Aromaticity of Cope and Claisen rearrangements

• Published in: Theoretical chemistry accounts 2023-05, Vol.142 (5), Article 40
• Main Authors: Castro, Thiago S., Martins, Guilherme F., de Alcântara Morais, Sara F., Ferreira, Daví A. C.
• Format: Article
• Language: English
• Subjects: Aromaticity; Atomic/Molecular Structure and Spectra; Chemical bonds; Chemistry; Chemistry and Materials Science; Density functional theory; Inorganic Chemistry; Investigations; Kinetics; Magnetochemistry; Organic Chemistry; Physical Chemistry; Regular Article; Theoretical and Computational Chemistry
• ISSN: 1432-881X; 1432-2234
• DOI: 10.1007/s00214-023-02975-0

Claisen and Cope rearrangements are [3,3]-sigmatropic rearrangements thermally activated that occur through six-membered transition states. Although extensively investigated for decades, little is known about the magnetochemistry of these rearrangements. In view of this, we carried out an investigation based on chemical-computational models through methods based on Density Functional Theory, QTAIM, Multicenter Bond Order, NCI, GIAO, and GIMIC. We demonstrated that CCR mechanisms are concerted in which the 6-membered cyclic transition states present high aromaticity character. The molar and anisotropic susceptibility, NICS(1), and NICSzz signals were verified to have a good correlation with J (nA T −1 ). However, these indices demonstrated insufficient to evaluate the magnitudes of aromaticity in these transition states. Among the magnetic and topological descriptors applied in this work, the magnetically induced current density ( J ) proved, this to be an excellent strategy for theoretically estimative of the aromaticity of the transition states involved in the investigated rearrangements. Among the rearrangements with chair conformation, it was noted that the higher aromaticity is associated with the less favoured kinetics. For chair conformations, Claisen rearrangement (TS2) presents J  = 8.63 nA T −1 and Cope (TS4) presents J  = 10.43 nA T −1 . However, the following order of aromaticity TS4 > TS2 > TS3 > TS1, with high paramagnetic currents in the boat conformations reducing the total current in the ring, suggests that it is not possible to establish a direct correlation between aromaticity and the kinetics of the Cope and Claisen rearrangements, since the most stable transition estate geometries are not necessarily the most aromatic.