Largeron, M, and M-B Fleury. 2012. A biologically inspired Cu(I)/topaquinone-like co-catalytic system for the highly atom-economical aerobic oxidation of primary amines to imines. Angewandte Chemie http://dx.doi.org/10.1002/anie.201200587.
A multistep process to produce intermediary chemicals for manufacturing can be done in one step with only three ingredients and little to no waste, report French chemists. Their method combines a starter catalyst with a small amount of copper and oxygen from the air to transform base compounds – called amines – into other chemical middlemen – known as imines. Manufacturers use imines extensively to produce products and drugs.
In comparison to current methods, the novel process is more efficient. It uses fewer chemicals, relies on safer chemicals, creates less waste and works under normal temperature and pressure. Additional studies will need to repeat and optimize the methods before they can be used on an industrial scale.
A major direction in the field of green chemistry is to develop ways to turn one chemical into another with no leftover atoms – and therefore little to no waste. This increases chemical efficiency. Calculating the number of atoms that go into the process and comparing them to what comes out in the end product is a way to measure chemical efficiency. These reactions – dubbed atom economy – strive to have the two equal each other.
Atom economy is a big shift away from the traditional processes that rely on additives known as activating reagents, which are added multiple times during a chemical synthesis to propel the reactions forward. Efficiency is reduced, since in each step, the activating reagent is not incorporated into the product and is lost as waste.
This is particularly troubling when producing pharmaceuticals where multiple steps are required to transform starting materials into commercial products. In these cases, activating groups increase chemical waste, since activation often requires an additional step in the overall sequence and frequently creates equal amounts of a byproduct.
Activating reagents are usually needed for oxidation reactions, which are particularly important in chemical production. During oxidation, hydrogen is removed from its parent molecule and is added to another. The process is chemically inefficient because it creates an equal amount of chemical waste for each molecule that is oxidized.
Researchers are looking to plants and animals for natural ways to improve efficiency during oxidation reactions. One way is to replace activating reagents with catalysts. Biological systems are extremely efficient and routinely use catalysts in reactions where the chemical industry uses activating reagents. Unlike activating reagents, catalysts can be used in very small quantities. This would dramatically reduce chemical waste, since small quantities of catalysts affect a reaction and they can be frequently recovered after a reaction.
In this study, researchers in France have accomplished this by replicating the activity of an important class of enzymes known as copper-containing amine oxidases (CuAOs). These enzymes control the concentration of nitrogen-containing molecules in numerous biological settings, including in people. The enzymes drive the oxidation of an amine to an imine. By carefully modulating the reaction conditions, the authors were able to produce a variety of imines from readily available starting materials under very mild conditions. Water was the only byproduct.
The authors note the reactions will need more tweaking before they can be used industrially. Nevertheless, the reaction conditions go a long way toward alleviating the typical waste associated with amine oxidation and provide a promising direction for future research.
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