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Green chemistry for chemical synthesis
Chemical synthesis is the foundation of chemical science and has made great contributions to the progress of human society. To name a few, 1) the invention of synthetic pesticide has solved the food shortage issue for the world's population, 2)the invention of synthetic antibiotics and drugs has greatly improved people’s health and 3) the synthesis of new functional materials has constantly improved people's living standards.
In the new era, we are challenged with the need to improve the efficiency of synthetic reaction to reduce waste and protect people’s health and the environment. Scientists suggest that the atom economy of synthetic reactions should be given high priority. Catalysts should make the reaction process more economical, energy-efficient and environmentally friendly. Moreover, greater emphasis should be placed on the 3R (reduction, recovery and reuse) of resources.
Atom Economy in Organic Synthesis
Atom economy is a measure of reactions’ efficiency. It is developed by Barry Trost and has been included in the 12 Principles of Green Chemistry. Atom economy aims to maximize incorporation of the atoms of the reactants into the desired product, with few or no atoms’ waste. The calculation formula is:
Atom economy differs from the reaction yield as it considers both the desired product and all used reagents and by-products. Here are some examples about maximizing atom economy:
By using the ruthenium catalyst, propargyl alcohols is isomerized into α, β-unsaturated carbonyl instead of traditional two-step stoichiometric reduction and oxidation sequence, which effectively realize the catalytic enantioselective total synthesis.
In the aqueous medium, γ, δ-unsaturated ketones and aldehydes can be formed by the addition of allyl alcohol to alkynes.
The stereoselective addition of primary alcohols to alkenes provides an atom-economic version of Grignard Reaction.
Direct conversion of C-H bond
In the presence of oxidant and transition metal catalyst, two different C–H bonds can directly generate C–C bonds through a cross-dehydrogenative coupling (CDC). For example, (NH)-indoles and tetrahydroisoquinolines are converted into alkaloids by this coupling reaction.
Synthesis without protection
The protection-deprotection of functional groups is widely used in organic synthesis, which increases the synthesis step and reduces the reaction efficiency. It was found that (+) -3-deoxy-d-glycero-d-galacto-nonulosonic acid (KDN) could be efficiently synthesized by indium-mediated allylation in water.
Cascade reaction refers to the incorporation of reactions to obtain the final product in one operation, which is an important part of green synthesis as well. For example, a palladium-catalyzed cascade reaction produces multi-rings in one step.
Bio-catalysis usually has high reaction efficiency and selectivity, and various catalysts (such as enzymes) have been utilized.
To overcome the new era’s challenge, it is of great significance to maximize atom economy and achieve green chemical synthesis, which can effectively reduce or even prevent pollution, so as to better protect people’s health and the environment.
- Li, Chao-Jun, and Barry M. Trost. "Green chemistry for chemical synthesis." Proceedings of the National Academy of Sciences 105, no. 36 (2008): 13197-13202.