A DFT study on secondary reaction pathways in the acid-catalysed Beckmann rearrangement of cyclohexanone oxime in aprotic solvent

Abstract

In recent studies regarding acid-catalysed Beckmann rearrangement of cyclohexanone oxime in aprotic solvents it has been observed that a quite surprising hydrolysis of the starting material, i.e., cyclohexanone oxime, occurs even if the experimental conditions and the water concentration in the reaction mixture can not justify such a side-reaction. Being this apparent hydrolysis critical for catalyst selectivity and poisoning, a computational DFT study on a possible secondary reaction pathway, involving a bi-molecular reaction between two cyclohexanone oxime molecules in acid media as the primary step, is here reported to explain the experimental results.

New insight on the mechanism of the catalytic hydrogenation of nitrobenzene to 4-aminophenol in CH3CN–H2O–CF3COOH as a reusable solvent system. Hydrogenation of nitrobenzene catalyzed by precious metals supported on carbon

“…The bases of the present research are the results in the Beckmann rearrangement of ketoximes to the corresponding amides in CH 3 CN-CF 3 COOH as solvent catalytic system [17][18][19][20]. The analogy between the Beckmann rearrangement with the process to 4-aminophenol via nitrobenzene hydrogenation originates from the idea that the N-phenylhydroxylamine (the intermediate of the process) (see Scheme 1) may undergo acid Bamberger rearrangement very easily in non-aqueous CF 3 COOH system.…”

“…Starting from these considerations, a single liquid phase process could be a catwalk to a more sustainable process by using easily reusable solvents and catalysts [17][18][19][20]. When products are solids or high boiling point liquids the use of a much lower boiling point solvent can be the key to improve the environmental sustainability of the whole process, because of products separation, catalyst and solvents reuse is made easy by filtration and evaporation.…”

Organocatalyzed Beckmann rearrangement of cyclohexanone oxime by trifluoroacetic acid in aprotic solvent

“…The presence of a strongly electrophilic nitrogen may induce side reactions in the presence of nucleophiles such as water and/ or the non protonated oxime [21,22]. As a matter of fact, a strong acid is required to ensure the complete protonation of the oxime, thus avoiding side reactions due to the non protonated oxime [21,22].…”

“…As a matter of fact, a strong acid is required to ensure the complete protonation of the oxime, thus avoiding side reactions due to the non protonated oxime [21,22]. The solvent may play an important role on aiding the hydrogen transfer as well as the concerted water extraction with the consequent rearrangement [18].…”

One-pot oximation–Beckmann rearrangement of ketones and aldehydes to amides of industrial interest: Acetanilide, caprolactam and acetaminophen

“…TFA has been also employed as catalyst for the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam [26][27][28]. In particular, a TFA/CH 3 CN mixture was used for a practical and high yielding synthesis of amides [26][27][28].…”