Exploring Catalysis in API Synthesis - Pharmaceutical Technology

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Exploring Catalysis in API Synthesis
Chemocatalysis and biocatalysis are important elements of an effective strategy for improving yield and stereoselectivty.


Pharmaceutical Technology
Volume 34, Issue 5, pp. 42-46


IMAGE: GETTY IMAGES
Catalysis is a crucial part of organic chemistry as a tool to improve reaction conditions, yield, stereochemistry, and to make desired products. Research in catalysis involves not only the development and application of new catalysts in specific reactions, but also centers on gaining a better understanding of the molecular science of catalysis.

Catalyst screening


Patricia Van Arnum
An important task in developing a chemical process is to identify the optimal catalyst by testing different metal and ligand combinations in various reactions. Combinatorial methods can be used in ligand synthesis to produce potential catalysts, and methods to screen potential catalysts are then needed. A group of Dutch researchers recently reported on using an approach that they dubbed, "the survival of the weakest," in which they used a selection method that focuses on the stability of the catalytic intermediate measured by electrospray mass spectrometry (1, 2). The researchers reported that the stability of the intermediate related inversely to the reactivity of the catalyst, which formed the basis of a catalyst-screening protocol in which less-abundant species represented the most-active catalysts. This screening method was used in the palladium-catalyzed allylic alkylation reaction using diphosphine and IndolPhos ligands (1).

Green approaches

Catalysis also provides the opportunity to develop more sustainable chemical processes. Depending on the reaction, such benefits, for example, would allow for byproduct reduction, improving reaction conditions such as temperature and pressure, and achieving better atom efficiency through better transformations. N. Raveendran Shiju, a researcher at the University of Amsterdam's Heterogeneous Catalysis and Sustainable Chemistry Group, recently reported on a new catalyst for ammoximation reactions, according to a Feb. 26, 2010, university press release. The catalyst, which is suitable for the production of fine chemicals, pharmaceutical intermediates, and nylon, offers the potential for a more economically attractive and greener process for ammoximation.

The current industrial ammoximation processes can generate as much as 2.5 kilograms of waste for every kilogram of end product, according to the release. The new heterogeneous catalyst reduces the level of waste, is easier to make, and uses fewer chemical additives in the process. The new catalyst could possibly be used for the production of paracetamol as well as caprolactam and nylon. The researchers filed patents for the technology in the United States and European Union, according to the release. They are further studying the relationship between the structure and activity at the catalyst's surface to further optimize the process.

Researchers at the University of Scotland reported on a new approach for separating the reaction products from a catalyst in a homogeneous reaction. The researchers used rhodium complexes formed from triphenylphosphine ligands functionalized with weakly basic amidine groups as highly active catalysts for the hydroformylation of alkenes. The approach involves tagging the coordinated phosphine ligands with weakly basic amidine groups to give organic soluble catalysts for performing reactions such as the conversion of alkenes to aldehydes rapidly in a solvent such as toluene. Addition of water and bubbling carbon dioxide causes the catalyst, but not the product, to switch into the aqueous phase as the protonated bicarbonate salt. After phase separation and the addition of a new organic phase, the catalyst can be switched back into the organic phase for use in a repeat reaction by bubbling it through nitrogen at 60 C. Losses of rhodium are minimal. The approach offers the potential of using more sustainable or less-energy intensive chemical processes (3, 4).


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