Fluorine effect on pericyclic and pseudopericyclic processes: Evidences and ab initio theory

Abstract

Electrocyclic ring opening (ERO) reactions of 2-pyrone, 2-pyranol and pyran and their fluoro compounds (1-6) have been studied at MP2/6-31G(d) level with special emphasis on the influence of fluorine on these pericyclic/pseudopericyclic processes. Calculations clearly predict that substitution of fluorine at C6 favour the reaction both kinetically and thermodynamically. Magnetic susceptibility anisotropy (Δχ aniso ), NICS(0), NBO and bond critical property (BCP) analyses clearly illustrate the following; 2-pyrone (1) and 6-fluoro-2-pyrone (2) reactions are pseudopericyclic; 6-fluoro-2-pyranol (reaction 4) corresponds to a borderline case; 2-pyranol (3) and pyran (5) and 6-fluoro pyran (6) reactions are clearly pericyclic in character. Correspondingly pseudeopericyclic reactions show up orbital disconnections and fluorine delays the occurrence of orbital disconnections on the reaction trajectory.

Can aromaticity trigger thermal [1,5]‐halogen shift towards the forbidden antarafacial mode?: A density functional case study

“…Interestingly, the W‐H forbidden ([A]) mode seems to be relatively economical compared with the supposedly the most favoured ([S]) pathway. This intriguing observation is consistent with the fact in our earlier paper . Although the issue was not brought to light in the paper for the purpose of not deviating from the focus of the paper, the geometry of TS (Figure ) very well shows that fluorine takes up antarafacial pathway.…”

“…But among the two stereochemical pathways [S]/[A], the [S] is preferred for both Cl and Br ( III & IV ) in contrast to F ( II ), and both pathways are very much separated by having large difference between them, which is indicated by ΔΔG ‡ (S‐A) in Table . It is very much believed and proved that fluorine always imparts a differential effects to wherever it is involved, and the marked shift in stereochemistry seen in the present paper would add more interest to the above differential property of fluorine. The pericyclic TSs are always intertwined with the concerted aromatic TS with cyclic array of delocalised electrons .…”

“…This intriguing observation is consistent with the fact in our earlier paper. [44] Although the issue was not brought to light in the paper [44] for the purpose of not deviating from the focus of the paper, the geometry of TS ( Figure 1) very well shows that fluorine takes up antarafacial pathway. Although fluorine shift shows a relatively large barrier, the bulkier halogens viz., chlorine and bromine show relatively lower barrier not only to II (X = F) but even it is lower than I (X = H) in the suprafacial mode.…”

Transannular Diels–Alder Reactivities of 14-Membered Macrocylic Trienes and Their Relationship with the Conformational Preferences of the Reactants: A Combined Quantum Chemical and Molecular Dynamics Study

“…But by including a suitable conformational constraint, even an exo approach is favored . Computational studies have played an instrumental role in understanding pericyclic reactions in general. ,− Two different mechanisms were initially proposed for Diels–Alder reaction, namely, the concerted and stepwise pathways. − Stepwise pathway process proceeds via a diradical intermediate, while concerted pathway proceeds through a single transition state. Both DFT and ab initio calculations have been extensively used to study both these mechanisms. − The concerted pathway has been shown to be favored over the pathway involving a diradical intermediate in general.…”