Understanding the sequence of the electronic flow along the HCN/CNH isomerization within a bonding evolution theory quantum topological framework

A Close Look to the Oxaphosphetane Formation along the Wittig Reaction: A [2+2] Cycloaddition?

“…In fact, the OPA formation has been evidenced to be a highly asynchronous process. Note also that the positive values exhibited by the J (ξ) descriptor , in such a region of the reaction coordinate point out specifically to such an initial stage in the formation of the new C–C bond . It should be emphasized in this point that it is precisely such a process yielding to the new C–C bond formation, which drives the final alkene product stereochemistry. − ,,, …”

“…The study and characterization of the electron density rearrangement along a chemical reaction provide key insights enabling a deeper rationalization and control of the entire process (see the Supporting Information for a revision of the conceptual details). ,,,, The detailed analysis of the topology of the ELF along the IRC of the reaction of ylide 6 and benzaldehyde 7 to yield OPA 8 (see Scheme ), both in vacuum and solution (THF) phases, reveals in fact the existence of eight elementary bifurcation catastrophes characterizing a given sequence for the rearrangement of the electron pairs (as represented by ELF) along the chosen reaction path. The topological catastrophes belong in the present case exclusively to the fold (F and F † ) and cusp (C and C † ) elementary types. ,, We recall in this point that in a fold catastrophe F † a point, which is not a critical one (i.e., a wandering point), is transformed into two critical points of different parities, increasing the number of basins by 1.…”

“…Within an orbital-based representation, such a chemical event has been simply rationalized in terms of “a long range weak interaction between a lone pair in the oxygen atom of the carbonyl group and an empty p orbital in the phosphorous atom” . As above revealed, BET provides a more clear detailed picture for bond formation along the path associated with the formation of the cyclic intermediate (without reference to orbitals), which is a fully functional rationalization of chemical bond making/forming processes in terms of the reorganization of electron pairing (as represented by ELF as an indirect measure of Pauli repulsion) along the chemical transformation on a given potential energy surface. ,,,− Complementarily, BET results reveal where the main chemical events become located along the single kinetic step of reaction. The above description is summarized in Table , which reports interatomic distances at the reaction center and associated basin valence electron populations at each of the turning configurations between the different SDDs.…”

“…Based on the ELF representation (which provides a powerful description of chemical bonding) such a reaction should not be considered to be a classical 22CA reaction. Instead, a different understanding of the mechanism in terms of a two-stage one-step process is proposed. ,,− The first stage corresponds to the formation of the C–C bond that occurs at the top of the barrier, followed for the P–O bond formation that occurs at the end of the reaction path connecting TS1 and OPA 8 . The topologically based results point out clearly and undoubtedly that these two key chemical events occur separately on different molecular configurations along the single kinetic reaction step.…”

“…Following ongoing interests aimed to enhance and refine our current understanding of key reaction mechanisms, − here, we focus on the changes in the topology of the electron localization function (ELF) along the first step of the Wittig reaction within the perspective of the so-called bonding evolution theory (BET) framework. − It has in fact argued that ELF provides a clear demarcation between chemical and nonchemical processes, and that the BET framework based on ELF provides a universal framework for understanding chemical bonding, which is formally supported on the rigorous mathematical foundation arising from the topological theory of dynamical systems. ,, In such a context, quantum topological approaches constitute the basis for a robust coordinate-space representation of bonding where bond-breaking and bond-forming processes are associated to well-characterized topological changes. , These changes can be identified and characterized using Thom’s elementary catastrophe theory principles, − providing thus a rich understanding of the associated electronic rearrangements. Within the BET perspective, only cusp, fold, and elliptic umbilic elementary catastrophes have been found to constitute fingerprints associated to the elementary processes driven a chemical transformation (i.e., electron density rearrangements, associated to bond breaking/bond forming processes, along the nuclei rearrangement). ,− Our goal in this work is to further contribute to provide a more complete picture for the sequence of the flow of the electron density pairs (as derived from the ELF framework) as the nature of bonding interactions evolves along the nuclei rearrangement associated to the key step of the simplest example for a Li salt-free Wittig reaction (Scheme ). As recently emphasized, the aim of conceptual density functional theory is “to develop a nonempirically, mathematically, and physically sound, density-based, quantum–mechanical theory for interpreting and predicting chemical phenomena, especially chemical reactions”.…”