Quantum-chemical modelling of proton transfer in the Beckmann rearrangement

Abstract

A model of proton transfer in the Beckmann rearrangement was developed by density functional theory (DFT) for the example of acetaldoxime. It was concluded that the Beckmann rearrangement of acetaldoxime may consist of three stages: Transfer of a proton from the imine carbon atom to the nitrogen atom with the formation of an acetaldoxime cation; formation of an enolic imine by an intermolecular mechanism; formation of acetamide (the final product of the Beckmann rearrangement) also by an intermolecular mechanism.The rearrangement of ketone oximes to the corresponding amides, known as the Beckmann rearrangement, is a widely used method for the synthesis of many amides and lactams [1][2][3][4]. It is assumed that the group in the anti position in relation to the oxime hydroxyl group migrates during the rearrangement process [5]. The rearrangement is caused by acidic reagents and takes place through intermediates containing an electron-deficient nitrogen atom. It is also considered that the Beckmann rearrangement is an intramolecular process [6-8].The quantum-chemical procedure of density functional theory (DFT) was used to model proton transfer in the Beckmann rearrangement [9, 10]. The calculations were performed by the "Priroda" software developed by Laikov and Ustynyuk [11] in the reaction coordinate routine using the PBE approximation [12] and its modification mPBE [13]. In addition, the BLYP functional [14,15] and the local density approximation [16] were used for purposes of comparison. The bond orders P ij and the charges at the atoms q i and also the bond angles and dihedral angles for the case of the transition state of acetaldoxime with hydrogen chloride are given in Tables 1 and 2. According to our proposed mechanism a five-membered ring of acetaldoxime with hydrogen chloride (I) (N-C-H 1 -Cl-H 6 -N) may be formed at the first stage of the Beckmann rearrangement (scheme). 1500040-5760/08/4403-0150