A retrospective view on curly arrows revealed by the bonding evolution theory and non-covalent interactions: reaction mechanisms meet quantum foundations.

The concept of chemical bonds and the detailed knowledge of how and where the breaking/formation processes occur are perhaps the central research fields in chemical sciences. However, the exact mechanisms underlying these processes still deserve an in-depth explanation. This perspective article systematically describes how the electronic distribution changes along the reaction pathway using Bonding Evolution Theory (BET) and Non-Covalent Interaction (NCI) analysis, highlighting their crucial role in the study of reaction mechanisms. Both methods enable on-the-fly topological classification for understanding how old bonds are broken and new bonds are formed. Specifically, the curly arrow representation is used to provide a detailed description of the molecular mechanism of chemical rearrangements based on the movements of electrons from the reactants to the products the corresponding transition structures and possible intermediates. Furthermore, this review examines various illustrative studies that offer the classic examples of internal proton transfer reaction, decarbonylation of unsaturated cyclic ketones, Mannich-type reactions, and thio-Claisen rearrangement. The analysis of the actual mechanism of these chemical reactions highlights the power of the ELF and NCI analysis to provide a physical basis for electron rearrangements along the reaction pathway. Finally, we address the current challenges and present perspectives for advancing the theoretical understanding and practical applications in this rapidly developing field.