaryl and aryl-alkyl carbodiimides via a dehydrosulfurisation of their corresponding thioureas is described. Herein, the classical method of oxidative desulfurisation of thiourea to carbodiimide involving toxic heavy metal oxides (HgO) has been replaced with an easily available, cost-effective and environmentally benign reagent, iodine. Simple reaction conditions, easy purification of the products and high yields are important attributes of the present methodology and perhaps the best alternative from a green chemistry perspective. The only limitation to this method however, is in the preparation of 1,3-dialkyl substituted carbodiimide.

Comparing the Traditional with the Modern: A Greener, Solvent-Free Dihydropyrimidone Synthesis

A microscale organic synthesis experiment is outlined where students undertake both a traditional and Biginelli preparation of a dihydropyrimidone, within the same three-hour session. Each method is straightforward, appropriate as part of a mid-level undergraduate laboratory, and performed individually or between a pair of students. Emphasis is placed on comparing approaches from a green chemistry perspective. The class probes concepts of catalytic reactivity, solvent-free synthesis, atom economy, and energy consumption to assess green improvements made by employing the modern strategy.

Organocatalytic reactions in water

Organocatalysts have emerged as a third major way of catalyzing a wide variety of reactions, besides metal catalysts and biocatalysts. They have gained tremendous importance because of their green chemistry perspective. The criteria for green chemistry would be largely fulfilled if the major component of the reaction mixture, i.e. the solvent, is water which is a suitable solvent in various biosynthetic reactions. In this feature article we have described reactions promoted by organocatalysts in a large excess of water, without any organic solvent or excess of any reactant. We have also explained the structural features required for organocatalysts to work well in aqueous media.

Green Chemistry Perspectives of Methane Conversion via Oxidative Methylation of Aromatics over Zeolite Catalysts

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Ecomateriais: desenvolvimento e aplicação de materiais porosos funcionais para proteção ambiental

Fundamental aspects of the conception and applications of ecomaterials, in particular porous materials in the perspective of green chemistry are discussed in this paper. General recommendations for description and classification of porous materials are reviewed briefly. By way of illustration, some case studies of materials design and applications in pollution detection and remediation are described. It is shown here how different materials developed by our groups, such as porous glasses, ecomaterials from biomass and anionic clays were programmed to perform specific functions. A discussion of the present and future of ecomaterials in green chemistry is presented along with important key goals.

Green chemistry perspectives of methane conversion via oxidative methylation of aromatics over zeolite catalysts

Methane gas is known to be the most destructive greenhouse gas. The current world reserves of natural gas, which contains mainly methane, are also still underutilized due to high transportation costs. Thus, considerable interest is presently shown in conversion of methane to transportable liquid fuels and chemicals of importance to the petrochemical industry. The catalytic methylation of aromatics is one possible new route for converting methane to more valuable higher hydrocarbons. This paper provides a general overview of the recent work that we and other researchers have done on the utilisation of methane for catalytic methylation of aromatic compounds and for direct coal liquefaction for the production of liquid hydrocarbons. In particular, the paper presents a detailed description of more recent substantial experimental evidence that we have provided for the requirement of oxygen as a stoichiometry reactant for benzene methylation with methane over moderately acidic zeolite catalysts. The reaction, which has been termed “oxidative methylation”, was thus postulated to involve a two-step mechanism involving intermediate methanol formation by methane partial oxidation, followed by benzene methylation with methanol in the second step. However, strongly acidic zeolites can cause cracking of benzene to yield methylated products in the absence of oxygen. The participation of methane and oxygen, and the effective use of zeolite catalysts in this methylation reaction definitely have some positive green chemistry implications. Thus, the results of these previous studies are also discussed in this review in light of the principles and tools of green chemistry. Various metrics were used to evaluate the greenness, cost-effectiveness, and material and energy efficiency of the oxidative methylation reaction.

Large-Scale Photochemical Reactions of Nanocrystalline Suspensions: A Promising Green Chemistry Method

Photochemical reactions in the solid state can be scaled up from a few milligrams to 10 grams by using colloidal suspensions of a photoactive molecular crystal prepared by the solvent shift method. Pure products are recovered by filtration, and the use of H(2)O as a suspension medium makes this method a very attractive one from a green chemistry perspective. Using the photodecarbonylation of dicumyl ketone (DCK) as a test system, we show that reaction efficiencies in colloidal suspensions rival those observed in solution. [reaction: see text]

The Kinetics and the Mechanism of the Reaction of 2-chloro-4,6-dialkylamino-1,3,5-triazines with Ozone

Eight 2-chloro-4,6-dialkylamino-1,3,5-triazines were reacted with ozone at pH 3. Alkyl groups were methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, hydrogen. Oxidative N-dealkylation and α-oxygenation to give an amide were obtained. Kinetic measurements were used to obtain second order rate constants and activation parameters. The entropy of activation and some semi empirical calculations suggests a mechanism occurring via addition to nitrogen concerted with hydrogen transfer. The α-oxygenation reaction is the metal-free oxidation of value in a green chemistry perspective.