Contributed by Mingxin Liu, graduate student at McGill University
Oxidation of aldehyde towards carboxylic acid is a very common reaction in organic chemistry. At the same time, it is also an unavoidable element of the bio-metabolic pathway. In cytoplasm, aldehydes are highly efficiently oxidized into the corresponding carboxylic acid under very mild (37oC) conditions, utilizing oxygen as the oxidant and enzyme as catalyst. Although this process has been running in our body every day, in industry and academia, methods to achieve this transformation still remain very scarce due to the fact that most oxidation conditions cannot halt the oxidation at the carboxylic acid stage. As a result, traditional oxidations of aldehydes usually yield a mixture of different oxidation products. Even till now, oxidation of aldehydes still relies on stoichiometric amounts of expensive and hazardous oxidants, and often require organic solvents which are also eco-unfriendly.
Professor Chao-Jun Li and his team at McGill University have been focusing on the study of achieving organic reactions in water for many years. Recently, they addressed the above-mentioned difficulties of achieving aldehyde oxidation by introducing a silver(I)-based homogeneous catalyst. With this catalyst, a wide variety of different aldehydes were successfully transformed into the corresponding carboxylic acids in an extremely high efficiency. The reaction only requires very little or even ppm level of silver catalyst loading, utilizes normal atmospheric oxygen (or oxygen gas) as the sole oxidant, and water as the sole solvent under very mild temperature (50oC). Over 50 examples of different aldehydes, including aliphatic, aromatic, and unsaturated aldehydes with different electronic properties and functionalities, have successfully been oxidized into the corresponding carboxylic acids in excellent to quantitative yields. Chromatography is generally not required in almost all cases. With only 2mg (~360ppm) catalyst loading, the gram scale reaction with benzaldehyde also gave 82% isolated yield, indicating the potential possibility of expanding this methodology to industrial level.
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