Comparative study of eco-friendly methoximation of aldehydes and ketones using MnCl₂.4H₂O and CeCl₃.7H₂O as catalysts

Methoximes are a special group among oxime ethers; they are notoriously the most widely used oxime derivatives both in chemical synthesis and in the pharmaceutical industry. For this reason, there is a continuous and permanent interest in the synthesis of methoximes.

Conventional methods towards their synthesis mostly rely on the reaction between methoxylamine and aldehydes or ketones; however, despite the significance of these protocols, most of them require hard conditions, which may be incompatible with highly functionalized or sensitive compounds.

We have recently advanced the state of the art by demonstrating that the methoximation of aldehydes and ketones with various substituents can be accelerated by CeCl₃.7H₂O as a suitable promoter. As part of our ongoing research program, we are interested in developing simple, efficient and eco-friendly transformations. Therefore, in pursuit of these interests, here we report an efficient approach for the methoximation of aldehyde and ketones using MnCl₂.4H₂O, MeONH₂.HCl and NaOAc in EtOH at 50 °C as solvent and compare the results with the previous approach. Under these mild conditions, expedient processes and good to excellent product yields were achieved.

More than 20 reactions were carried out with different aldehydes and ketones, generally in shorter reaction times and in some cases, even better yields were obtained when compared to the cerium-based catalysis. This is also advantageous, since manganese salts represent an even greener and inexpensive alternative, because manganese is of great abundance on Earth.